A0BFB4 (GSPATT00028266001) is a 381-amino acid protein kinase domain-containing protein from Paramecium tetraurelia, a ciliated protozoan. The protein contains a canonical protein kinase catalytic domain (Pfam PF00069, residues 96-348) with a conserved ATP-binding site (IPR017441) and a serine/threonine kinase active site (IPR008271), indicating it functions as a serine/threonine protein kinase that catalyzes ATP-dependent phosphorylation of protein substrates. It is classified in PANTHER family PTHR44167 (related to ovarian-specific serine/threonine-protein kinase LOK). P. tetraurelia possesses an extraordinarily expanded kinome of 2606 protein kinases (approximately 6.6% of its ~39,550 genes), the largest known relative to genome size. The dominant kinase group in P. tetraurelia is the calcium/calmodulin-dependent kinase family (970 members), reflecting the central role of Ca2+-dependent signaling in ciliate biology. Phylogenetic inference by PANTHER places A0BFB4 in a clade with autophagy-initiating kinases (ATG1/ULK1 family members), suggesting a potential role in autophagy regulation, although no gene-specific experimental data exist for this protein. The substrates and precise biological role of A0BFB4 remain unknown.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
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GO:0016020
membrane
|
IBA
GO_REF:0000033 |
KEEP AS NON CORE |
Summary: Membrane localization is inferred by phylogenetic annotation (IBA) from orthologs including ULK1, ATG1, and other kinases that associate with membranes during autophagy initiation. Membrane association is plausible for a kinase involved in autophagosome formation, as phagophore nucleation occurs at membrane sites. However, no direct localization data exist for A0BFB4, and 'membrane' (GO:0016020) is a very broad term. In P. tetraurelia, protein kinases localize to diverse membrane compartments including cilia, contractile vacuoles, and plasma membrane. Without specific evidence, this broad term is acceptable as a non-core annotation.
Reason: Membrane is a very broad cellular component term. The IBA inference from ATG1/ULK1 orthologs is plausible but nonspecific; no direct localization evidence exists for A0BFB4.
|
|
GO:0005737
cytoplasm
|
IBA
GO_REF:0000033 |
ACCEPT |
Summary: Cytoplasmic localization is inferred by phylogenetic annotation from a broad set of orthologs across yeast, fly, mouse, plant, and other organisms. ATG1/ULK1 family kinases and related kinases are predominantly cytoplasmic, and this is a reasonable default localization for a soluble protein kinase. Cytoplasmic localization is broadly conserved and applicable to a ciliate kinase.
Reason: Cytoplasm is a well-supported default localization for a soluble protein kinase. The IBA inference from diverse orthologs is robust, and cytoplasmic localization is expected for this class of kinases regardless of the specific biological role.
|
|
GO:0000045
autophagosome assembly
|
IBA
GO_REF:0000033 |
UNDECIDED |
Summary: Autophagosome assembly is inferred by phylogenetic annotation from ATG1/ULK1 orthologs (ULK1/O75385, ATG1/S000003148 in yeast, Atg1/FBgn0260945 in fly, Ulk1/MGI:1270126 in mouse). ATG1/ULK1 kinases are master regulators of autophagy initiation and are directly involved in phagophore nucleation and autophagosome formation. Autophagy is conserved across eukaryotes including ciliates; Paramecium uses both macroautophagy and microautophagy pathways. The PANTHER family PTHR44167 is annotated as LOK-related rather than ATG1/ULK1, so the phylogenetic placement of A0BFB4 within the ATG1/ULK1 clade relies on the PANTHER tree topology at the subfamily level. This annotation is plausible but depends entirely on the accuracy of the phylogenetic inference for this highly expanded ciliate kinome.
Reason: Autophagy is conserved in ciliates and the IBA orthologs are well-characterized ATG1/ULK1 family members. However, the PANTHER family (PTHR44167) is annotated as LOK-related kinase rather than ATG1/ULK1, raising uncertainty about whether the phylogenetic placement is correct for this protein from a massively expanded kinome (2606 kinases). Without gene-specific experimental evidence, the confidence in this specific functional assignment is moderate.
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|
GO:0010506
regulation of autophagy
|
IBA
GO_REF:0000033 |
UNDECIDED |
Summary: Regulation of autophagy is inferred from ATG1/ULK1 orthologs. ULK1 is a key regulatory kinase that integrates nutrient-sensing signals (via mTOR and AMPK) to initiate autophagy. This annotation is largely redundant with autophagosome assembly (GO:0000045) for an ATG1/ULK1-type kinase, as the kinase's role in autophagosome assembly IS the regulatory step. The same uncertainty about phylogenetic placement applies as for the autophagosome assembly annotation.
Reason: Same phylogenetic uncertainty as for autophagosome assembly. Additionally, this term is partially redundant with GO:0000045 for an ATG1/ULK1-type kinase, since the kinase initiates autophagy by directly assembling the autophagosome machinery. Without gene-specific evidence, cannot confidently assign.
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GO:0000407
phagophore assembly site
|
IBA
GO_REF:0000033 |
UNDECIDED |
Summary: The phagophore assembly site (PAS) is the subcellular location where autophagosome formation initiates. ATG1/ULK1 kinases localize to the PAS to nucleate phagophore formation. This is a specific subcellular localization that is directly tied to the autophagy function. The annotation depends on the same phylogenetic inference as the autophagy process annotations. Additionally, the PAS concept was originally defined in yeast; whether ciliates organize autophagy initiation at a morphologically equivalent structure is uncertain.
Reason: Depends on the same uncertain phylogenetic inference as the autophagy annotations. The phagophore assembly site is a specific structure first characterized in yeast; its precise equivalent in P. tetraurelia ciliates is not established.
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GO:0005776
autophagosome
|
IBA
GO_REF:0000033 |
UNDECIDED |
Summary: Autophagosome localization is inferred from ATG1/ULK1 orthologs that transiently associate with autophagosomes during their formation. This annotation is closely tied to the autophagosome assembly function annotation and depends on the same phylogenetic inference. Autophagosomes have been observed in Paramecium, so the structure itself exists in this organism.
Reason: Same phylogenetic uncertainty as the other autophagy-related annotations. While autophagosomes do exist in ciliates, the specific localization of A0BFB4 to this compartment is unverified.
|
|
GO:0004674
protein serine/threonine kinase activity
|
IBA
GO_REF:0000033 |
ACCEPT |
Summary: Protein serine/threonine kinase activity is inferred by phylogenetic annotation from a broad set of orthologs across yeast, fly, worm, mouse, human, and fission yeast. This molecular function annotation is strongly supported by the domain architecture: A0BFB4 contains a protein kinase domain (PF00069) with a Ser/Thr kinase active site (IPR008271) and conserved ATP-binding site (IPR017441). The Ser/Thr kinase active site signature specifically supports serine/threonine rather than tyrosine kinase activity. This is the most well-supported annotation for this protein.
Reason: Strongly supported by both phylogenetic inference and domain architecture. The protein has a canonical Ser/Thr kinase domain with conserved active-site and ATP-binding residues. This is the core molecular function of the protein.
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|
GO:0005829
cytosol
|
IBA
GO_REF:0000033 |
KEEP AS NON CORE |
Summary: Cytosol localization is inferred from ULK1 and ATG1 orthologs. Cytosol is a more specific term than cytoplasm (GO:0005737), denoting the soluble fraction excluding organelles. For a soluble kinase, cytosolic localization is reasonable but no direct fractionation or imaging data exist for A0BFB4. This annotation is largely redundant with the cytoplasm annotation.
Reason: Cytosol is more specific than cytoplasm and is plausible for a soluble kinase, but adds little information beyond the cytoplasm annotation. No direct localization data exist for A0BFB4.
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GO:0000166
nucleotide binding
|
IEA
GO_REF:0000104 |
MARK AS OVER ANNOTATED |
Summary: Nucleotide binding is inferred electronically from the UniRule annotation pipeline. While technically correct (kinases bind ATP, which is a nucleotide), this is a very high-level parent term that is subsumed by the more specific ATP binding (GO:0005524) annotation. This adds no information beyond what is already captured by the ATP binding and kinase activity annotations.
Reason: Nucleotide binding is a parent of ATP binding (GO:0005524), which is already annotated. This redundant high-level term adds no informational value.
|
|
GO:0004672
protein kinase activity
|
IEA
GO_REF:0000002 |
MARK AS OVER ANNOTATED |
Summary: Protein kinase activity is inferred from the InterPro Ser/Thr kinase active site signature (IPR008271). This is a parent term of protein serine/threonine kinase activity (GO:0004674), which is already annotated via both IBA and IEA. The more specific child term is preferred.
Reason: This is a parent term of GO:0004674 (protein serine/threonine kinase activity), which is already annotated with both IBA and IEA evidence. The more specific term is preferred; this redundant parent annotation adds no value.
|
|
GO:0004674
protein serine/threonine kinase activity
|
IEA
GO_REF:0000120 |
ACCEPT |
Summary: This is a second annotation of protein serine/threonine kinase activity, from the combined IEA pipeline (GO_REF:0000120) based on EC:2.7.11.1 and UniRule. This duplicates the IBA annotation of the same term (GO:0004674) with electronic rather than phylogenetic evidence, providing independent corroboration from domain architecture. Both the IBA phylogenetic inference and this IEA domain-based inference converge on the same core molecular function.
Reason: Protein serine/threonine kinase activity is strongly supported by both phylogenetic inference (IBA) and domain architecture (IEA). This IEA annotation provides independent corroboration of the core molecular function.
|
|
GO:0005524
ATP binding
|
IEA
GO_REF:0000120 |
ACCEPT |
Summary: ATP binding is inferred from the protein kinase domain (IPR000719), the ATP-binding site signature (IPR017441), and UniRule. All protein kinases bind ATP as a phosphate donor. A0BFB4 has a conserved ATP-binding site (residue 125 annotated as ATP-binding in UniProt). This is a well-supported, core molecular function annotation.
Reason: ATP binding is a fundamental requirement for kinase catalytic activity, strongly supported by the conserved ATP-binding site and domain architecture. This is a core molecular function annotation.
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|
GO:0016301
kinase activity
|
IEA
GO_REF:0000104 |
MARK AS OVER ANNOTATED |
Summary: Kinase activity is a high-level parent of protein kinase activity (GO:0004672), which is itself a parent of protein serine/threonine kinase activity (GO:0004674). This annotation is redundant with both the more specific protein kinase activity and protein serine/threonine kinase activity annotations.
Reason: Kinase activity (GO:0016301) is an ancestor of GO:0004674 (protein serine/threonine kinase activity), which is already annotated. Redundant high-level term with no additional informational value.
|
|
GO:0016740
transferase activity
|
IEA
GO_REF:0000104 |
MARK AS OVER ANNOTATED |
Summary: Transferase activity is a very high-level molecular function term. All kinases are transferases (they transfer phosphate groups). This annotation is fully subsumed by the more specific kinase activity, protein kinase activity, and protein serine/threonine kinase activity annotations.
Reason: Transferase activity (GO:0016740) is a distant ancestor of GO:0004674 (protein serine/threonine kinase activity). This is the most generic of the redundant hierarchy of MF terms annotated to this protein. Adds no informational value.
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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.
Critical Limitation: No specific literature was found for protein A0BFB4 (gene identifier GSPATT00028266001) from Paramecium tetraurelia. The functional annotation provided below is based on (1) bioinformatic inference from domain architecture and (2) contextual knowledge about the P. tetraurelia kinome and ciliate signaling biology. Direct experimental evidence for this specific gene product is not currently available in the scientific literature.
Protein A0BFB4 is annotated in UniProt as a "protein kinase domain-containing protein" encoded by the gene GSPATT00028266001 in Paramecium tetraurelia, a ciliated protozoan. The protein belongs to the protein kinase superfamily based on domain annotation. P. tetraurelia is notable for possessing an extraordinarily large kinome comprising 2606 unique protein kinases, representing approximately 6.6% of its ~39,550 genes—the largest known kinome relative to genome size at the time of characterization (bemm2009akinomeof pages 1-2, bemm2009akinomeof pages 2-3). This massive expansion of protein kinases reflects the organism's complex Ca²⁺-dependent signaling requirements for ciliary function, membrane trafficking, and sensory responses (bemm2009akinomeof pages 1-2, bemm2009akinomeof pages 2-3, plattner2017signallinginciliates pages 1-2).
| Feature | Description | Evidence Source |
|---|---|---|
| UniProt accession | A0BFB4; annotated as a protein kinase domain-containing protein with ORF name GSPATT00028266001 in Paramecium tetraurelia. | UniProt information provided in user prompt |
| Organism | Paramecium tetraurelia, a ciliate model organism with extensive kinase expansion and Ca2+-centered signaling biology. | (bemm2009akinomeof pages 1-2, plattner2017signallinginciliates pages 1-2, villalobo2022calmodulininparamecium pages 1-2) |
| Protein family | Belongs to the protein kinase superfamily; this superfamily comprises enzymes that transfer phosphate from ATP to protein substrates and is central to reversible signaling control. | (seok2021structuralinsightsinto pages 1-2, adams2001kineticandcatalytic pages 1-2) |
| Core catalytic domain | Contains a Pkinase / protein kinase domain; canonical Ser/Thr kinase domains are typically ~290 amino acids and provide the conserved catalytic fold for ATP binding and phosphotransfer. | (seok2021structuralinsightsinto pages 1-2, adams2001kineticandcatalytic pages 1-2) |
| PF00069 | Pkinase (PF00069): Pfam protein kinase domain consistent with eukaryotic protein kinases. | UniProt information provided in user prompt; general kinase-domain context (seok2021structuralinsightsinto pages 1-2, adams2001kineticandcatalytic pages 1-2) |
| IPR000719 | Prot_kinase_dom. (IPR000719): InterPro protein kinase catalytic domain annotation, supporting assignment to the eukaryotic protein kinase class. | UniProt information provided in user prompt; general kinase-domain context (seok2021structuralinsightsinto pages 1-2, adams2001kineticandcatalytic pages 1-2) |
| IPR017441 | Protein_kinase_ATP_BS. (IPR017441): annotation for a conserved ATP-binding site, consistent with ATP-dependent phosphotransfer activity. | UniProt information provided in user prompt; ATP-dependent kinase mechanism (seok2021structuralinsightsinto pages 1-2, adams2001kineticandcatalytic pages 1-2) |
| IPR008271 | Ser/Thr_kinase_AS. (IPR008271): annotation for a serine/threonine kinase active site, supporting inference that the protein most likely phosphorylates Ser/Thr residues rather than Tyr residues. | UniProt information provided in user prompt; Ser/Thr kinase biology (seok2021structuralinsightsinto pages 1-2, johnson2023anatlasof pages 1-2) |
| IPR011009 | Kinase-like_dom_sf. (IPR011009): kinase-like domain superfamily assignment, supporting conserved structural similarity to other kinase-family members. | UniProt information provided in user prompt; conserved kinase fold (seok2021structuralinsightsinto pages 1-2, adams2001kineticandcatalytic pages 1-2) |
| Likely enzymatic function | Most likely catalyzes transfer of the γ-phosphate of ATP to hydroxyl groups on serine/threonine residues in protein substrates; however, no A0BFB4-specific biochemical assay or substrate list was identified. | General kinase mechanism (seok2021structuralinsightsinto pages 1-2, adams2001kineticandcatalytic pages 1-2); no gene-specific paper found in retrieved literature |
| Substrate specificity status | Unknown for A0BFB4 specifically. In Ser/Thr kinases generally, specificity depends on short sequence motifs around the phosphoacceptor site and can involve both positive and negative selectivity. | (johnson2023anatlasof pages 1-2, adams2001kineticandcatalytic pages 1-2) |
| Gene-specific evidence status | No direct literature was found for A0BFB4 / GSPATT00028266001 in the retrieved sources; functional interpretation therefore depends mainly on domain architecture and kinome context rather than direct experiment. | Literature search results summarized from retrieved context set (bemm2009akinomeof pages 1-2, plattner2017signallinginciliates pages 1-2) |
| Kinome context in P. tetraurelia | P. tetraurelia contains 2606 unique protein kinases, corresponding to about 6.6% of its ~39,550 genes, described as the largest kinome reported at that time. | (bemm2009akinomeof pages 1-2, bemm2009akinomeof pages 2-3, bemm2009akinomeof pages 3-4) |
| Dominant kinase classes in species | The largest kinase group in P. tetraurelia is the calcium/calmodulin-dependent subfamily (970 members), indicating strong coupling between kinase signaling and Ca2+/calmodulin-regulated physiology. | (bemm2009akinomeof pages 2-3, plattner2017signallinginciliates pages 1-2) |
| Broader signaling context | In ciliates, especially Paramecium, serine/threonine kinases participate in ciliary regulation, membrane trafficking, exocytosis, contractile vacuole function, and Ca2+-dependent signaling. | (plattner2017signallinginciliates pages 1-2, plattner2017signallinginciliates pages 20-21, plattner2017signallinginciliates pages 19-20, yano2013proteomicanalysisof pages 1-2) |
| Localization evidence status | No direct subcellular localization data for A0BFB4 were identified. In P. tetraurelia more broadly, kinases can be associated with cilia/ciliary membrane, membranes, cytoplasm, or trafficking-related organelles. | (plattner2017signallinginciliates pages 1-2, yano2013proteomicanalysisof pages 1-2) |
Table: This table summarizes the verified identity, domain annotations, and inferred molecular properties of Paramecium tetraurelia protein A0BFB4, along with the broader kinome context needed for cautious functional annotation when gene-specific literature is lacking.
Based on domain architecture, protein A0BFB4 most likely functions as a serine/threonine protein kinase that catalyzes the transfer of the γ-phosphate group from ATP to hydroxyl groups on serine or threonine residues of target protein substrates (seok2021structuralinsightsinto pages 1-2, adams2001kineticandcatalytic pages 1-2). This inference is supported by the presence of:
Serine/threonine protein kinases catalyze phosphorylation through a highly conserved mechanism. The catalytic domain, typically ~290 amino acids, forms a bilobal structure consisting of a smaller N-terminal lobe and a larger C-terminal lobe (seok2021structuralinsightsinto pages 1-2, adams2001kineticandcatalytic pages 1-2). The active site resides in the cleft between these lobes and coordinates ATP binding via conserved residues, including a critical lysine that positions the phosphate groups and an aspartate involved in catalysis (adams2001kineticandcatalytic pages 1-2). Metal ions, typically Mg²⁺, are essential cofactors that facilitate phosphoryl transfer by stabilizing the negative charge on ATP phosphates and the transition state (adams2001kineticandcatalytic pages 1-2).
Most phosphorylation events in eukaryotes occur on serine residues (86%), followed by threonine (12%) and tyrosine (2%) (seok2021structuralinsightsinto pages 1-2). The reaction mechanism involves nucleophilic attack by the substrate hydroxyl group on the γ-phosphate of ATP, yielding ADP and a phosphorylated protein product. This reversible post-translational modification serves as a key regulatory mechanism controlling protein activity, localization, stability, and interactions (seok2021structuralinsightsinto pages 1-2).
The specific substrates of A0BFB4 are unknown. In general, serine/threonine kinases achieve substrate specificity through recognition of short linear motifs (typically 5-10 residues) surrounding the phosphoacceptor site (johnson2023anatlasof pages 1-2, adams2001kineticandcatalytic pages 1-2). A comprehensive profiling study of 303 human Ser/Thr kinases revealed that substrate specificity is substantially more diverse than previously appreciated and is driven extensively by negative selectivity—that is, kinases discriminate against certain amino acids at specific positions as much as they select for preferred residues (johnson2023anatlasof pages 1-2). Without experimental data, the precise substrate motif preference and cellular targets of A0BFB4 cannot be determined.
No direct localization data exist for protein A0BFB4. In Paramecium tetraurelia, protein kinases are distributed across multiple subcellular compartments reflecting their diverse regulatory roles. Proteomic analysis of ciliary membranes has identified Ca²⁺-dependent protein kinases and signaling proteins in cilia (yano2013proteomicanalysisof pages 1-2), while other kinases localize to the cytoplasm, cell membrane, or trafficking organelles including the contractile vacuole complex, phagosomes, and secretory vesicles (plattner2017signallinginciliates pages 1-2, plattner2017signallinginciliates pages 20-21, plattner2017signallinginciliates pages 19-20). The subcellular distribution of A0BFB4 would need to be determined experimentally through techniques such as immunofluorescence microscopy, GFP tagging, or subcellular fractionation followed by Western blotting.
P. tetraurelia possesses 2606 protein kinases organized into multiple subfamilies (bemm2009akinomeof pages 1-2, bemm2009akinomeof pages 2-3). The largest subfamily is the calcium/calmodulin-dependent kinase (CAMK) group with 970 members, underscoring the central importance of Ca²⁺ signaling in ciliate physiology (bemm2009akinomeof pages 2-3). Other major groups include AGC kinases (635 members, including PKA and PKG families) and CMGC kinases (322 members, including cyclin-dependent and mitogen-activated protein kinases) (bemm2009akinomeof pages 2-3). This kinome expansion reflects P. tetraurelia's complex requirements for:
(plattner2017signallinginciliates pages 1-2, plattner2017signallinginciliates pages 20-21, plattner2017signallinginciliates pages 19-20, yano2013proteomicanalysisof pages 1-2)
Calcium is a universal second messenger in Paramecium that regulates diverse cellular processes (plattner2017signallinginciliates pages 1-2, villalobo2022calmodulininparamecium pages 1-2). Calmodulin (CaM), a highly conserved Ca²⁺-binding protein, serves as a key sensor and transducer of Ca²⁺ signals (villalobo2022calmodulininparamecium pages 2-3, villalobo2022calmodulininparamecium pages 1-2). CaM is localized to cilia where it regulates ciliary beat frequency and direction (villalobo2022calmodulininparamecium pages 2-3, villalobo2022calmodulininparamecium pages 1-2). The massive expansion of CaM-dependent kinases in P. tetraurelia (970 members) reflects genome evolution shaped by the organism's reliance on Ca²⁺/CaM signaling for fundamental cellular activities (bemm2009akinomeof pages 2-3).
Paramecium cells utilize multiple mechanisms for Ca²⁺ regulation, including:
- Plasma membrane Ca²⁺ channels for Ca²⁺ influx
- Intracellular Ca²⁺-release channels (IP₃ receptors and ryanodine receptor-like proteins) on various organelles
- Ca²⁺-ATPases (SERCA-type pumps) for sequestration into stores
- Ca²⁺-binding proteins for rapid buffering
(plattner2017signallinginciliates pages 1-2, plattner2017signallinginciliates pages 20-21, plattner2017signallinginciliates pages 19-20)
Without gene-specific data, the precise biological role of A0BFB4 cannot be definitively assigned. However, based on its membership in the protein kinase superfamily and the cellular context of P. tetraurelia, plausible functions include:
Regulation of ciliary function: Protein phosphorylation is a key mechanism controlling ciliary beat patterns in response to environmental stimuli (plattner2017signallinginciliates pages 1-2, yano2013proteomicanalysisof pages 1-2)
Membrane trafficking regulation: Kinases regulate vesicle formation, transport, and fusion during endocytosis, exocytosis, and phagocytosis (plattner2017signallinginciliates pages 1-2, plattner2017signallinginciliates pages 20-21, plattner2017signallinginciliates pages 19-20)
Ca²⁺-dependent signaling: Given the predominance of CaM-kinases in the P. tetraurelia kinome, A0BFB4 could participate in Ca²⁺-regulated processes (bemm2009akinomeof pages 2-3)
Cell cycle regulation: CMGC kinases, including cyclin-dependent kinases, control cell division and nuclear dynamics (bemm2009akinomeof pages 2-3)
Metabolic regulation: Kinases modulate enzyme activities in response to cellular energy status (plattner2017signallinginciliates pages 1-2)
The P. tetraurelia lineage underwent at least two rounds of whole-genome duplication followed by extensive gene loss (bemm2009akinomeof pages 1-2, bemm2009akinomeof pages 2-3). Despite this massive gene turnover, the kinome was preferentially retained and expanded, suggesting strong selective pressure to maintain kinase diversity (bemm2009akinomeof pages 2-3). The kinome comprises only 40 distinct domain architectures, far fewer than the 134 architectures found in the human kinome, despite having five times as many kinases (bemm2009akinomeof pages 2-3, bemm2009akinomeof pages 3-4). This suggests that Paramecium kinases may achieve functional diversity through alternative mechanisms such as differential expression, subcellular targeting, or regulatory domain variations rather than through extensive domain shuffling (bemm2009akinomeof pages 2-3, bemm2009akinomeof pages 3-4).
Phylogenetic analysis of the P. tetraurelia kinome reveals independent lineage-specific expansions relative to human kinases, with particular amplification of kinase families important for ciliate-specific processes (bemm2009akinomeof pages 2-3, bemm2009akinomeof pages 3-4). Approximately 22% of P. tetraurelia kinases contain substitutions at predicted catalytic sites, suggesting they may function as pseudokinases with regulatory rather than catalytic roles, or they may have evolved altered substrate specificities (bemm2009akinomeof pages 2-3).
This report is constrained by the complete absence of gene-specific experimental literature for A0BFB4/GSPATT00028266001. All functional inferences are based on:
To establish the specific function of A0BFB4, future research should include:
Protein A0BFB4 from Paramecium tetraurelia is a predicted serine/threonine protein kinase based on domain architecture, belonging to one of the 2606 kinases that constitute the largest known kinome relative to genome size. While no direct experimental evidence exists for this specific protein, its domain composition indicates it likely catalyzes ATP-dependent phosphorylation of serine and/or threonine residues on protein substrates. Given P. tetraurelia's remarkable expansion of Ca²⁺/calmodulin-dependent kinases and the central role of protein phosphorylation in ciliate signaling—including ciliary regulation, membrane trafficking, exocytosis, and sensory transduction—A0BFB4 likely participates in one or more of these fundamental cellular processes. However, definitive functional assignment requires targeted experimental investigation including substrate identification, phenotypic analysis of loss-of-function mutants, and subcellular localization studies.
Key Citations:
- Bemm et al. 2009: Characterization of the 2606-member P. tetraurelia kinome (bemm2009akinomeof pages 1-2, bemm2009akinomeof pages 2-3, bemm2009akinomeof pages 3-4)
- Plattner 2017: Comprehensive review of signaling in ciliates (plattner2017signallinginciliates pages 1-2, plattner2017signallinginciliates pages 20-21, plattner2017signallinginciliates pages 19-20)
- Villalobo et al. 2022: Calmodulin and Ca²⁺ signaling in Paramecium (villalobo2022calmodulininparamecium pages 2-3, villalobo2022calmodulininparamecium pages 1-2)
- Adams 2001 & Seok 2021: Protein kinase catalytic mechanisms (seok2021structuralinsightsinto pages 1-2, adams2001kineticandcatalytic pages 1-2)
- Johnson et al. 2023: Substrate specificity atlas of human Ser/Thr kinases (johnson2023anatlasof pages 1-2)
- Yano et al. 2013: Proteomic analysis of P. tetraurelia cilia (yano2013proteomicanalysisof pages 1-2)
References
(bemm2009akinomeof pages 1-2): Felix Bemm, Roland Schwarz, Frank Förster, and Jörg Schultz. A kinome of 2600 in the ciliate paramecium tetraurelia. FEBS Letters, 583:3589-3592, Nov 2009. URL: https://doi.org/10.1016/j.febslet.2009.10.029, doi:10.1016/j.febslet.2009.10.029. This article has 16 citations and is from a peer-reviewed journal.
(bemm2009akinomeof pages 2-3): Felix Bemm, Roland Schwarz, Frank Förster, and Jörg Schultz. A kinome of 2600 in the ciliate paramecium tetraurelia. FEBS Letters, 583:3589-3592, Nov 2009. URL: https://doi.org/10.1016/j.febslet.2009.10.029, doi:10.1016/j.febslet.2009.10.029. This article has 16 citations and is from a peer-reviewed journal.
(plattner2017signallinginciliates pages 1-2): H. Plattner. Signalling in ciliates: long‐ and short‐range signals and molecular determinants for cellular dynamics. Biological Reviews, Feb 2017. URL: https://doi.org/10.1111/brv.12218, doi:10.1111/brv.12218. This article has 26 citations and is from a domain leading peer-reviewed journal.
(villalobo2022calmodulininparamecium pages 1-2): Eduardo Villalobo, Gabriel Gutiérrez, and Antonio Villalobo. Calmodulin in paramecium: focus on genomic data. Microorganisms, 10:1915, Sep 2022. URL: https://doi.org/10.3390/microorganisms10101915, doi:10.3390/microorganisms10101915. This article has 2 citations.
(seok2021structuralinsightsinto pages 1-2): Seung-Hyeon Seok. Structural insights into protein regulation by phosphorylation and substrate recognition of protein kinases/phosphatases. Life, 11:957, Sep 2021. URL: https://doi.org/10.3390/life11090957, doi:10.3390/life11090957. This article has 120 citations.
(adams2001kineticandcatalytic pages 1-2): Joseph A. Adams. Kinetic and catalytic mechanisms of protein kinases. Chemical reviews, 101 8:2271-90, Jul 2001. URL: https://doi.org/10.1021/cr000230w, doi:10.1021/cr000230w. This article has 921 citations and is from a highest quality peer-reviewed journal.
(johnson2023anatlasof pages 1-2): Jared L. Johnson, Tomer M. Yaron, Emily M. Huntsman, Alexander Kerelsky, Junho Song, Amit Regev, Ting-Yu Lin, Katarina Liberatore, Daniel M. Cizin, Benjamin M. Cohen, Neil Vasan, Yilun Ma, Konstantin Krismer, Jaylissa Torres Robles, Bert van de Kooij, Anne E. van Vlimmeren, Nicole Andrée-Busch, Norbert F. Käufer, Maxim V. Dorovkov, Alexey G. Ryazanov, Yuichiro Takagi, Edward R. Kastenhuber, Marcus D. Goncalves, Benjamin D. Hopkins, Olivier Elemento, Dylan J. Taatjes, Alexandre Maucuer, Akio Yamashita, Alexei Degterev, Mohamed Uduman, Jingyi Lu, Sean D. Landry, Bin Zhang, Ian Cossentino, Rune Linding, John Blenis, Peter V. Hornbeck, Benjamin E. Turk, Michael B. Yaffe, and Lewis C. Cantley. An atlas of substrate specificities for the human serine/threonine kinome. Nature, 613:759-766, Jan 2023. URL: https://doi.org/10.1038/s41586-022-05575-3, doi:10.1038/s41586-022-05575-3. This article has 762 citations and is from a highest quality peer-reviewed journal.
(bemm2009akinomeof pages 3-4): Felix Bemm, Roland Schwarz, Frank Förster, and Jörg Schultz. A kinome of 2600 in the ciliate paramecium tetraurelia. FEBS Letters, 583:3589-3592, Nov 2009. URL: https://doi.org/10.1016/j.febslet.2009.10.029, doi:10.1016/j.febslet.2009.10.029. This article has 16 citations and is from a peer-reviewed journal.
(plattner2017signallinginciliates pages 20-21): H. Plattner. Signalling in ciliates: long‐ and short‐range signals and molecular determinants for cellular dynamics. Biological Reviews, Feb 2017. URL: https://doi.org/10.1111/brv.12218, doi:10.1111/brv.12218. This article has 26 citations and is from a domain leading peer-reviewed journal.
(plattner2017signallinginciliates pages 19-20): H. Plattner. Signalling in ciliates: long‐ and short‐range signals and molecular determinants for cellular dynamics. Biological Reviews, Feb 2017. URL: https://doi.org/10.1111/brv.12218, doi:10.1111/brv.12218. This article has 26 citations and is from a domain leading peer-reviewed journal.
(yano2013proteomicanalysisof pages 1-2): Junji Yano, Anbazhagan Rajendran, Megan S. Valentine, Madhurima Saha, Bryan A. Ballif, and Judith L. Van Houten. Proteomic analysis of the cilia membrane of paramecium tetraurelia. Journal of proteomics, 78:113-22, Jan 2013. URL: https://doi.org/10.1016/j.jprot.2012.09.040, doi:10.1016/j.jprot.2012.09.040. This article has 45 citations and is from a peer-reviewed journal.
(villalobo2022calmodulininparamecium pages 2-3): Eduardo Villalobo, Gabriel Gutiérrez, and Antonio Villalobo. Calmodulin in paramecium: focus on genomic data. Microorganisms, 10:1915, Sep 2022. URL: https://doi.org/10.3390/microorganisms10101915, doi:10.3390/microorganisms10101915. This article has 2 citations.
id: A0BFB4
gene_symbol: A0BFB4
product_type: PROTEIN
status: DRAFT
taxon:
id: NCBITaxon:5888
label: Paramecium tetraurelia
description: >-
A0BFB4 (GSPATT00028266001) is a 381-amino acid protein kinase domain-containing
protein from Paramecium tetraurelia, a ciliated protozoan. The protein contains a
canonical protein kinase catalytic domain (Pfam PF00069, residues 96-348) with a
conserved ATP-binding site (IPR017441) and a serine/threonine kinase active site
(IPR008271), indicating it functions as a serine/threonine protein kinase that
catalyzes ATP-dependent phosphorylation of protein substrates. It is classified in
PANTHER family PTHR44167 (related to ovarian-specific serine/threonine-protein
kinase LOK). P. tetraurelia possesses an extraordinarily expanded kinome of 2606
protein kinases (approximately 6.6% of its ~39,550 genes), the largest known
relative to genome size. The dominant kinase group in P. tetraurelia is the
calcium/calmodulin-dependent kinase family (970 members), reflecting the central
role of Ca2+-dependent signaling in ciliate biology. Phylogenetic inference by
PANTHER places A0BFB4 in a clade with autophagy-initiating kinases (ATG1/ULK1
family members), suggesting a potential role in autophagy regulation, although no
gene-specific experimental data exist for this protein. The substrates and precise
biological role of A0BFB4 remain unknown.
existing_annotations:
- term:
id: GO:0016020
label: membrane
evidence_type: IBA
original_reference_id: GO_REF:0000033
qualifier: is_active_in
review:
summary: >-
Membrane localization is inferred by phylogenetic annotation (IBA) from
orthologs including ULK1, ATG1, and other kinases that associate with
membranes during autophagy initiation. Membrane association is plausible for
a kinase involved in autophagosome formation, as phagophore nucleation occurs
at membrane sites. However, no direct localization data exist for A0BFB4, and
'membrane' (GO:0016020) is a very broad term. In P. tetraurelia, protein
kinases localize to diverse membrane compartments including cilia, contractile
vacuoles, and plasma membrane. Without specific evidence, this broad term is
acceptable as a non-core annotation.
action: KEEP_AS_NON_CORE
reason: Membrane is a very broad cellular component term. The IBA inference from ATG1/ULK1 orthologs is plausible but nonspecific; no direct localization evidence exists for A0BFB4.
- term:
id: GO:0005737
label: cytoplasm
evidence_type: IBA
original_reference_id: GO_REF:0000033
qualifier: is_active_in
review:
summary: >-
Cytoplasmic localization is inferred by phylogenetic annotation from a broad
set of orthologs across yeast, fly, mouse, plant, and other organisms. ATG1/ULK1
family kinases and related kinases are predominantly cytoplasmic, and this is
a reasonable default localization for a soluble protein kinase. Cytoplasmic
localization is broadly conserved and applicable to a ciliate kinase.
action: ACCEPT
reason: Cytoplasm is a well-supported default localization for a soluble protein kinase. The IBA inference from diverse orthologs is robust, and cytoplasmic localization is expected for this class of kinases regardless of the specific biological role.
- term:
id: GO:0000045
label: autophagosome assembly
evidence_type: IBA
original_reference_id: GO_REF:0000033
qualifier: involved_in
review:
summary: >-
Autophagosome assembly is inferred by phylogenetic annotation from ATG1/ULK1
orthologs (ULK1/O75385, ATG1/S000003148 in yeast, Atg1/FBgn0260945 in fly,
Ulk1/MGI:1270126 in mouse). ATG1/ULK1 kinases are master regulators of
autophagy initiation and are directly involved in phagophore nucleation and
autophagosome formation. Autophagy is conserved across eukaryotes including
ciliates; Paramecium uses both macroautophagy and microautophagy pathways. The
PANTHER family PTHR44167 is annotated as LOK-related rather than ATG1/ULK1, so
the phylogenetic placement of A0BFB4 within the ATG1/ULK1 clade relies on the
PANTHER tree topology at the subfamily level. This annotation is plausible but
depends entirely on the accuracy of the phylogenetic inference for this highly
expanded ciliate kinome.
action: UNDECIDED
reason: >-
Autophagy is conserved in ciliates and the IBA orthologs are well-characterized
ATG1/ULK1 family members. However, the PANTHER family (PTHR44167) is annotated
as LOK-related kinase rather than ATG1/ULK1, raising uncertainty about whether
the phylogenetic placement is correct for this protein from a massively expanded
kinome (2606 kinases). Without gene-specific experimental evidence, the
confidence in this specific functional assignment is moderate.
- term:
id: GO:0010506
label: regulation of autophagy
evidence_type: IBA
original_reference_id: GO_REF:0000033
qualifier: involved_in
review:
summary: >-
Regulation of autophagy is inferred from ATG1/ULK1 orthologs. ULK1 is a key
regulatory kinase that integrates nutrient-sensing signals (via mTOR and AMPK)
to initiate autophagy. This annotation is largely redundant with autophagosome
assembly (GO:0000045) for an ATG1/ULK1-type kinase, as the kinase's role in
autophagosome assembly IS the regulatory step. The same uncertainty about
phylogenetic placement applies as for the autophagosome assembly annotation.
action: UNDECIDED
reason: >-
Same phylogenetic uncertainty as for autophagosome assembly. Additionally,
this term is partially redundant with GO:0000045 for an ATG1/ULK1-type kinase,
since the kinase initiates autophagy by directly assembling the autophagosome
machinery. Without gene-specific evidence, cannot confidently assign.
- term:
id: GO:0000407
label: phagophore assembly site
evidence_type: IBA
original_reference_id: GO_REF:0000033
qualifier: is_active_in
review:
summary: >-
The phagophore assembly site (PAS) is the subcellular location where
autophagosome formation initiates. ATG1/ULK1 kinases localize to the PAS to
nucleate phagophore formation. This is a specific subcellular localization
that is directly tied to the autophagy function. The annotation depends on the
same phylogenetic inference as the autophagy process annotations. Additionally,
the PAS concept was originally defined in yeast; whether ciliates organize
autophagy initiation at a morphologically equivalent structure is uncertain.
action: UNDECIDED
reason: >-
Depends on the same uncertain phylogenetic inference as the autophagy annotations.
The phagophore assembly site is a specific structure first characterized in yeast;
its precise equivalent in P. tetraurelia ciliates is not established.
- term:
id: GO:0005776
label: autophagosome
evidence_type: IBA
original_reference_id: GO_REF:0000033
qualifier: is_active_in
review:
summary: >-
Autophagosome localization is inferred from ATG1/ULK1 orthologs that transiently
associate with autophagosomes during their formation. This annotation is closely
tied to the autophagosome assembly function annotation and depends on the same
phylogenetic inference. Autophagosomes have been observed in Paramecium, so the
structure itself exists in this organism.
action: UNDECIDED
reason: >-
Same phylogenetic uncertainty as the other autophagy-related annotations. While
autophagosomes do exist in ciliates, the specific localization of A0BFB4 to
this compartment is unverified.
- term:
id: GO:0004674
label: protein serine/threonine kinase activity
evidence_type: IBA
original_reference_id: GO_REF:0000033
qualifier: enables
review:
summary: >-
Protein serine/threonine kinase activity is inferred by phylogenetic annotation
from a broad set of orthologs across yeast, fly, worm, mouse, human, and fission
yeast. This molecular function annotation is strongly supported by the domain
architecture: A0BFB4 contains a protein kinase domain (PF00069) with a
Ser/Thr kinase active site (IPR008271) and conserved ATP-binding site
(IPR017441). The Ser/Thr kinase active site signature specifically supports
serine/threonine rather than tyrosine kinase activity. This is the most
well-supported annotation for this protein.
action: ACCEPT
reason: >-
Strongly supported by both phylogenetic inference and domain architecture. The
protein has a canonical Ser/Thr kinase domain with conserved active-site and
ATP-binding residues. This is the core molecular function of the protein.
- term:
id: GO:0005829
label: cytosol
evidence_type: IBA
original_reference_id: GO_REF:0000033
qualifier: is_active_in
review:
summary: >-
Cytosol localization is inferred from ULK1 and ATG1 orthologs. Cytosol is a
more specific term than cytoplasm (GO:0005737), denoting the soluble fraction
excluding organelles. For a soluble kinase, cytosolic localization is reasonable
but no direct fractionation or imaging data exist for A0BFB4. This annotation
is largely redundant with the cytoplasm annotation.
action: KEEP_AS_NON_CORE
reason: >-
Cytosol is more specific than cytoplasm and is plausible for a soluble kinase,
but adds little information beyond the cytoplasm annotation. No direct
localization data exist for A0BFB4.
- term:
id: GO:0000166
label: nucleotide binding
evidence_type: IEA
original_reference_id: GO_REF:0000104
qualifier: enables
review:
summary: >-
Nucleotide binding is inferred electronically from the UniRule annotation
pipeline. While technically correct (kinases bind ATP, which is a nucleotide),
this is a very high-level parent term that is subsumed by the more specific
ATP binding (GO:0005524) annotation. This adds no information beyond what is
already captured by the ATP binding and kinase activity annotations.
action: MARK_AS_OVER_ANNOTATED
reason: >-
Nucleotide binding is a parent of ATP binding (GO:0005524), which is already
annotated. This redundant high-level term adds no informational value.
- term:
id: GO:0004672
label: protein kinase activity
evidence_type: IEA
original_reference_id: GO_REF:0000002
qualifier: enables
review:
summary: >-
Protein kinase activity is inferred from the InterPro Ser/Thr kinase active
site signature (IPR008271). This is a parent term of protein serine/threonine
kinase activity (GO:0004674), which is already annotated via both IBA and IEA.
The more specific child term is preferred.
action: MARK_AS_OVER_ANNOTATED
reason: >-
This is a parent term of GO:0004674 (protein serine/threonine kinase activity),
which is already annotated with both IBA and IEA evidence. The more specific
term is preferred; this redundant parent annotation adds no value.
- term:
id: GO:0004674
label: protein serine/threonine kinase activity
evidence_type: IEA
original_reference_id: GO_REF:0000120
qualifier: enables
review:
summary: >-
This is a second annotation of protein serine/threonine kinase activity, from
the combined IEA pipeline (GO_REF:0000120) based on EC:2.7.11.1 and UniRule.
This duplicates the IBA annotation of the same term (GO:0004674) with
electronic rather than phylogenetic evidence, providing independent
corroboration from domain architecture. Both the IBA phylogenetic inference and
this IEA domain-based inference converge on the same core molecular function.
action: ACCEPT
reason: >-
Protein serine/threonine kinase activity is strongly supported by both
phylogenetic inference (IBA) and domain architecture (IEA). This IEA annotation
provides independent corroboration of the core molecular function.
- term:
id: GO:0005524
label: ATP binding
evidence_type: IEA
original_reference_id: GO_REF:0000120
qualifier: enables
review:
summary: >-
ATP binding is inferred from the protein kinase domain (IPR000719), the
ATP-binding site signature (IPR017441), and UniRule. All protein kinases bind
ATP as a phosphate donor. A0BFB4 has a conserved ATP-binding site (residue 125
annotated as ATP-binding in UniProt). This is a well-supported, core molecular
function annotation.
action: ACCEPT
reason: >-
ATP binding is a fundamental requirement for kinase catalytic activity, strongly
supported by the conserved ATP-binding site and domain architecture. This is a
core molecular function annotation.
- term:
id: GO:0016301
label: kinase activity
evidence_type: IEA
original_reference_id: GO_REF:0000104
qualifier: enables
review:
summary: >-
Kinase activity is a high-level parent of protein kinase activity (GO:0004672),
which is itself a parent of protein serine/threonine kinase activity
(GO:0004674). This annotation is redundant with both the more specific protein
kinase activity and protein serine/threonine kinase activity annotations.
action: MARK_AS_OVER_ANNOTATED
reason: >-
Kinase activity (GO:0016301) is an ancestor of GO:0004674 (protein
serine/threonine kinase activity), which is already annotated. Redundant
high-level term with no additional informational value.
- term:
id: GO:0016740
label: transferase activity
evidence_type: IEA
original_reference_id: GO_REF:0000104
qualifier: enables
review:
summary: >-
Transferase activity is a very high-level molecular function term. All kinases
are transferases (they transfer phosphate groups). This annotation is fully
subsumed by the more specific kinase activity, protein kinase activity, and
protein serine/threonine kinase activity annotations.
action: MARK_AS_OVER_ANNOTATED
reason: >-
Transferase activity (GO:0016740) is a distant ancestor of GO:0004674 (protein
serine/threonine kinase activity). This is the most generic of the redundant
hierarchy of MF terms annotated to this protein. Adds no informational value.
core_functions:
- description: >-
Protein serine/threonine kinase that catalyzes ATP-dependent phosphorylation of
serine and threonine residues on protein substrates. The specific substrates and
biological pathway context are unknown. Phylogenetic inference suggests possible
involvement in autophagy regulation as an ATG1/ULK1-related kinase, but this has
not been experimentally verified.
molecular_function:
id: GO:0004674
label: protein serine/threonine kinase activity
locations:
- id: GO:0005737
label: cytoplasm
references:
- id: GO_REF:0000002
title: Gene Ontology annotation through association of InterPro records with GO terms
findings: []
- id: GO_REF:0000033
title: Annotation inferences using phylogenetic trees
findings: []
- id: GO_REF:0000104
title: Electronic Gene Ontology annotations created by transferring manual GO annotations
between related proteins based on shared sequence features
findings: []
- id: GO_REF:0000120
title: Combined Automated Annotation using Multiple IEA Methods
findings: []