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The cts1 gene of Schizosaccharomyces pombe encodes a CTP synthase (CTPS) enzyme (string-db.org). CTP synthase catalyzes the ATP-dependent amination of UTP to form CTP, using L-glutamine as the nitrogen donor (string-db.org) (pmc.ncbi.nlm.nih.gov). This reaction represents the final and rate-limiting step in de novo CTP biosynthesis, producing cytidine 5′-triphosphate (CTP) and L-glutamate (pmc.ncbi.nlm.nih.gov). The enzyme has a bifunctional mechanism: an N-terminal glutamine amidotransferase (GAT) domain hydrolyzes glutamine, and the resulting ammonia is channeled through an intramolecular tunnel to the C-terminal synthetase domain, where it is incorporated into UTP in an ATP-dependent condensation (pmc.ncbi.nlm.nih.gov). Notably, Cts1 can also utilize ammonia directly (in lieu of glutamine) as a substrate for UTP amination (string-db.org).
Regulation of Cts1 activity is crucial for nucleotide homeostasis. GTP acts as an allosteric activator of CTP synthase, binding to the GAT domain to stimulate efficient glutamine hydrolysis (pmc.ncbi.nlm.nih.gov). Conversely, CTP synthases are subject to feedback inhibition by their product CTP, preventing excessive accumulation of CTP. Proper control of CTP levels is vital – an inability to regulate CTP pools is associated with cellular dysfunction and malignancies (pmc.ncbi.nlm.nih.gov). Thus, Cts1 plays a key role in maintaining nucleotide balance, coupling glutamine metabolism to pyrimidine nucleotide synthesis. The enzyme is typically active as a homotetramer, and this oligomeric state is required for its catalytic function (pmc.ncbi.nlm.nih.gov). Overall, cts1’s molecular function is defined by CTP synthase activity (GO:0003883), driving de novo CTP production that fuels myriad cellular processes.
Cts1 is predominantly a cytosolic enzyme, consistent with its role in nucleotide biosynthesis in the cytoplasm. However, under certain conditions Cts1 exhibits a remarkable ability to assemble into filamentous subcellular structures called cytoophidia (“cellular snakes”). Fluorescence-tagging experiments have shown that endogenously tagged Cts1 (Ctp1–YFP) forms filamentous cytoophidia in S. pombe (pmc.ncbi.nlm.nih.gov). Each fission yeast cell generally contains two Cts1 filaments: a long, thick cytoophidium in the cytoplasm and a shorter, thinner filament associated with the nucleus (pmc.ncbi.nlm.nih.gov). The nuclear-associated filament (sometimes termed an “N-cytoophidium”) resides at the nuclear periphery or within the nucleus, while the other filament (C-cytoophidium) is in the cytosol (pmc.ncbi.nlm.nih.gov). These observations indicate that a fraction of Cts1 localizes to the nucleus or nuclear envelope region in addition to the cytosol. In microscopy images, the cytoplasmic filament often lies adjacent to the outside of the nucleus, whereas the nuclear filament is just inside the nuclear envelope (pmc.ncbi.nlm.nih.gov). This unique distribution suggests Cts1 may dynamically partition between the cytoplasm and nucleus, forming compartment-specific enzymatic filaments.
The cytoophidium structures are dynamic and cell-cycle regulated. Time-lapse imaging reveals that upon cell division, Cts1 filaments are asymmetrically inherited – typically only one of the two daughter cells inherits the cytoophidium (particularly the cytoplasmic filament), while the other daughter often does not (pmc.ncbi.nlm.nih.gov). This suggests Cts1 assemblies can disassemble and reassemble each cell cycle, or redistribute unevenly between daughters. The physiological significance of this asymmetric inheritance is still under investigation, but it offers a unique example of a metabolic enzyme showing structured segregation during division (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). It is clear that Cts1’s location is not uniform: it can exist as a diffuse cytosolic pool or in highly organized filamentous compartments, reflecting a layer of spatial regulation on its activity. In gene ontology terms, Cts1 is localized to the cytosol (GO:0005829) and has also been observed in the nucleus (GO:0005634) in the form of nuclear filaments.
As the sole CTP synthase in fission yeast, Cts1 is essential for pyrimidine nucleotide biosynthesis. It enables the de novo CTP biosynthetic process (GO:0006241) by producing CTP from UTP (pmc.ncbi.nlm.nih.gov). This biochemical function situates Cts1 at the heart of several broader biological processes. CTP is a critical building block for RNA and DNA synthesis; thus Cts1 activity is indirectly required for DNA replication and transcription by supplying one of the four ribonucleotides needed for RNA (and ultimately DNA via dCTP) (pmc.ncbi.nlm.nih.gov). Cells unable to synthesize CTP will deplete their nucleotide pools and arrest in proliferation. Indeed, CTP synthase is considered an “essential” enzyme for cell viability (pmc.ncbi.nlm.nih.gov). In S. pombe, deletion of cts1+ is lethal (no viable knockout can be recovered), indicating that Cts1 is required for cell survival. Consistent with this, Cts1 is sometimes referred to as an essential metabolic enzyme in fission yeast (pmc.ncbi.nlm.nih.gov). When Cts1 function is lost or chemically inhibited, cells cannot sustain DNA/RNA production and will exhibit halted cell cycle progression and loss of viability.
Beyond nucleic acid synthesis, CTP is also required for various metabolic pathways, such as phospholipid biosynthesis. CTP serves as a donor of cytidylyl groups in the synthesis of phosphatidylcholine, CDP-diacylglycerol, and other membrane phospholipids. Thus, Cts1 activity contributes to membrane biogenesis and overall lipid metabolism. For example, cardiolipin and phosphatidylcholine pathways rely on CTP, linking Cts1 to the general process of membrane formation. In summary, the biological role of Cts1 can be encapsulated by its involvement in nucleotide metabolic processes (providing CTP for nucleic acid synthesis) and by extension in processes like DNA replication, RNA transcription, and membrane lipid production that depend on adequate CTP supply (pmc.ncbi.nlm.nih.gov) (pubmed.ncbi.nlm.nih.gov). Given these central roles, it is not surprising that S. pombe cells strictly require Cts1 for growth and proliferation.
Cts1 (CTP synthase) is a ~600 amino acid protein comprised of two major domains with distinct functions. The N-terminal domain (~the first 140 residues) is a glutamine amidotransferase (GATase) domain, which contains the active site cysteine responsible for glutamine hydrolysis (pmc.ncbi.nlm.nih.gov). This domain belongs to the class I amidotransferase family and provides the glutaminase activity: it binds L-glutamine and catalyzes the removal of the amide nitrogen, generating glutamate and ammonia. Key conserved motifs in this domain (including a catalytic Cys-His-Glu triad) facilitate glutamine binding and cleavage, a feature shared with other glutamine-dependent enzymes (pmc.ncbi.nlm.nih.gov).
The C-terminal domain constitutes the synthetase domain, which binds the substrate UTP and co-substrate ATP, and carries out the actual UTP aminase (ligase) reaction to produce CTP (pmc.ncbi.nlm.nih.gov). This domain contains the pockets for UTP and ATP, as well as sites for allosteric regulators. The ammonia released in the GAT domain is funneled through an internal channel to the synthetase active site, where it reacts with the UTP, in a mechanism coordinated with ATP hydrolysis (pmc.ncbi.nlm.nih.gov). Structural studies (e.g. cryo-EM of Drosophila CTPS) show that CTPS undergoes conformational changes upon ligand binding—particularly, binding of GTP at an allosteric site on the GAT domain induces a catalytically active conformation that couples the two active sites (pmc.ncbi.nlm.nih.gov). The enzyme’s architecture thus includes a regulatory allosteric site (for GTP) and likely a product inhibition site (for CTP) that modulate its activity.
Functionally, Cts1 operates as a homotetramer. Four identical Cts1 subunits assemble into a ring-shaped tetramer, which is the active form needed for catalysis (pmc.ncbi.nlm.nih.gov). These tetramers can further polymerize end-to-end into long filaments (cytoophidia) in vivo. No additional protein components are required for cytoophidium formation – it is a polymer of Cts1 itself. Each monomer contributes to extensive inter-subunit interfaces; for example, the tetramerization involves interactions between the synthetase domains of neighboring subunits. Filament assembly likely involves a stacking of tetramers in a helical or linear manner. The filamentous form does not represent a distinct domain but is a higher-order structural state. It has been proposed that filament formation can sequester Cts1 in inactive or partially active form, serving as a regulatory mechanism (though in some organisms filaments may retain activity) (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). In summary, Cts1’s structural features include two catalytic domains (GATase and synthase) and the inherent ability to oligomerize into enzymatically active tetramers and further into filamentous assemblies.
Because cts1 is a fission yeast gene, it is not directly implicated in human disease. Nevertheless, its function as CTP synthase has clear relevance to human health via its orthologs. Human CTPS1 (the functional human counterpart of yeast Cts1) is crucial for immune cell proliferation. Loss-of-function mutations in human CTPS1 cause a severe immunodeficiency syndrome, due to an inability of activated T-lymphocytes and B-lymphocytes to proliferate (pubmed.ncbi.nlm.nih.gov). Patients with CTPS1 deficiency have life-threatening immunological defects: their T/B cells cannot expand in response to antigen because they cannot sufficiently synthesize CTP for DNA/RNA, leading to defective clonal expansion (pubmed.ncbi.nlm.nih.gov). This underscores the central role of CTP synthase in supporting cell division. The immunodeficiency phenotype can be rescued in vitro by supplementing nucleosides (cytidine) or by reintroducing wild-type CTPS1, confirming that the proliferative defect is specifically due to loss of CTP synthesis capacity (pubmed.ncbi.nlm.nih.gov). Thus, CTPS1 is absolutely required for rapidly dividing cells (like immune blasts), paralleling the essential requirement for Cts1 in dividing yeast cells.
Beyond rare genetic deficiencies, CTP synthase is also relevant in the context of cancer and antimicrobial therapy. CTPS1 is overexpressed in many cancers, as tumor cells have high demand for nucleotide synthesis (pmc.ncbi.nlm.nih.gov). Dysregulated nucleotide pools can contribute to genomic instability and uncontrolled growth; indeed, CTPS1 is one of the most upregulated metabolic enzymes in certain malignancies (pmc.ncbi.nlm.nih.gov). For this reason, CTPS is being explored as a target for anti-cancer drugs (pmc.ncbi.nlm.nih.gov). Several inhibitors of CTP synthase (such as 3-deazauridine, cyclopentenyl cytosine, and DON) have shown anti-proliferative effects. In a yeast context, inhibition of Cts1 mimics a “starvation” for CTP and triggers cell cycle arrest. For example, drugs that inhibit CTP synthase or mutations that lower its activity cause S. pombe cells to stop dividing and often enlarge (a typical response to cell cycle arrest in fission yeast). Phenotypically, a cts1 temperature-sensitive mutant or partial loss-of-function might display slow growth, cell elongation (due to G2 arrest from nucleotide depletion), or sensitivity to DNA-damaging agents (because of impaired dCTP supply for DNA repair). Furthermore, cts1 was identified in a screen for calcineurin-related functions in a distant fungus (Cryptococcus neoformans, though there CTS1 refers to a different gene) – this highlights that naming overlaps exist but the S. pombe cts1 specifically encodes CTP synthase, not directly tied to calcineurin in yeast. In summary, while cts1 per se is a yeast gene, its homologs are involved in critical disease-related pathways: immune cell proliferation and cancer cell metabolism. This conservation of function makes Cts1 a potential antifungal target as well – an inhibitor that selectively targets fungal CTP synthase would be lethal to yeast cells while potentially sparing the human enzyme if designed correctly.
Under normal nutrient-rich conditions, cts1 is expressed in vegetatively growing S. pombe cells at levels sufficient to meet metabolic needs. It is generally considered a house-keeping gene, since a constant supply of CTP is required for ongoing cellular processes. Consistent with this, cts1+ mRNA and protein are present throughout the cell cycle and across different growth conditions. In one study, disruption of the TOR (Target of Rapamycin) signaling pathway in S. pombe did not significantly alter cts1 transcript or protein levels, suggesting that nutrient signaling does not acutely regulate cts1 expression (pmc.ncbi.nlm.nih.gov). Specifically, knockout of TORC1/TORC2 subunits shortened Cts1 filaments but the total Cts1–YFP protein level remained relatively unchanged under TOR-inhibited conditions (pmc.ncbi.nlm.nih.gov). This indicates that cts1 expression is relatively stable and not strongly down-regulated by TOR, even though TOR affects the enzyme’s assembly state (filament length).
However, Cts1 activity and assembly state do respond to growth conditions. During exponential log-phase growth (nutrient-rich, actively dividing cells), Cts1 is highly active and nearly all cells display cytoophidia, implying abundant enzyme and/or high flux through the pathway (www.mdpi.com). By contrast, in stationary phase or nutrient-depleted conditions, S. pombe cells disassemble Cts1 filaments – in stationary-phase cultures, the previously prevalent cytoophidia disappear from fission yeast cells (www.mdpi.com). This disappearance correlates with a reduced demand for CTP when cells are quiescent. It is likely that cts1 expression or Cts1 enzyme activity is down-modulated as cells enter stationary phase or starve, though the filaments’ absence could also result from product feedback (high CTP levels in non-dividing cells may inhibit filament formation, causing Cts1 to remain diffuse). Thus, while cts1 mRNA/protein levels don’t dramatically fluctuate in reported experiments, the functional state of Cts1 is regulated: active growth promotes Cts1 polymerization (and presumably high enzymatic throughput), whereas nutrient limitation or growth arrest leads to Cts1 depolymerization and possibly reduced activity.
Regulation of cts1 can also be considered in the context of the cell cycle and developmental cues. Entry into S-phase (DNA synthesis) likely requires upregulation of nucleotide biosynthesis genes. Although specific cell-cycle regulation of cts1 in fission yeast has not been heavily reported, one can infer parallels from other systems. In human T-cells, CTPS1 expression is low in resting (G0) cells and is rapidly up-regulated upon mitogenic stimulation (when cells enter the cell cycle) (pubmed.ncbi.nlm.nih.gov). Likewise, S. pombe likely increases nucleotide biosynthetic capacity when cells commit to division or when apropriate growth signals are present. There may be transcriptional regulators ensuring cts1 expression meets demand (for example, in budding yeast, pyrimidine biosynthesis genes are co-regulated by Pyr1/Ppr1, although fission yeast uses different regulatory networks). Overall, cts1 exhibits a constitutive expression pattern with adjustments tied to growth state: it is highly active during rapid growth and dialed back during quiescence. Post-translational modifications might also regulate Cts1 (in other species, protein kinase A phosphorylation of CTPS has been observed), but such regulation in S. pombe is not yet well characterized.
CTP synthase is an ancient and highly conserved enzyme, reflecting its fundamental role in biology. The cts1 gene of fission yeast has clear orthologs in virtually all organisms, from bacteria to humans. At the sequence level, Cts1 shares significant homology with CTP synthases in other species. For instance, S. pombe Cts1 is homologous to E. coli PyrG (CTP synthase) and to the budding yeast enzymes Ura7 and Ura8. (In fact, budding yeast has two CTP synthase isoforms, Ura7 and Ura8, due to a genome duplication, whereas S. pombe and most other eukaryotes have a single cts1+ gene) (www.mdpi.com). Despite the duplication, the yeast enzymes perform the same function and even form similar filaments. Key catalytic residues and domain architectures are strictly conserved. For example, the glutamine-binding site cysteine and the ATP/UTP-binding motifs in the synthetase domain are present in all species’ CTPS enzymes. This conservation underscores that the mechanism of CTP biosynthesis and its regulation by GTP/CTP is under strong purifying selection – any major deviation would be detrimental to nucleotide balance.
The phenomenon of CTP synthase filamentation (cytoophidia) is also evolutionarily conserved. Researchers have observed CTPS polymers in bacteria, yeast, flies, and human cells (pmc.ncbi.nlm.nih.gov). The first discoveries of cytoophidia were made almost simultaneously in bacteria, Drosophila, and budding yeast, and subsequently in mammalian cells (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). S. pombe was also shown to form Cts1 filaments, reinforcing that this ability to compartmentalize into filaments is a conserved property of CTPS (pmc.ncbi.nlm.nih.gov). This suggests an important biological function for filamentation has been preserved (possibly related to enzyme regulation or cellular organization of metabolism). Additionally, the requirement of GTP for glutamine-dependent activity is conserved from E. coli to eukaryotes, indicating the allosteric regulation mechanisms appeared early in evolution (pmc.ncbi.nlm.nih.gov). Human CTP synthases (CTPS1 and CTPS2) are about ~60% identical in sequence to yeast Cts1 and can functionally complement yeast mutants, highlighting deep conservation of function. In summary, cts1 and its encoded enzyme exemplify evolutionary conservation at multiple levels: sequence, structure, mechanism, and even higher-order assembly are all maintained across the tree of life. This makes CTP synthase a useful model for studying enzyme regulation and polymerization in a broad biological context.
Based on the characterized functions and properties of cts1, the following Gene Ontology terms are applicable (supported by experimental evidence from the literature):
Each of these GO annotations for cts1 can be supported by experimental findings. For instance, the CTP synthase activity is supported by biochemical characterization of the enzyme’s reaction (pmc.ncbi.nlm.nih.gov). The CTP biosynthetic process involvement is evidenced by the essential requirement of cts1 for nucleotide synthesis and cell replication (pmc.ncbi.nlm.nih.gov). Localization to cytosol and nucleus is confirmed by fluorescence microscopy of Cts1–YFP in vivo (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). These annotations together encapsulate the gene’s role in metabolism (MF/BP) and its intracellular localization (CC), providing a concise summary of cts1’s function for Gene Ontology curation.
References: The above information is drawn from a variety of studies and databases. Key references include biochemical analyses of CTP synthase’s activity and mechanism (pmc.ncbi.nlm.nih.gov) (string-db.org), cell biology studies demonstrating Cts1’s filament formation and localization (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov), and medical research highlighting the importance of CTPS in cell proliferation and disease (pubmed.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). These sources provide strong evidence for annotating cts1 with the GO terms listed, ensuring that each annotation is backed by experimental observations (IDA: Inferred from Direct Assay, and ISS: Inferred from Sequence Similarity where appropriate). This comprehensive understanding of cts1 will facilitate its accurate curation in the Gene Ontology database.