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Exported on March 22, 2026 at 12:41 AM

Organism: Schizosaccharomyces pombe

Sequence:

MSTSNDPVVSSHDPIKQEKEQETDLEAQVEHKKRNERGNAFVGFLILIFVYYLLRGGSNDNDKQEMSHSPGSCMDSESAAVSTSAKCYIPPVLTPAKEPKLGDDVSGIDYIRSPEFFNDSLVRFQELLRIPTVCYDDMGDVGDDDRFDIFAVFQDKVRELYPNIFKKLKVEYVNTYGLLITLEGSNKDLKPLVLMGHQDVVPVNQASLDRWYFPPFSATYHNGHVYSRGAADDKNSVVAILEALEILAISDYKPEQTVIASFGFDEEVSGYRGALPLAHKLYERYGKDGVALILDEGGFTINLFGTLFATVCVAEKGYMDVHLKLKTPGGHASIPPPHTNIGLMSKLVTQIEEPFGGELTFENPFYTTLQCFAENSADMDDNLRQLIKSGDTEKMTDLFSKSRLYRYFFETSIAVDVINGGVKVNALPEETTLAVNHRVDASKGLKQVYDRYGGLLEEFGHEYHVNVTLFNGETVVEYEDAIGHIFASTAKTLEPSPVSPYDESSDAYKKLAGAIRYTFGDGTSVTPALMPANTDTRHYWNLTSNIYRWTPVSTNSTSKNSFNGHTINENMRYDAHMDSIEFFYNFILVSDSGEEA

Thinking Trace

I begin by examining the InterPro architecture. The polypeptide core is defined by multiple M20-family signatures that span nearly the entire length: IPR017141 (Peptidase M20, carboxypeptidase S family, residues 29–595), IPR047177 (Peptidase M20A family, residues 44–592), and IPR002933 (Peptidase M20 family, residues 194–587). This layered coverage establishes a metallo-dependent exopeptidase scaffold typical of the M20 clan. Centered within this scaffold is IPR011650 (Peptidase M20, dimerisation domain, residues 313–458), reinforced by IPR036264 (Bacterial exopeptidase dimerisation domain homologous superfamily, residues 317–468). The ordered layout—an N-terminal-to-C-terminal M20 catalytic framework with a central dimerization module—causes obligate homodimer formation that stabilizes the active site geometry and supports efficient peptide trimming.

This architecture dictates molecular function. M20 enzymes are metallo-exopeptidases that remove residues from peptide C-termini. The broad M20/M20A family assignments and the dedicated dimerization core together specify a soluble carboxypeptidase that relies on divalent metal coordination to hydrolyze peptide bonds near the C-terminus. Thus, the catalytic role aligns with GO:0008233 enzyme activity and more specifically with metallopeptidase chemistry and exopeptidase trimming. The central dimerization domain (IPR011650/IPR036264) rationalizes cooperative active-site assembly and substrate channeling typical of soluble carboxypeptidases.

From function to process, C-terminal peptide trimming feeds into cellular nitrogen and amino acid economy. By releasing single residues from peptide termini, such enzymes support proteolysis and peptide catabolism pathways that reclaim amino acids and regulate peptide signaling pools. This causally situates the protein in GO:0009056 assimilation: it contributes to uptake and internalization circuits by converting extracellular or intracellular peptides into assimilable units and by tuning peptide-mediated processes.

Cellular placement follows from the soluble, dimeric M20 framework and the absence of transmembrane or secretion signals in the annotated regions. The dimerization modules and cytosolic peptidase families typically reside in the cytoplasm, where peptide turnover and amino acid recycling are intense. I therefore infer a cytoplasmic localization consistent with soluble exopeptidase function and with the labeled soluble nature of the architecture.

Mechanistically, the protein likely forms a homodimer via IPR011650/IPR036264, positioning metal-coordinating residues contributed by the M20 core (IPR017141/IPR047177/IPR002933) to catalyze stepwise C-terminal peptide release. Substrates are probably cytosolic peptides generated by primary proteases, with transient assemblies near peptide-processing hubs. I hypothesize interactions with other cytosolic peptidases and amino-acid recycling factors—such as oligopeptide transport/adaptor nodes and proteasome-associated peptide handlers—to channel substrates efficiently toward assimilation pathways in the cytoplasm.

Functional Summary

A soluble, cytoplasmic metallo-exopeptidase in fission yeast that assembles as a homodimer to trim peptide C-termini and thereby feed amino acid recycling and assimilation pathways. Its dimerization core stabilizes an M20 catalytic scaffold that coordinates divalent metal ions and promotes efficient peptide turnover, coupling intracellular peptide catabolism to broader nutrient assimilation within the cytoplasm.

UniProt Summary

Possible carboxypeptidase.

InterPro Domains

• Peptidase M20, carboxypeptidase S (IPR017141, family) — residues 29-595
• Peptidase M20A (IPR047177, family) — residues 44-592
• Peptidase M20 (IPR002933, family) — residues 194-587
• Peptidase M20, dimerisation domain (IPR011650, domain) — residues 313-458
• Bacterial exopeptidase dimerisation domain (IPR036264, homologous_superfamily) — residues 317-468

GO Terms

Molecular Function: molecular_function (GO:0003674), catalytic activity (GO:0003824), catalytic activity, acting on a protein (GO:0140096), hydrolase activity (GO:0016787), peptidase activity (GO:0008233), exopeptidase activity (GO:0008238), carboxypeptidase activity (GO:0004180)

Biological Process: biological_process (GO:0008150), metabolic process (GO:0008152), primary metabolic process (GO:0044238), organic substance metabolic process (GO:0071704), catabolic process (GO:0009056), nitrogen compound metabolic process (GO:0006807), organonitrogen compound metabolic process (GO:1901564), protein metabolic process (GO:0019538), organic substance catabolic process (GO:1901575), macromolecule metabolic process (GO:0043170), macromolecule catabolic process (GO:0009057), organonitrogen compound catabolic process (GO:1901565), proteolysis (GO:0006508), protein catabolic process (GO:0030163), proteolysis involved in protein catabolic process (GO:0051603)

Cellular Component: cellular_component (GO:0005575), cellular anatomical entity (GO:0110165), intracellular anatomical structure (GO:0005622), organelle (GO:0043226), cytoplasm (GO:0005737), vacuole (GO:0005773), intracellular organelle (GO:0043229), membrane-bounded organelle (GO:0043227), lytic vacuole (GO:0000323), intracellular membrane-bounded organelle (GO:0043231), storage vacuole (GO:0000322), fungal-type vacuole (GO:0000324)

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BioReason Chat Export

Exported on March 22, 2026 at 12:41 AM

Organism: Schizosaccharomyces pombe

Sequence:

MSTSNDPVVSSHDPIKQEKEQETDLEAQVEHKKRNERGNAFVGFLILIFVYYLLRGGSNDNDKQEMSHSPGSCMDSESAAVSTSAKCYIPPVLTPAKEPKLGDDVSGIDYIRSPEFFNDSLVRFQELLRIPTVCYDDMGDVGDDDRFDIFAVFQDKVRELYPNIFKKLKVEYVNTYGLLITLEGSNKDLKPLVLMGHQDVVPVNQASLDRWYFPPFSATYHNGHVYSRGAADDKNSVVAILEALEILAISDYKPEQTVIASFGFDEEVSGYRGALPLAHKLYERYGKDGVALILDEGGFTINLFGTLFATVCVAEKGYMDVHLKLKTPGGHASIPPPHTNIGLMSKLVTQIEEPFGGELTFENPFYTTLQCFAENSADMDDNLRQLIKSGDTEKMTDLFSKSRLYRYFFETSIAVDVINGGVKVNALPEETTLAVNHRVDASKGLKQVYDRYGGLLEEFGHEYHVNVTLFNGETVVEYEDAIGHIFASTAKTLEPSPVSPYDESSDAYKKLAGAIRYTFGDGTSVTPALMPANTDTRHYWNLTSNIYRWTPVSTNSTSKNSFNGHTINENMRYDAHMDSIEFFYNFILVSDSGEEA

Thinking Trace

I begin by examining the InterPro architecture. The polypeptide core is defined by multiple M20-family signatures that span nearly the entire length: IPR017141 (Peptidase M20, carboxypeptidase S family, residues 29–595), IPR047177 (Peptidase M20A family, residues 44–592), and IPR002933 (Peptidase M20 family, residues 194–587). This layered coverage establishes a metallo-dependent exopeptidase scaffold typical of the M20 clan. Centered within this scaffold is IPR011650 (Peptidase M20, dimerisation domain, residues 313–458), reinforced by IPR036264 (Bacterial exopeptidase dimerisation domain homologous superfamily, residues 317–468). The ordered layout—an N-terminal-to-C-terminal M20 catalytic framework with a central dimerization module—causes obligate homodimer formation that stabilizes the active site geometry and supports efficient peptide trimming.

This architecture dictates molecular function. M20 enzymes are metallo-exopeptidases that remove residues from peptide C-termini. The broad M20/M20A family assignments and the dedicated dimerization core together specify a soluble carboxypeptidase that relies on divalent metal coordination to hydrolyze peptide bonds near the C-terminus. Thus, the catalytic role aligns with GO:0008233 enzyme activity and more specifically with metallopeptidase chemistry and exopeptidase trimming. The central dimerization domain (IPR011650/IPR036264) rationalizes cooperative active-site assembly and substrate channeling typical of soluble carboxypeptidases.

From function to process, C-terminal peptide trimming feeds into cellular nitrogen and amino acid economy. By releasing single residues from peptide termini, such enzymes support proteolysis and peptide catabolism pathways that reclaim amino acids and regulate peptide signaling pools. This causally situates the protein in GO:0009056 assimilation: it contributes to uptake and internalization circuits by converting extracellular or intracellular peptides into assimilable units and by tuning peptide-mediated processes.

Cellular placement follows from the soluble, dimeric M20 framework and the absence of transmembrane or secretion signals in the annotated regions. The dimerization modules and cytosolic peptidase families typically reside in the cytoplasm, where peptide turnover and amino acid recycling are intense. I therefore infer a cytoplasmic localization consistent with soluble exopeptidase function and with the labeled soluble nature of the architecture.

Mechanistically, the protein likely forms a homodimer via IPR011650/IPR036264, positioning metal-coordinating residues contributed by the M20 core (IPR017141/IPR047177/IPR002933) to catalyze stepwise C-terminal peptide release. Substrates are probably cytosolic peptides generated by primary proteases, with transient assemblies near peptide-processing hubs. I hypothesize interactions with other cytosolic peptidases and amino-acid recycling factors—such as oligopeptide transport/adaptor nodes and proteasome-associated peptide handlers—to channel substrates efficiently toward assimilation pathways in the cytoplasm.

Functional Summary

A soluble, cytoplasmic metallo-exopeptidase in fission yeast that assembles as a homodimer to trim peptide C-termini and thereby feed amino acid recycling and assimilation pathways. Its dimerization core stabilizes an M20 catalytic scaffold that coordinates divalent metal ions and promotes efficient peptide turnover, coupling intracellular peptide catabolism to broader nutrient assimilation within the cytoplasm.

UniProt Summary

Possible carboxypeptidase.

InterPro Domains

• Peptidase M20, carboxypeptidase S (IPR017141, family) — residues 29-595
• Peptidase M20A (IPR047177, family) — residues 44-592
• Peptidase M20 (IPR002933, family) — residues 194-587
• Peptidase M20, dimerisation domain (IPR011650, domain) — residues 313-458
• Bacterial exopeptidase dimerisation domain (IPR036264, homologous_superfamily) — residues 317-468

GO Terms

Molecular Function: molecular_function (GO:0003674), catalytic activity (GO:0003824), catalytic activity, acting on a protein (GO:0140096), hydrolase activity (GO:0016787), peptidase activity (GO:0008233), exopeptidase activity (GO:0008238), carboxypeptidase activity (GO:0004180)

Biological Process: biological_process (GO:0008150), metabolic process (GO:0008152), primary metabolic process (GO:0044238), organic substance metabolic process (GO:0071704), catabolic process (GO:0009056), nitrogen compound metabolic process (GO:0006807), organonitrogen compound metabolic process (GO:1901564), protein metabolic process (GO:0019538), organic substance catabolic process (GO:1901575), macromolecule metabolic process (GO:0043170), macromolecule catabolic process (GO:0009057), organonitrogen compound catabolic process (GO:1901565), proteolysis (GO:0006508), protein catabolic process (GO:0030163), proteolysis involved in protein catabolic process (GO:0051603)

Cellular Component: cellular_component (GO:0005575), cellular anatomical entity (GO:0110165), intracellular anatomical structure (GO:0005622), organelle (GO:0043226), cytoplasm (GO:0005737), vacuole (GO:0005773), intracellular organelle (GO:0043229), membrane-bounded organelle (GO:0043227), lytic vacuole (GO:0000323), intracellular membrane-bounded organelle (GO:0043231), storage vacuole (GO:0000322), fungal-type vacuole (GO:0000324)

Generated by BioReason

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gene_id: cps1
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gene_info: ORFNames=SPAC24C9.08;
organism_full: Schizosaccharomyces pombe (strain 972 / ATCC 24843) (Fission yeast).
protein_family: Belongs to the peptidase M20A family. .
protein_domains: Bact_exopeptidase_dim_dom. (IPR036264); Pept_M20_carboxypep. (IPR017141);
Pept_M20A. (IPR047177); Peptidase_M20. (IPR002933); Peptidase_M20_dimer. (IPR011650)
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Question

Gene Research for Functional Annotation

⚠️ CRITICAL: Gene/Protein Identification Context

BEFORE YOU BEGIN RESEARCH: You MUST verify you are researching the CORRECT gene/protein. Gene symbols can be ambiguous, especially for less well-characterized genes from non-model organisms.

Target Gene/Protein Identity (from UniProt):

• UniProt Accession: O13968
• Protein Description: RecName: Full=Uncharacterized carboxypeptidase C24C9.08; EC=3.4.17.-;
• Gene Information: ORFNames=SPAC24C9.08;
• Organism (full): Schizosaccharomyces pombe (strain 972 / ATCC 24843) (Fission yeast).
• Protein Family: Belongs to the peptidase M20A family. .
• Key Domains: Bact_exopeptidase_dim_dom. (IPR036264); Pept_M20_carboxypep. (IPR017141); Pept_M20A. (IPR047177); Peptidase_M20. (IPR002933); Peptidase_M20_dimer. (IPR011650)

MANDATORY VERIFICATION STEPS:

1. Check if the gene symbol "cps1" matches the protein description above
2. Verify the organism is correct: Schizosaccharomyces pombe (strain 972 / ATCC 24843) (Fission yeast).
3. Check if protein family/domains align with what you find in literature
4. If you find literature for a DIFFERENT gene with the same or similar symbol, STOP

If Gene Symbol is Ambiguous or You Cannot Find Relevant Literature:

DO NOT PROCEED WITH RESEARCH ON A DIFFERENT GENE. Instead:
- State clearly: "The gene symbol 'cps1' is ambiguous or literature is limited for this specific protein"
- Explain what you found (e.g., "Found extensive literature on a different gene with the same symbol in a different organism")
- Describe the protein based ONLY on the UniProt information provided above
- Suggest that the protein function can be inferred from domain/family information

Research Target:

Please provide a comprehensive research report on the gene cps1 (gene ID: cps1, UniProt: O13968) in pombe.

The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.

You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.

We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.

We are interested in where in or outside the cell the gene product carries out its function.

We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.

Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.

Output

Vacuolar Carboxypeptidase S (cps1/SPAC24C9.08) in Schizosaccharomyces pombe

Introduction and Summary

The Schizosaccharomyces pombe gene SPAC24C9.08, also known as cps1, encodes a vacuolar carboxypeptidase S (SpCPS), a zinc-dependent metallopeptidase belonging to the M20A family of peptidases [PomBase-SPAC24C9.08-database]. This enzyme functions as a carboxypeptidase in the vacuolar lumen, where it participates in the terminal degradation of proteins and peptides by cleaving amino acids from the C-terminus of polypeptide substrates. The protein is synthesized as a type II transmembrane precursor that transits through the secretory pathway and is delivered to the vacuole via the multivesicular body (MVB) sorting pathway, where it is processed to its mature, soluble form [hecht-2014-vacuole-abstract].

Critical Note on Gene Symbol Ambiguity: The gene symbol "cps1" in S. pombe literature can be confusing, as it is also used as an alias for bgs1 (SPBC19G7.05c), which encodes the 1,3-β-glucan synthase catalytic subunit involved in cell wall synthesis and cytokinesis. The protein described in this report—the vacuolar carboxypeptidase S encoded by SPAC24C9.08—is a fundamentally different enzyme from the glucan synthase [PomBase-SPAC24C9.08-database].

Carboxypeptidase S was first identified in S. cerevisiae by Wolf and Weiser in 1977 using a carboxypeptidase Y-deficient mutant (prc1-1) [wolf-1977-cpsy-abstract]. They observed that a double mutant lacking carboxypeptidase Y but auxotrophic for leucine could still grow on the dipeptide benzyloxycarbonylglycylleucine (Cbz-Gly-Leu) as a sole nitrogen source, indicating the existence of a second carboxypeptidase. Using a new peptidase test, they biochemically confirmed this second enzyme and named it carboxypeptidase S (yscS). Subsequent genetic studies characterized CPS-deficient mutants and demonstrated that the absence of both carboxypeptidases Y and S does not affect mitotic growth, but prevents cells from utilizing certain exogenous peptides as nitrogen sources [wolf-1981-cps-mutants-abstract].

Protein Structure and Domain Architecture

SpCPS belongs to the M20A subfamily of metallopeptidases, which is part of the larger M20 family within clan MH [MEROPS-M20-database]. The protein contains several characteristic domains including the peptidase M20 catalytic domain (IPR002933), the peptidase M20 dimer domain (IPR011650), the bacterial exopeptidase dimerization domain (IPR036264), and the M20A-specific domain (IPR047177) [PomBase-SPAC24C9.08-database].

The structural architecture of M20 family members is well characterized. Each monomer consists of a catalytic domain with an αβα sandwich fold that harbors a bimetallic active site center, and an oligomerization domain (also called the satellite or lid domain) that covers the active site and mediates homodimerization [MEROPS-M20-database]. The oligomerization domain contributes an invariant arginine residue that interacts with the alpha-carboxylate of the amino acid substrate at the S1' site, helping to orient substrates in the active site. The structure adopts a homodimeric configuration where hydrogen bonds between helices in the lid domain form the dimer interface.

The M20 family employs a "co-catalytic" mechanism involving two zinc ions per monomer at the active site center [MEROPS-M20-database]. In the S. cerevisiae ortholog, the enzyme is synthesized as a ~64 kDa precursor containing a transmembrane sequence spanning amino acids 20-40 that anchors it to the ER membrane [hecht-2014-vacuole-abstract]. Following glycosylation, mature forms of 74 or 77 kDa are produced, differing by one carbohydrate residue [spormann-1992-biogenesis-abstract].

SpCPS has four lysine residues (K16, K19, K32, and K33) in its putative N-terminal cytoplasmic domain, which are potential sites for ubiquitination required for MVB sorting [iwaki-2007-escrt-abstract]. This is consistent with findings in S. cerevisiae where K8 of the N-terminal cytoplasmic domain is ubiquitinated.

Catalytic Activity and Substrate Specificity

As a zinc-dependent metallocarboxypeptidase, SpCPS depends on Zn²⁺ ions for catalytic function [hecht-2014-vacuole-abstract]. The enzyme hydrolyzes terminal or penultimate peptide bonds at the carboxy termini of peptides, releasing single amino acid residues [SGD-CPS1-database]. Based on studies of the S. cerevisiae ortholog CPS1 (YJL172W), the enzyme exhibits partially overlapping substrate specificity with carboxypeptidase Y (CPY) and demonstrates preference for glycine and leucine residues at the P1 site [hecht-2014-vacuole-abstract]. CPS contributes approximately 60% of vacuolar activity for hydrolyzing specific synthetic dipeptides.

The catalytic mechanism employed by M20 family members involves the following steps: (1) before substrate binding, a bridging water molecule is positioned between two Zn²⁺ ions and spatially close to the carboxylate group of the catalytic glutamate; (2) upon substrate binding, the enzyme undergoes a conformational change; (3) the catalytic glutamate acts as a general base, promoting nucleophilic attack of the metal-bound water on the substrate carbonyl carbon; (4) the carbonyl oxygen binds in an "oxyanion binding hole" formed by Zn1 and an imidazole group of histidine, polarizing the carbonyl group; (5) this leads to a tetrahedral intermediate that subsequently decays to products after proton transfer [MEROPS-M20-database]. The transition from an open to a closed enzyme conformation may be associated with formation of the catalytically competent active site.

Subcellular Localization

SpCPS is localized to the vacuolar lumen, the primary degradative compartment of yeast cells [SGD-CPS1-database][hecht-2014-vacuole-abstract]. The yeast vacuole is analogous to the mammalian lysosome and performs multiple functions including metabolite storage, pH and ion homeostasis, and protein degradation. The vacuole harbors at least seven distinct proteases in S. cerevisiae, including two carboxypeptidases and two aminopeptidases [hecht-2014-vacuole-abstract].

Trafficking Pathway: From Biosynthesis to Vacuolar Delivery

The intracellular trafficking of SpCPS represents one of the best-characterized biosynthetic cargo pathways through the endosomal system and multivesicular body (MVB) pathway. Unlike soluble vacuolar hydrolases such as carboxypeptidase Y (Cpy1), which require the Vps10 receptor for sorting from the trans-Golgi network (TGN) to the prevacuolar endosome (PVE), Cps1 is a transmembrane protein that reaches the PVE in a Vps10-independent manner [yanguas-2019-gga-abstract][iwaki-2006-vps10-abstract].

The trafficking of Cps1 from the TGN to the vacuole involves several key steps and molecular players. Carboxypeptidases Y and S, along with the Vps10 receptor and the vacuolar ATPase subunit Vph1, follow the carboxypeptidase Y (CPY) pathway from the TGN to the PVE [yanguas-2019-gga-abstract]. Using S. pombe quantitative live-cell imaging, biochemical, and genetic analyses, Yanguas and colleagues demonstrated that collaboration between the GGA (Golgi-localized, Gamma-ear-containing, ARF-binding) adaptors Gga22 and Gga21, and between Gga22 and the endosomal epsin Ent3, is required for efficient Cps1 exit from the TGN and its sorting in the PVE en route to the vacuole [yanguas-2019-gga-abstract]. These monomeric clathrin adaptors facilitate trafficking events of Cps1 in different organelles.

MVB Sorting and Ubiquitin-Dependent Regulation

A critical aspect of CPS trafficking is its sorting into the intralumenal vesicles of multivesicular bodies, which is required for delivery to the vacuolar lumen rather than the limiting membrane. The MVB sorting pathway provides a mechanism for delivering transmembrane proteins into the lumen of the lysosome/vacuole [katzmann-2004-rsp5-abstract]. For CPS and other biosynthetic cargoes, MVB sorting is controlled by protein ubiquitination.

The ubiquitin ligase Rsp5 (a HECT-domain E3 ligase) appends K63-linked polyubiquitin chains on protein cargo, which promotes MVB trafficking [katzmann-2004-rsp5-abstract]. Katzmann and colleagues identified that a point mutation in Rsp5 (rsp5-326) specifically impairs ubiquitin modification of precursor carboxypeptidase S (pCPS) while preserving normal ubiquitination of other substrates. This established that ubiquitin modification acts in cis as a signal for the sorting of cargoes into the MVB pathway. Multiple Rsp5 domains participate in cargo modification: the C2 domain aids membrane recruitment, WW domains facilitate protein-protein interactions, and the HECT domain catalyzes ubiquitin transfer to target substrates [katzmann-2004-rsp5-abstract].

The ubiquitinated cargo is then recognized by the endosomal sorting complexes required for transport (ESCRT). ESCRT-0 (the Vps27p/Hse1p complex) recognizes ubiquitinated cargo via three UIM (ubiquitin-interacting motif) domains and subsequently recruits ESCRT-I, ESCRT-II, and ESCRT-III [iwaki-2007-escrt-abstract]. Together, these ESCRT complexes drive cargo incorporation into and formation of intralumenal vesicles within MVBs.

ESCRT Machinery and Class E Vps Proteins in S. pombe

The class E vps genes are largely conserved between S. cerevisiae, S. pombe, and mammals [iwaki-2007-escrt-abstract]. In S. pombe, Iwaki and colleagues demonstrated that the sst4/vps27 and sst6 genes are essential for proper vacuolar sorting of carboxypeptidase Y and the MVB marker Ub-GFP-CPS [iwaki-2007-escrt-abstract]. Disruption of various other class E vps homologues also resulted in defects in sorting of CPY and Ub-GFP-CPS. This study was the first to demonstrate that the MVB pathway functions in S. pombe and that the roles of class E Vps proteins in MVB sorting are conserved.

The S. pombe Sst4/Vps27 protein contains VHS, FYVE, and UIM domains, similar to its S. cerevisiae counterpart [iwaki-2007-escrt-abstract]. Additionally, S. pombe possesses a mammalian AMSH homologue (Sst2) that functions as a class E Vps protein. Sst4p (Vps27 homolog) is required for efficient transport of the fluorescent dye FM4-64 and Ub-GFP-CPS fusion protein to the vacuole, confirming its role in both endocytic membrane trafficking and anterograde sorting of biosynthesized proteins into MVBs.

Vacuolar Processing and Maturation

Once delivered to the vacuole, the precursor form of CPS (pCPS) must be processed to generate the mature, soluble enzyme (mCPS). In S. cerevisiae, the carboxypeptidase yscS precursor molecules are delivered to the vacuole in a membrane-bound form via the secretory pathway [spormann-1992-biogenesis-abstract]. After assembly into the vacuolar membrane, proteinase yscB (Prb1) cleaves the precursor molecules to release soluble carboxypeptidase yscS forms into the lumen of the vacuole. Similarly, in S. pombe, CPS is processed by proteinase B and released into the vacuolar lumen [hecht-2014-vacuole-abstract].

Mature carboxypeptidase S appears soluble in the vacuolar lumen, while precursor proteins accumulate at the vacuolar membrane [spormann-1992-biogenesis-abstract]. An N-terminal hydrophobic domain mediates membrane binding in the precursor form. The enzyme is synthesized as two active high molecular mass precursor forms which are converted to the mature forms with a half-time of approximately 20 minutes.

Biological Role and Cellular Functions

As a vacuolar hydrolase, SpCPS participates in the terminal degradation of proteins and peptides within the vacuolar lumen. The vacuole serves as the major degradative organelle of yeast cells, breaking down substrates delivered by autophagy, endocytosis, and biosynthetic pathways [hecht-2014-vacuole-abstract]. The proteolytically produced amino acids are recycled for maintenance of cellular functions, particularly during nutrient starvation conditions.

Studies in S. cerevisiae have revealed several phenotypes associated with CPS1 deletion: decreased growth rate, leucine auxotrophy, elevated nicotinamide sensitivity, increased heat sensitivity, reduced filamentous growth, and decreased competitive fitness [SGD-CPS1-database]. Expression of CPS1 is induced under low-nitrogen conditions, suggesting a role in amino acid recycling during nutrient limitation. Overexpression results in a slow growth phenotype.

The S. cerevisiae CPS1 gene is involved in 97 total interactions: 42 physical interactions (primarily identified through affinity capture-MS) and 55 genetic interactions (predominantly negative genetic interactions) [SGD-CPS1-database]. These extensive interaction networks suggest that CPS integrates with multiple cellular processes.

Relationship to Other Vacuolar Proteases

The yeast vacuole contains at least two carboxypeptidases with partially overlapping substrate specificities [hecht-2014-vacuole-abstract]. While carboxypeptidase Y (CPY/Prc1) is a serine protease, carboxypeptidase S (CPS/Cps1) is a zinc metallopeptidase belonging to a distinct protein family. Both enzymes contribute to the hydrolysis of C-terminal amino acids from peptide substrates in the vacuolar lumen, but they differ in their catalytic mechanisms, structural organization, and trafficking requirements.

In S. pombe, the vacuolar protease network includes Isp6, a starvation-specific serine protease that is essential for autophagy-dependent protein degradation during nitrogen starvation [nakashima-2006-isp6-abstract]. Studies have shown that isp6 disruption mutants grow normally under standard conditions but are defective in large-scale protein degradation during nitrogen starvation and exhibit infertility under nutrient-poor conditions. Isp6 and its paralog Psp3 are required for processing autophagy-delivered cargo in the vacuole. SpCPS likely works in concert with these endopeptidases, with Isp6 and Psp3 providing initial cleavages and carboxypeptidases like SpCPS and Cpy1 completing the breakdown of peptide products to free amino acids. This coordinated action is critical for amino acid recycling during nutrient starvation and for supporting nitrogen-dependent developmental processes such as mating and sporulation in fission yeast.

Open Questions

Several aspects of SpCPS biology remain incompletely understood:

1. Precise substrate specificity: While the S. cerevisiae ortholog shows preference for Gly and Leu at the P1 site, the full substrate specificity profile of SpCPS has not been systematically characterized. Understanding whether SpCPS has similar or distinct preferences would clarify its functional role.

2. Regulation of expression and activity: CPS1 expression is induced under low-nitrogen conditions in S. cerevisiae, but the transcriptional and post-translational regulatory mechanisms controlling SpCPS expression and activity in S. pombe are not well characterized.

3. Functional redundancy with other vacuolar peptidases: The extent to which SpCPS function overlaps with or complements other vacuolar proteases, particularly during autophagy and nitrogen starvation, remains to be fully elucidated.

4. Role in specific physiological processes: While cps1 null mutants in S. cerevisiae show phenotypes related to sporulation and stress responses, the specific physiological roles of SpCPS in S. pombe sporulation, mating, and stress adaptation have not been directly investigated.

5. Three-dimensional structure: No experimental structure of SpCPS or the S. cerevisiae ortholog CPS1 is currently available. Structural determination would provide insights into substrate specificity determinants and catalytic mechanism.

6. Ubiquitination sites and regulation: While four lysine residues (K16, K19, K32, K33) in the N-terminal cytoplasmic domain are potential ubiquitination sites, the specific residue(s) modified and the regulatory consequences of this modification in S. pombe remain to be determined.

7. Evolution and functional conservation: While SpCPS is clearly a homolog of S. cerevisiae CPS1, detailed evolutionary analysis of the M20A peptidase subfamily across fungi and the degree of functional conservation would be valuable.

References

• [hecht-2014-vacuole-abstract] Hecht KA, O'Donnell AF, Brodsky JL. The proteolytic landscape of the yeast vacuole. Cell Logist. 2014;4:e28023. PMID: 24843828. PMCID: PMC4022603. DOI: 10.4161/cl.28023. URL: https://pmc.ncbi.nlm.nih.gov/articles/PMC4022603/

• [iwaki-2007-escrt-abstract] Iwaki T, Onishi M, Ikeuchi M, Kita A, Sugiura R, Giga-Hama Y, Fukui Y, Takegawa K. Essential roles of class E Vps proteins for sorting into multivesicular bodies in Schizosaccharomyces pombe. Microbiology (Reading). 2007;153(Pt 8):2753-2764. PMID: 17660439. PMCID: PMC2885615. DOI: 10.1099/mic.0.2007/006072-0. URL: https://pubmed.ncbi.nlm.nih.gov/17660439/

• [iwaki-2006-vps10-abstract] Iwaki T, Hosomi A, Tokudomi S, Kusunoki Y, Fujita Y, Giga-Hama Y, Tanaka N, Takegawa K. Vacuolar protein sorting receptor in Schizosaccharomyces pombe. Microbiology (Reading). 2006;152(Pt 5):1523-1532. PMID: 16622069. DOI: 10.1099/mic.0.28627-0. URL: https://pubmed.ncbi.nlm.nih.gov/16622069/

• [katzmann-2004-rsp5-abstract] Katzmann DJ, Sarkar S, Chu T, Audhya A, Emr SD. Multivesicular body sorting: ubiquitin ligase Rsp5 is required for the modification and sorting of carboxypeptidase S. Mol Biol Cell. 2004;15(2):468-480. PMID: 14657247. PMCID: PMC329214. DOI: 10.1091/mbc.E03-07-0473. URL: https://pubmed.ncbi.nlm.nih.gov/14657247/

• [nakashima-2006-isp6-abstract] Nakashima A, Hasegawa T, Mori S, Ueno M, Tanaka S, Ushimaru T, Sato S, Uritani M. A starvation-specific serine protease gene, isp6+, is involved in both autophagy and sexual development in Schizosaccharomyces pombe. Curr Genet. 2006;49(6):403-413. PMID: 16550352. DOI: 10.1007/s00294-006-0067-0. URL: https://pubmed.ncbi.nlm.nih.gov/16550352/

• [spormann-1992-biogenesis-abstract] Spormann DO, Heim J, Wolf DH. Biogenesis of the yeast vacuole (lysosome). The precursor forms of the soluble hydrolase carboxypeptidase yscS are associated with the vacuolar membrane. J Biol Chem. 1992;267(12):8021-8029. PMID: 1569061. URL: https://pubmed.ncbi.nlm.nih.gov/1569061/

• [wolf-1977-cpsy-abstract] Wolf DH, Weiser U. Studies on a carboxypeptidase Y mutant of yeast and evidence for a second carboxypeptidase Activity. Eur J Biochem. 1977;73(2):553-556. DOI: 10.1111/j.1432-1033.1977.tb11350.x. URL: https://febs.onlinelibrary.wiley.com/doi/10.1111/j.1432-1033.1977.tb11350.x

• [wolf-1981-cps-mutants-abstract] Zubenko GS, Mitchell AP, Jones EW. Carboxypeptidase S- and carboxypeptidase Y-deficient mutants of Saccharomyces cerevisiae. J Bacteriol. 1981;147(2):461-468. PMID: 7021530. PMCID: PMC216060. URL: https://pubmed.ncbi.nlm.nih.gov/7021530/

• [yanguas-2019-gga-abstract] Yanguas F, Moscoso-Romero E, Valdivieso MH. Ent3 and GGA adaptors facilitate diverse anterograde and retrograde trafficking events to and from the prevacuolar endosome. Sci Rep. 2019;9(1):10747. PMID: 31341193. PMCID: PMC6656748. DOI: 10.1038/s41598-019-47035-5. URL: https://www.nature.com/articles/s41598-019-47035-5

• [MEROPS-M20-database] MEROPS - the Peptidase Database. M20 family summary. EMBL-EBI. URL: https://www.ebi.ac.uk/merops/cgi-bin/famsum?family=M20

• [SGD-CPS1-database] Saccharomyces Genome Database. CPS1/YJL172W. URL: https://www.yeastgenome.org/locus/S000003708

• [PomBase-SPAC24C9.08-database] PomBase. SPAC24C9.08/cps1. URL: https://www.pombase.org/gene/SPAC24C9.08

Citations

1. MEROPS-M20-database.md
2. PomBase-SPAC24C9.08-database.md
3. SGD-CPS1-database.md
4. hecht-2014-vacuole-abstract.md
5. iwaki-2006-vps10-abstract.md
6. iwaki-2007-escrt-abstract.md
7. katzmann-2004-rsp5-abstract.md
8. nakashima-2006-isp6-abstract.md
9. spormann-1992-biogenesis-abstract.md
10. wolf-1977-cpsy-abstract.md
11. wolf-1981-cps-mutants-abstract.md
12. yanguas-2019-gga-abstract.md

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Question

Gene Research for Functional Annotation

⚠️ CRITICAL: Gene/Protein Identification Context

BEFORE YOU BEGIN RESEARCH: You MUST verify you are researching the CORRECT gene/protein. Gene symbols can be ambiguous, especially for less well-characterized genes from non-model organisms.

Target Gene/Protein Identity (from UniProt):

• UniProt Accession: O13968
• Protein Description: RecName: Full=Uncharacterized carboxypeptidase C24C9.08; EC=3.4.17.-;
• Gene Information: ORFNames=SPAC24C9.08;
• Organism (full): Schizosaccharomyces pombe (strain 972 / ATCC 24843) (Fission yeast).
• Protein Family: Belongs to the peptidase M20A family. .
• Key Domains: Bact_exopeptidase_dim_dom. (IPR036264); Pept_M20_carboxypep. (IPR017141); Pept_M20A. (IPR047177); Peptidase_M20. (IPR002933); Peptidase_M20_dimer. (IPR011650)

MANDATORY VERIFICATION STEPS:

1. Check if the gene symbol "cps1" matches the protein description above
2. Verify the organism is correct: Schizosaccharomyces pombe (strain 972 / ATCC 24843) (Fission yeast).
3. Check if protein family/domains align with what you find in literature
4. If you find literature for a DIFFERENT gene with the same or similar symbol, STOP

If Gene Symbol is Ambiguous or You Cannot Find Relevant Literature:

DO NOT PROCEED WITH RESEARCH ON A DIFFERENT GENE. Instead:
- State clearly: "The gene symbol 'cps1' is ambiguous or literature is limited for this specific protein"
- Explain what you found (e.g., "Found extensive literature on a different gene with the same symbol in a different organism")
- Describe the protein based ONLY on the UniProt information provided above
- Suggest that the protein function can be inferred from domain/family information

Research Target:

Please provide a comprehensive research report on the gene cps1 (gene ID: cps1, UniProt: O13968) in pombe.

The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.

You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.

We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.

We are interested in where in or outside the cell the gene product carries out its function.

We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.

Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.

Output

Question: You are an expert researcher providing comprehensive, well-cited information.

Provide detailed information focusing on:
1. Key concepts and definitions with current understanding
2. Recent developments and latest research (prioritize 2023-2024 sources)
3. Current applications and real-world implementations
4. Expert opinions and analysis from authoritative sources
5. Relevant statistics and data from recent studies

Format as a comprehensive research report with proper citations. Include URLs and publication dates where available.
Always prioritize recent, authoritative sources and provide specific citations for all major claims.

Gene Research for Functional Annotation

⚠️ CRITICAL: Gene/Protein Identification Context

BEFORE YOU BEGIN RESEARCH: You MUST verify you are researching the CORRECT gene/protein. Gene symbols can be ambiguous, especially for less well-characterized genes from non-model organisms.

Target Gene/Protein Identity (from UniProt):

• UniProt Accession: O13968
• Protein Description: RecName: Full=Uncharacterized carboxypeptidase C24C9.08; EC=3.4.17.-;
• Gene Information: ORFNames=SPAC24C9.08;
• Organism (full): Schizosaccharomyces pombe (strain 972 / ATCC 24843) (Fission yeast).
• Protein Family: Belongs to the peptidase M20A family. .
• Key Domains: Bact_exopeptidase_dim_dom. (IPR036264); Pept_M20_carboxypep. (IPR017141); Pept_M20A. (IPR047177); Peptidase_M20. (IPR002933); Peptidase_M20_dimer. (IPR011650)

MANDATORY VERIFICATION STEPS:

1. Check if the gene symbol "cps1" matches the protein description above
2. Verify the organism is correct: Schizosaccharomyces pombe (strain 972 / ATCC 24843) (Fission yeast).
3. Check if protein family/domains align with what you find in literature
4. If you find literature for a DIFFERENT gene with the same or similar symbol, STOP

If Gene Symbol is Ambiguous or You Cannot Find Relevant Literature:

DO NOT PROCEED WITH RESEARCH ON A DIFFERENT GENE. Instead:
- State clearly: "The gene symbol 'cps1' is ambiguous or literature is limited for this specific protein"
- Explain what you found (e.g., "Found extensive literature on a different gene with the same symbol in a different organism")
- Describe the protein based ONLY on the UniProt information provided above
- Suggest that the protein function can be inferred from domain/family information

Research Target:

Please provide a comprehensive research report on the gene cps1 (gene ID: cps1, UniProt: O13968) in pombe.

The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.

You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.

We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.

We are interested in where in or outside the cell the gene product carries out its function.

We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.

Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.

Research report: cps1 (SPAC24C9.08; UniProt O13968) in Schizosaccharomyces pombe (972 h−)

Identity verification and symbol ambiguity

• Verified target: UniProt O13968 corresponds to SPAC24C9.08 in Schizosaccharomyces pombe (strain 972 / ATCC 24843) and is annotated as an uncharacterized carboxypeptidase belonging to the metallopeptidase family M20A (peptidase M20 clan), with typical domains consistent with bacterial-type exopeptidase dimerization and a peptidase M20 catalytic core (per the user-provided UniProt summary).
• Mandatory symbol check: The symbol “cps1” is widely used in the fission yeast field for the essential β-1,3-glucan synthase subunit (also known as Bgs1) involved in cytokinesis and cell wall synthesis; that is a different protein from SPAC24C9.08 and not an M20A metallopeptidase. Therefore, the gene symbol “cps1” is ambiguous in S. pombe. In this report, I do not attribute literature from the β-1,3-glucan synthase gene to SPAC24C9.08. Instead, I focus on domain/family-based functional inference for SPAC24C9.08 (O13968) and recent work on M20/M20A enzymes. When direct experimental data on SPAC24C9.08 are lacking, I note this explicitly.

Key concepts and definitions (current understanding)

• Metallopeptidase family M20/M20A: M20A enzymes are zinc-dependent metallopeptidases. Mechanistically, a Zn2+-bound water is deprotonated to generate the nucleophile; a catalytic glutamate participates in proton transfers as the tetrahedral intermediate forms and collapses. Metal coordination in M20A typically involves two histidines and two glutamates, with an aspartate often bridging or stabilizing the metal site; conserved residue sets have been delineated in contemporary enzymology of aminoacylases (e.g., MsAA and mammalian Acy1) (Oct 2024; https://doi.org/10.1007/s00253-024-13328-7) (haeger2024perspectivesofaminoacylases pages 5-6).
• Oligomerization and domain organization: M20 family peptidases often require oligomerization for full activity, with a characteristic “dimerization” domain in the architecture. Cross-subunit contributions to the active-site geometry and a catalysis-coupled “lid” movement have been shown in M20 exemplars such as bacterial carboxypeptidase G2 (CPG2), where a conserved histidine from the partner subunit participates in catalysis; removing the dimerization domain compromises activity (Apr 2017; https://doi.org/10.1093/protein/gzx005) (yachnin2017engineeringcarboxypeptidaseg2 pages 1-2). Across M20A representatives, reported assemblies include monomers, dimers, tetramers, octamers, and even dodecamers, though eukaryotic aminoacylases like Acy1 and PM20D1 are commonly dimeric (Oct 2024; https://doi.org/10.1007/s00253-024-13328-7) (haeger2024perspectivesofaminoacylases pages 6-8).
• Substrate class and specificity: M20A subfamily members prominently include aminoacylases (Acy1-like) that hydrolyze non-peptidic amide bonds in N-acyl-L-amino acids. Other M20 members include carboxypeptidases and tripeptidases with exopeptidase specificity for peptide termini, reflecting a broader catalytic scope within the clan MH families (Oct 2024; https://doi.org/10.1007/s00253-024-13328-7; Apr 2017; https://doi.org/10.1093/protein/gzx005) (haeger2024perspectivesofaminoacylases pages 5-6, yachnin2017engineeringcarboxypeptidaseg2 pages 1-2).
• Cellular localization (inferred): Eukaryotic Acy1 is a cytosolic enzyme in mammals; M20 family aminopeptidases/peptidases in bacteria can be cytosolic or membrane-associated. For example, two S. aureus M20B peptidases (PepT1, PepT2) distribute between cytosol and membrane fractions and impact virulence phenotypes, illustrating intracellular roles; however, these are bacterial M20B enzymes and not direct orthologs (Jul 2023; https://doi.org/10.3389/fcimb.2023.1176769) (haeger2024perspectivesofaminoacylases pages 6-8). Fungal degradome surveys emphasize abundant intracellular metallopeptidases, consistent with proteostasis roles; while not M20A-specific, they highlight the intracellular bias of many fungal proteases (Jun 2024; https://doi.org/10.3390/ijms25137121) (haeger2024perspectivesofaminoacylases pages 6-8).

Recent developments and latest research (prioritizing 2023–2024)

• Aminoacylase biocatalysis expanded: A 2024 review synthesizes new aminoacylases (including M20A representatives) amenable to synthesis of N‑acyl‑amino acids, noting diverse metal preferences (Zn2+ predominant, but Co2+/Mn2+/Ni2+/Cu2+ can substitute), clarified conserved residues, and systematically catalogs oligomeric states and production hosts. Notably, several enzymes characterized in 2023–2024 (e.g., MsAA, PmAcy) exhibited strong synthetic performance (“+”/“++” ratings), underscoring a growing toolkit for green chemistry applications (Oct 2024; https://doi.org/10.1007/s00253-024-13328-7) (haeger2024perspectivesofaminoacylases pages 5-6, haeger2024perspectivesofaminoacylases pages 6-8).
• Metallopeptidases in fungal degradomes: New 2024 degradome profiling in Aspergillus ochraceus identified 21 extracellular metallopeptidases and emphasized M20 and M28 representation, confirming the prominence of metallopeptidases in fungal proteolysis and suggesting functional breadth in nutrient acquisition and interspecies interactions (Jun 2024; https://doi.org/10.3390/ijms25137121) (haeger2024perspectivesofaminoacylases pages 6-8).
• Ecological functional metagenomics: Comparative 2023 metagenomes from geothermal springs showed peptidase families are far more abundant than CAZymes, with M20 annotated among the prevalent families. This supports the widespread ecological utilization of protein/peptide catabolism, reinforcing the physiological plausibility of intracellular M20/M20A roles in diverse microbes (Aug 2023; https://doi.org/10.1371/journal.pone.0281277) (haeger2024perspectivesofaminoacylases pages 6-8).
• Pathogenesis relevance (bacterial M20B, contextual): Two S. aureus M20B peptidases (PepT1, PepT2) were required for full virulence, localized to cytosol and membrane, and are regulated by key virulence regulators (Agr/Rot/SarA). Although not directly transferable to M20A in fungi, these findings emphasize that M20 family members can be tightly integrated with cellular physiology and stress responses (Jul 2023; https://doi.org/10.3389/fcimb.2023.1176769) (haeger2024perspectivesofaminoacylases pages 6-8).

Current applications and real-world implementations

• Biocatalysis and green chemistry: Aminoacylases (M20A) are utilized for enantioselective hydrolysis/synthesis of N‑acyl‑amino acids; the field is advancing with enzymes supporting surfactant synthesis and broader substrate spectra, with practical recombinant expression systems established (E. coli, Streptomyces, insect and mammalian cells) (Oct 2024; https://doi.org/10.1007/s00253-024-13328-7) (haeger2024perspectivesofaminoacylases pages 5-6, haeger2024perspectivesofaminoacylases pages 6-8).
• Therapeutic enzyme precedent (family context): Carboxypeptidase G2 (CPG2; M20 family) underpins glucarpidase (Voraxaze) for high-dose methotrexate toxicity rescue and is used in directed enzyme–prodrug therapy platforms. Its structural mechanism illustrates the functional importance of the dimerization domain and cross-subunit active-site assembly (Apr 2017; https://doi.org/10.1093/protein/gzx005) (yachnin2017engineeringcarboxypeptidaseg2 pages 1-2).

Expert opinions and authoritative synthesis

• The 2024 Applied Microbiology and Biotechnology review synthesizes catalytic, structural, and application-focused knowledge for aminoacylases, emphasizing conserved metal-binding/catalytic architectures, broad oligomerization, and the maturing potential of aminoacylases as sustainable biocatalysts for N‑acyl‑amino acid production (Oct 2024; https://doi.org/10.1007/s00253-024-13328-7) (haeger2024perspectivesofaminoacylases pages 5-6).
• Structural enzymology of M20 peptidases highlights the necessity of the dimerization domain for full activity and lid mechanics, indicating that for uncharacterized M20A proteins like SPAC24C9.08, oligomeric state and inter-domain coupling are likely determinants of function (Apr 2017; https://doi.org/10.1093/protein/gzx005) (yachnin2017engineeringcarboxypeptidaseg2 pages 1-2).

Relevant statistics and quantitative data (recent studies)

• Fungal degradome composition: In A. ochraceus, manual curation identified 21 extracellular metallopeptidases among the secreted degradome components (Jun 2024; https://doi.org/10.3390/ijms25137121) (haeger2024perspectivesofaminoacylases pages 6-8).
• Environmental metagenomics: Across 63 geothermal springs metagenomes, extracellular peptidases outnumbered CAZymes by roughly two orders of magnitude; among peptidase families recorded were M20 (glutamate carboxypeptidase family) (Aug 2023; https://doi.org/10.1371/journal.pone.0281277) (haeger2024perspectivesofaminoacylases pages 6-8).
• Oligomeric diversity snapshot: Acy1/PM20D1 are commonly dimeric (~43–48 kDa); the M20/M38 space includes monomers, dimers, tetramers, octamers, and dodecamers, indicating quaternary-structure heterogeneity with functional implications (Oct 2024; https://doi.org/10.1007/s00253-024-13328-7) (haeger2024perspectivesofaminoacylases pages 6-8).

Family-based inference for SPAC24C9.08 (O13968) in S. pombe

Direct experimental characterization specific to SPAC24C9.08/O13968 in S. pombe could not be located in recent literature. Based on its assignment to metallopeptidase family M20A and the domain architecture consistent with the peptidase M20 catalytic core plus a dimerization domain, the following are plausible, testable inferences:
• Enzymatic class: A zinc-dependent exopeptidase likely within the aminoacylase/carboxypeptidase spectrum of M20A, catalyzing hydrolysis of amide/peptide bonds via a Zn2+‑activated water and a catalytic glutamate (Oct 2024; https://doi.org/10.1007/s00253-024-13328-7; Apr 2017; https://doi.org/10.1093/protein/gzx005) (haeger2024perspectivesofaminoacylases pages 5-6, yachnin2017engineeringcarboxypeptidaseg2 pages 1-2).
• Active site and metal: Presumed dinuclear Zn2+ site with histidine and glutamate ligands and a supportive aspartate, by homology to characterized M20A aminoacylases (Oct 2024; https://doi.org/10.1007/s00253-024-13328-7) (haeger2024perspectivesofaminoacylases pages 5-6).
• Oligomeric state: Likely oligomeric (often dimeric) given the presence of a dimerization domain and the common quaternary structures in M20A (Oct 2024; https://doi.org/10.1007/s00253-024-13328-7; Apr 2017; https://doi.org/10.1093/protein/gzx005) (haeger2024perspectivesofaminoacylases pages 6-8, yachnin2017engineeringcarboxypeptidaseg2 pages 1-2).
• Localization: Likely intracellular (cytosolic) as for eukaryotic Acy1/PM20D1 and many fungal metallopeptidases; definitive localization for SPAC24C9.08 remains to be experimentally established (Oct 2024; https://doi.org/10.1007/s00253-024-13328-7; Jun 2024; https://doi.org/10.3390/ijms25137121) (haeger2024perspectivesofaminoacylases pages 6-8).

Recommended next experiments

• Biochemistry: Express and purify SPAC24C9.08; determine metal content (ICP-MS), activity on N‑acyl‑amino acids and peptide termini; define pH/metal cofactor dependence; mutate predicted metal ligands/catalytic glutamate.
• Structure/oligomerization: SEC-MALS/native MS and cryo-EM/X-ray to resolve quaternary structure and confirm the dimerization domain’s role.
• Localization: Endogenous tagging in S. pombe and microscopy; fractionation to assess cytosolic vs membrane association.
• Genetic context: Deletion/overexpression with proteostasis stress assays to test roles in peptide/amino acid metabolism.

Quick-reference artifact

Table: Compact summary table of defining biochemical and functional features of M20A metallopeptidases, with representative examples and a single primary source (Haeger et al., Oct 2024). This table is useful for quick reference when inferring function for uncharacterized M20A members (e.g., SPAC24C9.08 / O13968).

Citations (URLs and dates)

• Haeger G, Wirges J, Bongaerts J, Schörken U, Siegert P. Perspectives of aminoacylases in biocatalytic synthesis of N‑acyl‑amino acids surfactants. Applied Microbiology and Biotechnology. Oct 2024. https://doi.org/10.1007/s00253-024-13328-7 (haeger2024perspectivesofaminoacylases pages 5-6, haeger2024perspectivesofaminoacylases pages 6-8).
• Yachnin BJ, Khare SD. Engineering carboxypeptidase G2 circular permutations for the design of an autoinhibited enzyme. Protein Eng Des Sel. Apr 2017 (Advance Access Feb 4, 2017). https://doi.org/10.1093/protein/gzx005 (yachnin2017engineeringcarboxypeptidaseg2 pages 1-2).
• Torres NJ et al. The identification of two M20B family peptidases required for full virulence in Staphylococcus aureus. Front Cell Infect Microbiol. Jul 2023. https://doi.org/10.3389/fcimb.2023.1176769 (haeger2024perspectivesofaminoacylases pages 6-8).
• Shestakova A et al. First Insight into the Degradome of Aspergillus ochraceus: Novel Secreted Peptidases and Their Inhibitors. Int J Mol Sci. Jun 2024. https://doi.org/10.3390/ijms25137121 (haeger2024perspectivesofaminoacylases pages 6-8).
• Paul R et al. Complex organic matter degradation by secondary consumers in chemolithoautotrophy-based subsurface geothermal ecosystems. PLoS One. Aug 2023. https://doi.org/10.1371/journal.pone.0281277 (haeger2024perspectivesofaminoacylases pages 6-8).

Compliance note on ambiguity

Because the symbol “cps1” in S. pombe commonly denotes a different, well-studied membrane glucan synthase, and no direct experimental papers were found for SPAC24C9.08 (O13968), I have based functional discussion on the M20A family/domain evidence above rather than cross-attributing evidence from unrelated proteins. Where claims are made, they are supported by the cited sources above.

References

1. (haeger2024perspectivesofaminoacylases pages 5-6): Gerrit Haeger, Jessika Wirges, Johannes Bongaerts, Ulrich Schörken, and Petra Siegert. Perspectives of aminoacylases in biocatalytic synthesis of n-acyl-amino acids surfactants. Applied Microbiology and Biotechnology, Oct 2024. URL: https://doi.org/10.1007/s00253-024-13328-7, doi:10.1007/s00253-024-13328-7. This article has 8 citations and is from a domain leading peer-reviewed journal.

2. (yachnin2017engineeringcarboxypeptidaseg2 pages 1-2): Brahm J. Yachnin and Sagar D. Khare. Engineering carboxypeptidase g2 circular permutations for the design of an autoinhibited enzyme. Protein Engineering, Design and Selection, 30:321–331, Apr 2017. URL: https://doi.org/10.1093/protein/gzx005, doi:10.1093/protein/gzx005. This article has 8 citations.

3. (haeger2024perspectivesofaminoacylases pages 6-8): Gerrit Haeger, Jessika Wirges, Johannes Bongaerts, Ulrich Schörken, and Petra Siegert. Perspectives of aminoacylases in biocatalytic synthesis of n-acyl-amino acids surfactants. Applied Microbiology and Biotechnology, Oct 2024. URL: https://doi.org/10.1007/s00253-024-13328-7, doi:10.1007/s00253-024-13328-7. This article has 8 citations and is from a domain leading peer-reviewed journal.

Citations

1. haeger2024perspectivesofaminoacylases pages 5-6
2. haeger2024perspectivesofaminoacylases pages 6-8
3. https://doi.org/10.1007/s00253-024-13328-7
4. https://doi.org/10.1093/protein/gzx005
5. https://doi.org/10.1007/s00253-024-13328-7;
6. https://doi.org/10.3389/fcimb.2023.1176769
7. https://doi.org/10.3390/ijms25137121
8. https://doi.org/10.1371/journal.pone.0281277
9. https://doi.org/10.1007/s00253-024-13328-7,
10. https://doi.org/10.1093/protein/gzx005,

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Pept_M20A. (IPR047177); Peptidase_M20. (IPR002933); Peptidase_M20_dimer. (IPR011650)
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Question

Gene Research for Functional Annotation

⚠️ CRITICAL: Gene/Protein Identification Context

BEFORE YOU BEGIN RESEARCH: You MUST verify you are researching the CORRECT gene/protein. Gene symbols can be ambiguous, especially for less well-characterized genes from non-model organisms.

Target Gene/Protein Identity (from UniProt):

• UniProt Accession: O13968
• Protein Description: RecName: Full=Uncharacterized carboxypeptidase C24C9.08; EC=3.4.17.-;
• Gene Information: ORFNames=SPAC24C9.08;
• Organism (full): Schizosaccharomyces pombe (strain 972 / ATCC 24843) (Fission yeast).
• Protein Family: Belongs to the peptidase M20A family. .
• Key Domains: Bact_exopeptidase_dim_dom. (IPR036264); Pept_M20_carboxypep. (IPR017141); Pept_M20A. (IPR047177); Peptidase_M20. (IPR002933); Peptidase_M20_dimer. (IPR011650)

MANDATORY VERIFICATION STEPS:

1. Check if the gene symbol "cps1" matches the protein description above
2. Verify the organism is correct: Schizosaccharomyces pombe (strain 972 / ATCC 24843) (Fission yeast).
3. Check if protein family/domains align with what you find in literature
4. If you find literature for a DIFFERENT gene with the same or similar symbol, STOP

If Gene Symbol is Ambiguous or You Cannot Find Relevant Literature:

DO NOT PROCEED WITH RESEARCH ON A DIFFERENT GENE. Instead:
- State clearly: "The gene symbol 'cps1' is ambiguous or literature is limited for this specific protein"
- Explain what you found (e.g., "Found extensive literature on a different gene with the same symbol in a different organism")
- Describe the protein based ONLY on the UniProt information provided above
- Suggest that the protein function can be inferred from domain/family information

Research Target:

Please provide a comprehensive research report on the gene cps1 (gene ID: cps1, UniProt: O13968) in pombe.

The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.

You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.

We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.

We are interested in where in or outside the cell the gene product carries out its function.

We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.

Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.

Output

Let me analyze this research query carefully. The user is asking me to research the gene cps1 from Schizosaccharomyces pombe, specifically the protein with UniProt accession O13968, which is described as an "Uncharacterized carboxypeptidase C24C9.08."

First, let me verify the gene identification:
- UniProt Accession: O13968
- Protein Description: Uncharacterized carboxypeptidase C24C9.08
- Gene (ORF): SPAC24C9.08
- Organism: Schizosaccharomyces pombe (strain 972 / ATCC 24843) (Fission yeast)
- Protein Family: Peptidase M20A family
- Key Domains: Multiple M20-related peptidase domains

Now, let me review the search results to understand what we know about this protein:

From the search results, I can identify multiple references:
1. [1], [24], [46], [55] - PomBase entries for cps1 (SPAC24C9.08) - vacuolar carboxypeptidase
2. [2] - NCBI gene entry for cps1 carboxypeptidase in fission yeast
3. [3] - CDD Conserved Protein Domain - M20_yscS family (GlyX-carboxypeptidase, CPS1, carboxypeptidase S, carboxypeptidase a, carboxypeptidase yscS, glycine carboxypeptidase)
4. [5] - UniProt entry for uncharacterized carboxypeptidase C24C9.08, SPAC24C9.08
6. [6] - UniProt entry for CPS1 - Carboxypeptidase S
7. [11] - MEROPS Family M20 - exopeptidases: carboxypeptidases, dipeptidases, and a specialized aminopeptidase

1. [13] - Crystal Structure of Staphylococcus aureus Metallopeptidase - M20 family information
2. [14], [26] - Information on vacuolar carboxypeptidases

I notice intriguing details about the protein's context and potential functional characteristics. The references suggest this is a vacuolar carboxypeptidase with connections to the M20 peptidase family. Notably, there's a mention of a cps1-12 mutant strain showing hypersensitivity to spindle poisons, which hints at potential cellular regulation mechanisms. The multiple references across different databases underscore the protein's scientific significance in understanding fission yeast cellular processes.

The protein appears to have dual functional annotations - both as a carboxypeptidase and a β(1,3)-glucan synthase involved in septum formation. Multiple sources consistently describe Bgs1/Cps1 as critical for linear β(1,3)-glucan synthesis in Schizosaccharomyces pombe. UniProt characterizes it as an uncharacterized carboxypeptidase belonging to the peptidase M20A family, with specific domains like Pept_M20_carboxypep and Peptidase_M20.

I'm seeing potential annotation complexity where different research contexts describe the same gene with varying functional interpretations. The literature predominantly references cps1 in relation to cell wall synthesis and septation, suggesting a potential discrepancy between UniProt's annotation and experimental research findings.

The key unresolved question centers on whether these are truly distinct genes or represent a single gene with multiple functional descriptions. Further investigation would be needed to definitively clarify this protein's precise molecular role and potential functional diversity.

Critically, multiple research sources consistently associate the cps1 gene with glucan synthase activity, which contrasts with the UniProt entry's carboxypeptidase classification. This suggests either an evolving understanding of the protein's function or potential annotation inconsistencies across different scientific databases.

The organism context remains consistent - all references are specifically about Schizosaccharomyces pombe - which provides a stable taxonomic framework for evaluating these protein characterizations.

Detailed examination reveals two distinct proteins sharing similar nomenclature:
1. A vacuolar carboxypeptidase (M20 family, SPAC24C9.08)
2. A β(1,3)-glucan synthase (Bgs1/Cps1p)

These proteins, while sharing nomenclatural overlap, appear to be functionally distinct entities within the same organism. The UniProt entry O13968 definitively describes the vacuolar carboxypeptidase variant, which should be the primary focus for precise protein characterization.

Research will systematically explore the protein's structural, functional, and cellular characteristics, emphasizing its unique role in vacuolar protein processing and potential metabolic contributions.

The comprehensive investigation will trace its molecular identity, enzymatic mechanisms, and broader biological significance within Schizosaccharomyces pombe's cellular environment.

CPS1 Vacuolar Carboxypeptidase (SPAC24C9.08) in Schizosaccharomyces pombe: A Comprehensive Functional Annotation

The cps1 gene (SPAC24C9.08) in the fission yeast Schizosaccharomyces pombe encodes an uncharacterized vacuolar carboxypeptidase belonging to the peptidase M20A family of metallopeptidases[1][5]. This zinc-dependent exopeptidase represents a key component of the vacuolar proteolytic system responsible for the final stages of protein degradation and amino acid recycling during cellular catabolism and autophagy. The enzyme functions in concert with other vacuolar hydrolases to complete the degradation of both constitutive and autophagic substrates, thereby generating free amino acids that support cellular metabolism under both nutrient-replete and nutrient-stressed conditions. Unlike the homonymous bgs1 gene (also historically referred to as cps1 in some contexts), which encodes a β(1,3)-glucan synthase involved in cell wall synthesis and septation[37][49], the SPAC24C9.08-encoded Cps1 protein represents a distinct proteolytic enzyme with specific roles in vacuolar protein hydrolysis. The characterization of this vacuolar carboxypeptidase contributes to our understanding of how yeast cells achieve efficient protein turnover and maintain cellular homeostasis through sophisticated compartmentalized proteolysis.

Protein Classification Within the Peptidase M20 Family

The Cps1 protein encoded by SPAC24C9.08 belongs to the Peptidase M20A subfamily, which comprises a diverse group of metalloexopeptidases with broad substrate specificity and participation in multiple metabolic pathways[11][13]. The M20 family of metallopeptidases is characterized by the presence of metal-coordinating residues that form a catalytic center typically containing one or two zinc ions, along with conserved structural domains that facilitate substrate binding and enzymatic turnover[13]. The specific domain architecture of the M20A subfamily includes a catalytic domain harboring the zinc coordination site and a regulatory lid domain that modulates substrate access to the active site[20]. This structural arrangement is shared among diverse carboxypeptidases across prokaryotic and eukaryotic organisms, enabling adaptation to multiple substrate specificities while maintaining the fundamental amide hydrolysis reaction mechanism[13].

The classification of Cps1 within the M20A subfamily places it in a functional category of enzymes that catalyze the removal of amino acids from the carboxy terminus of protein and peptide substrates[11][42]. This exopeptidase activity is particularly important for the complete degradation of proteins in the vacuole, where proteolysis is compartmentalized from the cytoplasm to prevent unwanted proteolytic damage to cytoplasmic proteins[14][26]. The M20 family has been extensively studied through structural and biochemical analyses, particularly for bacterial members such as carboxypeptidase G2 (CPG2) and the Staphylococcus aureus metallopeptidase Sapep, which have provided detailed insights into the catalytic mechanism and substrate recognition principles that are likely shared by the eukaryotic Cps1 protein[13][20].

The domain structure of Cps1 includes multiple functional regions that orchestrate its proteolytic activity. The primary catalytic domain contains the characteristic metal-binding motifs seen throughout the M20 family, typically involving histidine and glutamate residues that coordinate the zinc ions essential for catalysis[20][23]. Additionally, Cps1 contains a dimerization domain (IPR011650) that is found in many M20 family members and potentially enables the formation of functional dimers or higher-order oligomers[47]. The presence of a bacterial exopeptidase dimerization domain (IPR036264) further suggests that Cps1 may function as a multimeric enzyme, consistent with structural studies of other M20 carboxypeptidases that demonstrate dimer formation contributes to substrate binding and catalytic efficiency[20].

Enzymatic Function and Metal Ion Catalysis

Cps1, as a member of the M20 metallopeptidase family, catalyzes the hydrolytic removal of amino acids from the carboxy terminus of peptide and protein substrates through a zinc-dependent mechanism[11][13][23]. The zinc ion in the active site serves as the primary catalyst, polarizing the peptide carbonyl carbon and facilitating nucleophilic attack by an activated water molecule[23][41]. In the tetrahedral coordination geometry characteristic of zinc metalloproteases, the metal ion is bound by three amino acid ligands (typically histidine and glutamate residues) plus a catalytic water molecule[23][41]. During substrate binding, the water molecule is displaced and the carbonyl oxygen of the substrate forms a direct coordination bond with the zinc ion, stabilizing the transition state of the reaction[23].

The catalytic mechanism of M20 carboxypeptidases proceeds through well-characterized chemical steps that have been elucidated through structural and kinetic studies of bacterial homologues and other zinc metalloproteases[13][20][23]. Upon substrate binding, the zinc ion transitions from pentacoordinate geometry (three protein ligands, water, and a substrate oxygen) to hexacoordinate geometry as the catalytic water attacks the scissile carbonyl carbon[23]. This nucleophilic attack is facilitated by polarization of the carbonyl carbon by the zinc ion and activation of the water molecule by nearby glutamate or aspartate residues that serve as general base catalysts[23][41]. The resulting tetrahedral intermediate is stabilized by the zinc ion and by hydrogen bonding from other active site residues, ultimately leading to protonation of the leaving group and peptide bond cleavage[23]. The liberated carboxyl-terminal amino acid is released, and a new water molecule coordinates to the metal ion to restore the resting enzyme state[23][41].

While the detailed substrate specificity of Cps1 remains to be experimentally determined, comparative analysis with characterized M20 family members provides insights into likely substrate preferences. The M20 family exhibits remarkable diversity in substrate specificity despite maintaining a conserved catalytic mechanism, with different family members showing preferences for particular amino acids at the P1 position (the residue immediately carboxy-terminal to the scissile bond)[16][42]. Some M20 carboxypeptidases show broad specificity for multiple amino acids, while others are more restricted, particularly in their preference for acidic residues or specific hydrophobic amino acids[16]. The vacuolar location of Cps1 and its classification as an exopeptidase suggest it likely plays a role in the final degradation steps of vacuolar proteolysis, where it would remove amino acids sequentially from multiple protein substrates generated by prior endoprotease cleavage events[14][26]. This sequential exopeptidase activity is characteristic of the terminal degradation phase of autophagy, wherein initially processed autophagic cargo requires complete hydrolysis to free amino acids[14][26].

Cellular Localization and Vacuolar Targeting

The Cps1 carboxypeptidase is targeted to and functions within the vacuole, the cellular compartment functionally equivalent to the mammalian lysosome[1][5][24]. Vacuolar localization segregates Cps1 and other proteolytic enzymes from the cytoplasm, thereby protecting cytoplasmic proteins and other cellular components from unwanted degradation while maintaining the appropriate compartmentalization of proteolysis[26]. The delivery of Cps1 to the vacuole involves the secretory pathway, wherein newly synthesized Cps1 protein is transported through the endoplasmic reticulum (ER) and Golgi apparatus before sorting into vesicles that ultimately fuse with the vacuole[14][26]. This secretory trafficking pathway for vacuolar proteases represents one of the well-characterized protein sorting systems in yeast, and multiple key components of this pathway have been identified through genetic and biochemical approaches[26][32].

Like other vacuolar carboxypeptidases in yeast and fungi, Cps1 is initially synthesized as a precursor protein that undergoes proteolytic processing to generate the mature, catalytically active form[17][26][32]. The propeptide at the amino terminus of the nascent Cps1 precursor likely contains information required for recognition by the vacuolar protein sorting machinery[26][32]. Studies in Saccharomyces cerevisiae have demonstrated that the carboxypeptidase Y precursor contains a vacuolar targeting signal residing in its amino-terminal propeptide, consisting of a short tetrapeptide sequence (QRPL) that is recognized by the Vps10 sorting receptor in the Golgi apparatus[26][32]. Although the precise targeting signal within the S. pombe Cps1 propeptide has not been explicitly determined, sequence homology suggests that analogous recognition mechanisms likely operate, with the propeptide containing the necessary sorting determinants[45][48].

The multivesicular body (MVB) pathway represents one route by which vacuolar proteins can be delivered to the vacuole. This pathway involves the formation of multivesicular bodies that bud into the vacuolar lumen, thereby delivering their cargo proteins including proteases and other hydrolases[26]. In Saccharomyces cerevisiae, the zinc metallopeptidase Cps1 is known to be delivered via the MVB pathway, becoming associated with intravacuolar multivesicular bodies before its release into the vacuolar lumen[26]. Upon arrival in the vacuole, Cps1 undergoes maturation through removal of its inhibitory propeptide by the vacuolar protease Pep4 (protease A, a vacuolar aspartic protease) and subsequent processing by other vacuolar carboxypeptidases, ultimately generating the mature enzyme[14][26]. This proteolytic maturation step is essential for generating the catalytically competent form of Cps1 and represents an important control point in the regulation of vacuolar proteolytic capacity[14][26].

Role in Vacuolar Protein Degradation and Autophagy

The vacuole functions as the primary degradative compartment in yeast cells, responsible for breaking down proteins derived from multiple sources including cytoplasmic turnover, ER-associated degradation, and autophagic delivery of bulk cytoplasmic content[26][51][54]. During autophagy, cytoplasmic components including proteins, organelles, and other macromolecules are engulfed into double-membraned autophagic vesicles that subsequently fuse with the vacuole[26][51]. The resulting autophagic bodies within the vacuolar lumen are then degraded by the resident vacuolar hydrolases, including proteases such as Cps1, which collectively catalyze the complete breakdown of autophagic cargo[26][51]. This proteolytic degradation generates free amino acids and other metabolic intermediates that are exported back into the cytoplasm for reuse in biosynthetic processes, thereby supporting protein synthesis and other metabolic pathways especially under nutrient-limited conditions[26][51][54].

The terminal stages of autophagy—the breakdown of autophagic bodies and the degradation of their contents—require the coordinated action of multiple classes of vacuolar proteases. While the broad-specificity endoproteases such as Pep4 and the serine carboxypeptidase Prc1 initiate the degradation process by making internal cleavages and removing propeptides from zymogenic forms of vacuolar enzymes, the exopeptidases including Cps1 complete the degradation process by sequentially removing amino acids from peptide products[14][26]. Studies in S. cerevisiae demonstrate that the zinc metallopeptidase Cps1 specifically participates in the final degradation steps of autophagy, as deletion of CPS1 results in accumulation of partially degraded autophagic material and defects in the recovery of cellular amino acid pools during starvation[14][26]. Although direct experimental evidence for Cps1 function in S. pombe autophagy remains limited, the functional conservation of autophagy machinery across yeast species strongly suggests that the S. pombe Cps1 carboxypeptidase plays an analogous role in degrading autophagic cargo[51].

The regulation of Cps1 activity and expression is coupled to nutrient availability and cellular stress conditions that trigger autophagy. During nitrogen starvation and other nutrient-limiting conditions that induce autophagy, cells activate the autophagy machinery and increase the expression of autophagy-related proteins[26][51]. Concurrently, the vacuolar proteolytic system, including Cps1, is upregulated to enhance the capacity for cargo degradation and amino acid recycling[26]. The increased vacuolar proteolytic activity supports the synthesis of additional autophagic machinery proteins such as Atg8, which are required for continued autophagic flux[26]. Thus, Cps1 and the broader vacuolar proteolytic system are integral to the cellular strategy of nutrient recycling during periods of metabolic stress, enabling survival and maintenance of essential biosynthetic processes when external nutrient availability is limited[26][51].

Comparative Analysis with Homologous Carboxypeptidases

The M20 family of metallopeptidases exhibits substantial diversity in structure and substrate specificity despite sharing a conserved catalytic mechanism and active site architecture[13][20][27]. This diversity is exemplified by members of the M20 family that catalyze distinct reactions including carboxypeptidase activity (removal of C-terminal amino acids), dipeptidase activity (hydrolysis of dipeptides), and aminopeptidase-like activities (removal of N-terminal amino acids)[11][42]. Crystal structures and biochemical characterization of M20 family members have revealed that variations in the three-dimensional arrangement of the catalytic and regulatory lid domains allow for modulation of active site geometry, thereby enabling different substrate specificities and reaction types to be catalyzed by structurally related enzymes[13][20][27].

In eukaryotes, vacuolar carboxypeptidases similar to Cps1 have been identified in Saccharomyces cerevisiae, Arabidopsis thaliana, and mammalian cells, where they perform analogous proteolytic functions in lysosomal and vacuolar degradation pathways[14][17][26]. The S. cerevisiae Cps1 protein, also known as a zinc metallopeptidase carboxypeptidase, shares significant sequence identity and functional homology with the S. pombe SPAC24C9.08 Cps1 protein[14][26]. Both proteins are vacuolar zinc-dependent carboxypeptidases that participate in the final stages of proteolytic degradation and autophagy[14][26]. The conservation of these proteins across distantly related yeast species indicates that vacuolar zinc metallopeptidases represent an ancient and essential component of the eukaryotic proteolytic system[17][26].

The plant protein AtCPY, the Arabidopsis thaliana ortholog of carboxypeptidase Y, similarly functions as a vacuolar serine carboxypeptidase involved in protein degradation[17]. Interestingly, misfolded forms of AtCPY are recognized by the ER quality control system (ERAD pathway) and targeted for proteasomal degradation, demonstrating that even vacuolar proteases themselves are subject to proteolytic quality control mechanisms[17]. Mammalian carboxypeptidases such as the glutamate carboxypeptidase II (also known as N-acetylated-alpha-linked acidic dipeptidase 2), a member of the M28 metallopeptidase family, show functional analogies to M20 family carboxypeptidases and participate in the cleavage of terminal amino acids from substrates in specialized cellular compartments[8]. The structural and functional conservation of carboxypeptidase catalysis across these organisms underscores the fundamental importance of exopeptidase-mediated degradation in cellular protein turnover and metabolic regulation.

Distinction from the Bgs1/Cps1 β(1,3)-Glucan Synthase

An important clarification must be made regarding nomenclature: the gene SPAC24C9.08 encoding the vacuolar carboxypeptidase Cps1 represents a distinct gene from the bgs1 gene (also historically referred to as cps1 in earlier literature), which encodes a β(1,3)-glucan synthase essential for cell wall synthesis and cytokinesis[25][31][37][40][49]. This nomenclatural distinction reflects the evolutionary history of Schizosaccharomyces pombe genetics research, wherein the cps1 designation was originally assigned to the chlorpropham-supersensitive mutant that was later recognized to be a mutation in the bgs1 gene[19][22][57]. The bgs1/cps1 gene encodes a large integral membrane protein that catalyzes the synthesis of linear β(1,3)-glucan, the major structural polymer of the primary septum formed during cytokinesis[25][31][37][40][49].

The bgs1/cps1 β(1,3)-glucan synthase is a completely distinct protein from the SPAC24C9.08-encoded vacuolar carboxypeptidase discussed in this report. Whereas the Bgs1/Cps1p glucan synthase is an integral membrane protein involved in biosynthetic processes (specifically cell wall polymer synthesis), the SPAC24C9.08 Cps1 is a soluble vacuolar protease involved in catabolic processes (protein degradation)[37][40][49][52]. The β(1,3)-glucan synthase Bgs1/Cps1p contains multiple transmembrane domains characteristic of polymerase and synthase enzymes, whereas the carboxypeptidase Cps1 (SPAC24C9.08) contains domains characteristic of soluble metalloproteases[37][40][49]. These proteins have entirely different functions in cellular physiology and respond to different regulatory signals. Mutations in the bgs1/cps1 glucan synthase result in cell wall integrity defects, hypersensitivity to cell wall-damaging agents, and cytokinetic defects[19][22][25][31][40][57], whereas mutations in the SPAC24C9.08 carboxypeptidase would be expected to impair protein degradation and autophagy, though such mutations have not been extensively characterized.

The historical conflation of these two genes under the cps1 nomenclature underscores the importance of careful gene symbol verification when consulting older literature, particularly from the pre-genomic era when gene names were often based on phenotypic characterization of mutants rather than biochemical characterization of gene products[19][22][49][57]. Modern genomic databases such as PomBase and UniProt have clarified this nomenclatural situation by assigning systematic ORF names (SPAC24C9.08 for the vacuolar carboxypeptidase) and precise functional annotations based on sequence homology and domain architecture[1][5][24][46].

Current Characterization Status and Functional Inference

The SPAC24C9.08-encoded Cps1 carboxypeptidase is currently designated as "uncharacterized" in the primary UniProt database entry, reflecting the limited experimental characterization of this specific protein in the peer-reviewed literature[5]. However, the functional role of this enzyme can be reliably inferred from multiple lines of evidence: its classification within the well-characterized M20A peptidase family, its vacuolar localization, its structural homology to characterized vacuolar carboxypeptidases in other organisms, and the known roles of related carboxypeptidases in autophagy and protein degradation[1][5][11][13][26][42].

The peptidase M20 family belongs to the broader clan MH of metallopeptidases and is defined by characteristic active site residues and structural features that collectively define the carboxypeptidase reaction mechanism[11][42][44]. Members of this family have been extensively studied through structural biology, biochemistry, and genetic approaches, providing a comprehensive understanding of how M20 carboxypeptidases achieve their catalytic activity and recognize substrates[13][20][23][27]. Given that the SPAC24C9.08 Cps1 protein contains all of the conserved active site residues and structural domains characteristic of M20 carboxypeptidases, it is highly probable that this protein catalyzes zinc-dependent hydrolytic removal of amino acids from peptide substrates[1][5][44].

The vacuolar localization of Cps1, as indicated by sequence-based targeting predictions and database annotations, is consistent with a role in vacuolar protein degradation and autophagy[1][5][24][51][54]. The vacuole in Schizosaccharomyces pombe, like the vacuoles in other fungi and the lysosomes in mammalian cells, is the primary site of controlled proteolysis in the cell[26][51][54]. The compartmentalization of proteolytic enzymes such as Cps1 within the vacuole allows for extensive protein degradation without risking damage to cytoplasmic proteins and cellular structures[26][51][54]. Thus, the combination of M20 carboxypeptidase characteristics and vacuolar localization strongly suggests that Cps1 functions as a vacuolar exopeptidase participating in the final stages of protein degradation pathways including autophagy[1][5][26][51].

The precise kinetic parameters, substrate specificity, and in vivo substrates of the SPAC24C9.08 Cps1 protein remain to be experimentally determined through biochemical and genetic approaches. Future work employing direct enzymatic assays with purified recombinant protein, site-directed mutagenesis of proposed active site residues, and genetic analysis of cps1 deletion or catalytic mutant strains would provide detailed information about the specific substrates recognized by this enzyme, its preference for particular amino acids at the P1 position, and its quantitative contribution to vacuolar protein degradation capacity relative to other carboxypeptidases in S. pombe[26][51].

Integration into the Vacuolar Proteolytic System

The S. pombe Cps1 carboxypeptidase functions as part of a sophisticated multi-enzyme proteolytic system within the vacuole. Beyond Cps1, the vacuolar lumen contains numerous other proteolytic enzymes including endoproteases (Pep4, an aspartic protease analogous to yeast protease A), serine carboxypeptidases (Prc1 and potentially other serine carboxypeptidases), and additional metallopeptidases[14][26][51]. The different proteases in this system have complementary specificities: the endoproteases make internal cleavages within proteins, generating peptide intermediates; the broad-specificity carboxypeptidases remove residues sequentially from peptide termini; and specialized carboxypeptidases remove specific amino acids depending on their substrate specificity[14][26][51]. This compartmentalization and functional diversification of proteolytic activities ensures complete degradation of proteins to free amino acids[14][26][51].

The activation of vacuolar proteases requires removal of inhibitory propeptides, a process that is tightly regulated and coordinated to ensure that proteolytic activity is maintained only when appropriate[14][26]. The initial activation of Pep4 (protease A) is rate-limiting, as this enzyme subsequently activates other zymogenic proteases within the vacuole by removing their inhibitory propeptides[14][26]. The coordinated activation and balanced activity of multiple proteases ensures efficient protein degradation while preventing excessive proteolysis that might damage essential vacuolar proteins[14][26]. The regulation of Cps1 activity through its propeptide-mediated zymogenic storage represents one mechanism by which vacuolar proteolytic capacity is regulated in response to the availability of substrates (proteins requiring degradation)[14][26].

Conclusion and Significance

The Cps1 protein encoded by the SPAC24C9.08 gene in Schizosaccharomyces pombe represents a vacuolar zinc-dependent metallopeptidase belonging to the M20A family of carboxypeptidases[1][5][24]. Although currently designated as uncharacterized in primary databases, the functional role of Cps1 can be reliably inferred from its sequence homology to well-characterized M20 family carboxypeptidases, its vacuolar localization, and the known roles of related enzymes in autophagy and protein degradation[1][5][11][13][26][42]. This enzyme participates in the terminal phase of vacuolar proteolysis, sequentially removing amino acids from peptide substrates generated by prior endoprotease activity, thereby generating free amino acids that support cellular metabolism during both nutrient-replete and nutrient-starved conditions[26][51][54].

The characterization of vacuolar carboxypeptidases such as Cps1 contributes to the broader understanding of autophagy and cellular homeostasis, processes that are fundamental to cell survival and that are dysregulated in human diseases including cancer, neurodegeneration, and metabolic disorders[26][51]. The use of Schizosaccharomyces pombe as a model system for studying autophagy and protein degradation has yielded important insights into the molecular mechanisms of these processes that are highly conserved from yeast to humans[51]. Future detailed characterization of the SPAC24C9.08 Cps1 carboxypeptidase through biochemical and genetic approaches would provide specific information about its substrate specificity, kinetic parameters, and quantitative contribution to vacuolar proteolysis in fission yeast. Such studies would not only advance our understanding of this specific enzyme but would also contribute to the broader goal of understanding how cells achieve complete and controlled protein degradation through the orchestrated action of multiple classes of proteolytic enzymes functioning within distinct cellular compartments. The distinction between the vacuolar carboxypeptidase Cps1 (SPAC24C9.08) and the β(1,3)-glucan synthase historically called Cps1/Bgs1p exemplifies the importance of careful nomenclature and highlights how genomic and proteomic approaches have clarified the functional roles of distinct yeast gene products that were previously confused under overlapping nomenclature.

Citations

1. https://www.pombase.org/gene/SPAC24C9.08
2. https://www.ncbi.nlm.nih.gov/gene/2541555
3. https://www.ncbi.nlm.nih.gov/Structure/cdd/cd05674
4. https://pubmed.ncbi.nlm.nih.gov/9401022/
5. https://www.uniprot.org/uniprotkb/O13968/entry
6. https://www.uniprot.org/uniprotkb/P27614/entry
7. https://pmc.ncbi.nlm.nih.gov/articles/PMC179237/
8. https://www.ncbi.nlm.nih.gov/Structure/cdd/wrpsb.cgi?seqinput=NP_004467.1
9. https://pmc.ncbi.nlm.nih.gov/articles/PMC1885245/
10. https://pombase.org/gene/SPAC24C9.08
11. https://www.ebi.ac.uk/merops/cgi-bin/famsum?family=M20
12. https://flybase.org/reports/FBgn0039052.html
13. https://pmc.ncbi.nlm.nih.gov/articles/PMC2937973/
14. https://www.molbiolcell.org/doi/10.1091/mbc.E17-08-0516
15. https://academic.oup.com/nar/article/38/suppl_1/D227/3112316
16. https://pubmed.ncbi.nlm.nih.gov/20138839/
17. https://pmc.ncbi.nlm.nih.gov/articles/PMC179726/
18. https://journals.asm.org/doi/pdf/10.1128/jb.179.24.7653-7662.1997
19. https://pmc.ncbi.nlm.nih.gov/articles/PMC2998127/
20. https://pmc.ncbi.nlm.nih.gov/articles/PMC3904585/
21. https://chem.libretexts.org/Bookshelves/Biological_Chemistry/Supplemental_Modules_(Biological_Chemistry)/Enzymes/Metalloproteases
22. https://pmc.ncbi.nlm.nih.gov/articles/PMC7388972/
23. https://pmc.ncbi.nlm.nih.gov/articles/PMC5110390/
24. https://onlinelibrary.wiley.com/doi/full/10.1046/j.1365-2443.2002.00557.x
25. https://pubmed.ncbi.nlm.nih.gov/29514932/
26. https://febs.onlinelibrary.wiley.com/doi/10.1111/febs.14001
27. https://journals.asm.org/doi/10.1128/ec.2.3.510-520.2003
28. https://pmc.ncbi.nlm.nih.gov/articles/PMC2116205/
29. https://pmc.ncbi.nlm.nih.gov/articles/PMC4153625/
30. https://pmc.ncbi.nlm.nih.gov/articles/PMC4688203/
31. https://www.yeastgenome.org/locus/S000004912
32. https://pmc.ncbi.nlm.nih.gov/articles/PMC2670112/
33. https://pubmed.ncbi.nlm.nih.gov/6943549/
34. https://pmc.ncbi.nlm.nih.gov/articles/PMC10394242/
35. https://pmc.ncbi.nlm.nih.gov/articles/PMC5289844/
36. https://pubmed.ncbi.nlm.nih.gov/12556522/
37. https://www.ebi.ac.uk/interpro/entry/interpro/IPR002933
38. https://journals.asm.org/doi/pdf/10.1128/jb.179.13.4179-4189.1997
39. https://www.uniprot.org/uniprotkb/A0A166C4K0/entry
40. https://pubmed.ncbi.nlm.nih.gov/9209031/
41. https://pubmed.ncbi.nlm.nih.gov/12356913/
42. https://pubmed.ncbi.nlm.nih.gov/7763978/
43. https://pmc.ncbi.nlm.nih.gov/articles/PMC8997447/
44. https://journals.asm.org/doi/10.1128/ec.00187-10
45. https://pubmed.ncbi.nlm.nih.gov/3373337/
46. https://www.tandfonline.com/doi/full/10.1080/15548627.2018.1545821
47. https://pmc.ncbi.nlm.nih.gov/articles/PMC3613171/
48. https://research.bioinformatics.udel.edu/iptmnet/entry/O13968/
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BioReason-Pro RL Review: cps1 (S. pombe)

Source: cps1-deep-research-bioreason-rl.md

• Correctness: 3/5
• Completeness: 3/5

Functional Summary Review

The BioReason functional summary describes cps1 (SPAC24C9.08) as:

A soluble, cytoplasmic metallo-exopeptidase in fission yeast that assembles as a homodimer to trim peptide C-termini and thereby feed amino acid recycling and assimilation pathways. Its dimerization core stabilizes an M20 catalytic scaffold that coordinates divalent metal ions and promotes efficient peptide turnover, coupling intracellular peptide catabolism to broader nutrient assimilation within the cytoplasm.

The identification of the M20 family metallopeptidase domains and the di-zinc catalytic mechanism is correct. The homodimer architecture inferred from the dimerization domain is reasonable. The general claim about carboxypeptidase activity is consistent with the phylogenetic inference from S. cerevisiae CPS1.

However, there are important caveats that the summary misses:

1. Localization is wrong. The summary states "cytoplasmic" localization. The curated review establishes that SPAC24C9.08 localizes to the fungal-type vacuole (GO:0000324), confirmed by HDA evidence (PMID:16823372). For S. cerevisiae CPS1, the enzyme is synthesized as a type II transmembrane precursor and processed to a soluble form in the vacuolar lumen. BioReason's cytoplasmic assignment directly contradicts the experimental evidence.

2. Biochemical uncertainty not acknowledged. The curated review carefully notes that "the precise biochemical activity of SPAC24C9.08 has not been experimentally determined" and that the M20A subfamily has diverged to include enzymes with non-peptidase activities (e.g., mammalian PM20D1 functions as an N-fatty-acyl-amino acid synthase/hydrolase). BioReason confidently states carboxypeptidase function without any caveat.

3. Gene symbol ambiguity not mentioned. The curated review flags that "cps1" in S. pombe literature also refers to bgs1, a completely different glucan synthase -- an important nomenclature issue.

The identification of the M20/M20A family domains, dimerization module, and metallopeptidase chemistry is accurate at the structural level.

Comparison with interpro2go

The interpro2go predictions include carboxypeptidase activity (GO:0004180), which BioReason's summary essentially recapitulates. However, BioReason incorrectly places the enzyme in the cytoplasm rather than the vacuole, which is worse than the interpro2go cellular component predictions that correctly include vacuole (GO:0005773) and fungal-type vacuole (GO:0000324). BioReason repeats the interpro2go functional prediction while introducing a localization error.

Notes on thinking trace

The trace correctly identifies M20 family signatures and deduces metallopeptidase chemistry. The error arises from inferring cytoplasmic localization based on "absence of transmembrane or secretion signals in the annotated regions" -- but the protein actually has a type II transmembrane precursor form that is processed in the vacuole. This demonstrates a systematic weakness in localization prediction from domain architecture alone.