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UniProt ID: P40054
Directory alias: SER3
Enzymatic Activity: The SER3 gene of Saccharomyces cerevisiae encodes D-3-phosphoglycerate dehydrogenase 1, an enzyme that catalyzes the first step of the phosphorylated L-serine biosynthesis pathway (www.ncbi.nlm.nih.gov). Specifically, Ser3p oxidizes 3-phospho-D-glycerate (a glycolytic intermediate) to 3-phosphohydroxypyruvate (also called 3-phosphonooxypyruvate), with the concurrent reduction of NAD^+ to NADH (pmc.ncbi.nlm.nih.gov). This phosphoglycerate dehydrogenase activity (GO:0004617) initiates de novo L-serine production from carbohydrate metabolites (pmc.ncbi.nlm.nih.gov). In addition to its primary activity, Ser3p also exhibits alpha-ketoglutarate reductase activity (GO:0061759) – it can reversibly reduce α-ketoglutarate to D-2-hydroxyglutarate (and oxidize D-2-hydroxyglutarate back to α-ketoglutarate) (www.ncbi.nlm.nih.gov) (www.yeastgenome.org). The dual specificity means Ser3p links central carbon metabolism with amino acid synthesis, potentially influencing redox balance. Indeed, deletion of both SER3 and its paralog SER33 disrupts NAD^+/NADH-dependent metabolism, reflecting the enzyme’s important role in cellular redox homeostasis (pubmed.ncbi.nlm.nih.gov). Notably, like other D-3-phosphoglycerate dehydrogenases, Ser3p requires NAD^+ as a cofactor and produces NADH, tying serine biosynthesis to the cell’s redox state (pubmed.ncbi.nlm.nih.gov).
Molecular Mechanism and Regulation: In many organisms, 3-phosphoglycerate dehydrogenases are subject to feedback inhibition by L-serine, although regulatory mechanisms can differ by species (pmc.ncbi.nlm.nih.gov). Yeast Ser3p is part of this conserved enzyme family; enzymatically, it likely follows a similar ordered binding mechanism where 3-phosphoglycerate and NAD^+ must bind for catalysis to proceed (pmc.ncbi.nlm.nih.gov). However, evidence suggests yeast mainly regulates serine synthesis at the gene-expression level (see Expression Patterns and Regulation below). Ser3p is known to be post-translationally phosphorylated on multiple serine residues near its N-terminus by the TORC1 kinase signaling pathway (pmc.ncbi.nlm.nih.gov). This phosphorylation – which occurs in a Pib2-dependent manner (Pib2 being a TORC1-associated regulator) – is rapidly responsive to nutrient conditions (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Both Ser3p and Ser33p are direct targets of TORC1: under nutrient-rich conditions, TORC1 phosphorylates Ser3/Ser33, whereas starvation or rapamycin treatment leads to their dephosphorylation (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Pib2 physically interacts with Ser3/Ser33, recruiting them to TORC1 for modification (pmc.ncbi.nlm.nih.gov). While the functional consequence of Ser3p phosphorylation is still being explored, a phospho-mimetic vs. phospho-null Ser33 was shown to affect growth on serine/glycine-deficient media (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov), suggesting that TORC1-mediated phosphorylation modulates enzyme activity or stability as part of nutrient sensing. In summary, Ser3p catalyzes a key redox reaction in serine biosynthesis and is integrated into cellular signaling networks that fine-tune metabolic flux.
Ser3p is a cytosolic enzyme, consistent with its role in central metabolism. High-throughput localization studies and GO annotations indicate Ser3p is found in the cytoplasm (GO:0005737) (www.ncbi.nlm.nih.gov). No signal peptides or organelle-targeting sequences are present, and the protein lacks membrane-spanning domains, supporting a soluble cytosolic localization. This is in line with other enzymes of amino acid biosynthesis, which typically operate in the cytosol of yeast cells. Furthermore, the enzymatic substrates (3-phosphoglycerate from glycolysis) and products (3-phosphohydroxypyruvate, serine) are cytosolic metabolites, so a cytosolic locale is functionally logical. It has not been reported to localize to mitochondria, peroxisomes, or other organelles; rather, Ser3p likely diffuses in the cytosol or may associate with cytosolic metabolic complexes. Some metabolic enzymes in yeast form multi-enzyme complexes or reside in metabolons, but specific interaction partners of Ser3p in vivo are not well characterized. Current evidence thus supports that Ser3p primarily resides in the cytosol, where it can readily access glycolytic intermediates and contribute to serine and glycine biosynthesis (www.ncbi.nlm.nih.gov). This localization is also supported by the lack of any known organelle-targeting motifs in the SER3 sequence and by global proteomic studies of yeast protein location. In summary, Ser3p functions in the cytoplasm, aligning with the cellular compartment where serine biosynthetic processes occur.
L-Serine and Glycine Biosynthesis: Ser3p plays an essential role in the L-serine biosynthetic process (GO:0006564) (www.ncbi.nlm.nih.gov). It catalyzes the committed first step of the phosphorylated pathway of serine synthesis, converting 3-phosphoglycerate (3PG) into 3-phosphohydroxypyruvate. This pathway consists of three enzymes: Ser3p/Ser33p (3-phosphoglycerate dehydrogenase), Ser1p (phosphoserine aminotransferase), and Ser2p (phosphoserine phosphatase) (pmc.ncbi.nlm.nih.gov). Together, they produce L-serine from the glycolytic intermediate 3PG. Because serine is a precursor to glycine (via serine hydroxymethyltransferase) and other metabolites (such as cysteine and one-carbon tetrahydrofolate units), SER3 also indirectly supports glycine biosynthesis and one-carbon metabolism. Indeed, the serine/glycine biosynthetic pathways in yeast are interconnected (“serine-glycine superpathway”), and a block in SER3 can lead to auxotrophy for both serine and glycine (pubmed.ncbi.nlm.nih.gov). Experimental genetics confirmed this linkage: yeast cells lacking both SER3 and SER33 cannot synthesize serine or glycine de novo and thus require exogenous serine or glycine for growth (pubmed.ncbi.nlm.nih.gov). Even a single functional 3-phosphoglycerate dehydrogenase is typically sufficient for viability on rich or minimal glucose media, but complete loss of the first step disrupts the entire serine pathway, underscoring Ser3p’s central role.
Metabolic Flexibility: The requirement for the Ser3p-mediated pathway is conditional on carbon source and nutrient availability (pubmed.ncbi.nlm.nih.gov). In fermentative growth on glucose, the phosphoglycerate pathway is the dominant route for serine and glycine synthesis (the alternative glyoxylate-dependent pathway is repressed by high glucose) (pubmed.ncbi.nlm.nih.gov). Under these conditions, Ser3p/Ser33p activity is critical. In contrast, during growth on non-fermentable carbon sources (e.g. ethanol), yeast can utilize a glyoxylate bypass for glycine/serine biosynthesis, and expression of SER3 (and other SER genes) is greatly reduced (pubmed.ncbi.nlm.nih.gov). This indicates Ser3p is involved in metabolic adaptation: it is highly needed in glycolytic (high glucose) conditions but less so when cells rely on gluconeogenic substrates. Additionally, the enzyme’s secondary activity in converting α-ketoglutarate to D-2-hydroxyglutarate ties SER3 to 2-hydroxyglutarate metabolic process (a normally minor metabolic side-pathway). While yeast does not typically accumulate 2-hydroxyglutarate, under certain stress or metabolic perturbations, Ser3p could help detoxify excess D-2-hydroxyglutarate by oxidizing it back to α-ketoglutarate (www.ncbi.nlm.nih.gov). This reaction links amino acid metabolism with the TCA cycle, suggesting a role in maintaining metabolic balance when unusual metabolites accumulate.
Additional Processes: By contributing to serine production, Ser3p influences downstream processes that depend on serine and glycine. These include protein synthesis (serine incorporation into proteins), nucleotide and phospholipid biosynthesis (serine is a precursor for purines, pyrimidines, and phosphatidylserine), and one-carbon unit generation (serine hydroxymethyltransferase converts serine to glycine and one-carbon units for folate cycle). Therefore, SER3 is indirectly involved in one-carbon metabolism and nucleotide biosynthetic processes, although these are not direct GO annotations for Ser3p but rather consequences of serine availability. In summary, SER3 is chiefly involved in amino acid biosynthesis – specifically the production of serine (and by extension glycine) – and thereby underpins multiple biosynthetic and metabolic processes within the cell (pmc.ncbi.nlm.nih.gov) (pubmed.ncbi.nlm.nih.gov).
Yeast Phenotypes: In S. cerevisiae, loss of SER3 alone causes relatively mild phenotypes under typical lab conditions due to the presence of the paralog SER33. However, SER3 was originally identified through mutants requiring serine for growth (“Serine requiring” mutants) (www.yeastgenome.org) (www.yeastgenome.org). A double knockout of SER3 and SER33 renders yeast auxotrophic for serine (and glycine), leading to inability to grow on media lacking these amino acids (pubmed.ncbi.nlm.nih.gov). This auxotrophy is the primary phenotype associated with SER3 dysfunction – cells cannot perform de novo serine biosynthesis and thus display growth defects unless serine or glycine is supplemented. Additionally, because the Ser3p reaction helps regenerate NADH, ser3Δ ser33Δ mutants show perturbations in redox metabolism, evidenced by abnormal balances of pathway substrates and products (pubmed.ncbi.nlm.nih.gov). Such redox imbalance may manifest in altered growth rates or sensitivity to stress conditions that challenge NAD^+/NADH homeostasis. No lethal phenotypes are associated with SER3 deletion alone, but fitness may decrease in high-serine-demand scenarios (e.g. rapid proliferation) or in certain nutrient environments.
Human Disease Associations: While yeast itself does not suffer “diseases,” the human ortholog of SER3 provides insight into the enzyme’s importance in health. The human enzyme phosphoglycerate dehydrogenase (PHGDH) performs the same first step of serine biosynthesis, and mutations in PHGDH are known to cause Neu-Laxova syndrome 1 and other serine biosynthesis disorders (www.ncbi.nlm.nih.gov). Neu-Laxova syndrome is a rare congenital metabolic disorder characterized by severe developmental defects (including microcephaly, skin and limb abnormalities) and is often fatal; it results from loss-of-function mutations in the serine biosynthetic pathway, most frequently in PHGDH (www.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). More moderate PHGDH deficiencies can lead to PHGDH-related amino acid metabolic disorder, where patients suffer from low serine/glycine levels, neurological symptoms (seizures, psychomotor retardation), and must be treated with serine supplementation (pmc.ncbi.nlm.nih.gov). These human conditions underscore the fundamental role of this enzymatic activity in organisms – a role conserved in yeast SER3. There is also interest in PHGDH overexpression in cancer: certain cancers upregulate serine biosynthesis to support rapid cell growth, and PHGDH has been identified as a contributor to tumor cell proliferation (pmc.ncbi.nlm.nih.gov). While yeast SER3 is not directly linked to human disease, it is used as a model to study basic aspects of serine metabolism that are relevant to these conditions. For instance, yeast lacking serine biosynthesis can mimic aspects of serine-starvation stress, and yeast Ser3p has been studied to understand enzymatic regulation that might inform cancer metabolism. In summary, mutations in the human counterpart PHGDH cause serious metabolic diseases (Neu-Laxova syndrome, serine deficiency disorders) (www.ncbi.nlm.nih.gov), highlighting by homology that SER3 performs a crucial, conserved metabolic function.
Ser3p is a 469 amino acid protein with a multi-domain architecture typical of D-3-phosphoglycerate dehydrogenases. It contains an N-terminal region that binds NAD^+ (a Rossmann-fold domain for nucleotide binding) and a central catalytic domain that binds the substrate 3-phosphoglycerate (pmc.ncbi.nlm.nih.gov). In addition, the C-terminal portion of Ser3p likely forms a regulatory domain. Many enzymes in this family have a small regulatory domain (often an ACT domain) that binds serine or other effectors to modulate activity (pmc.ncbi.nlm.nih.gov). D-3-phosphoglycerate dehydrogenases across species are classified into types based on domain composition: Type II enzymes (like yeast Ser3p) have the ACT regulatory domain appended, whereas Type I have an additional oligomerization domain (ASB domain) and Type III lack any regulatory domain (pmc.ncbi.nlm.nih.gov). The yeast enzyme is predicted to be feedback-inhibited by serine binding to its ACT domain, although in vivo this regulation may be overshadowed by transcriptional control. Structurally, homologous enzymes (e.g. E. coli and human PHGDH) form homotetramers or homodimers, with each subunit contributing an active site and (for those with ACT domains) allosteric sites that sense serine levels (pmc.ncbi.nlm.nih.gov). By analogy, Ser3p likely functions as a homomultimer (the exact oligomeric state in yeast has not been experimentally confirmed, but bacterial and human PGDHs form tetramers). Key conserved residues in the active site include a catalytic histidine and arginine that interact with the substrate’s phosphate and hydroxyl groups (pmc.ncbi.nlm.nih.gov), and these residues are conserved in yeast Ser3p, suggesting a similar catalytic mechanism as other PGDHs.
Ser3p also has sites of post-translational modification. Several serine residues near the N-terminus serve as phosphorylation sites for TORC1 signaling (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). These phosphosites (e.g., Ser20, Ser22, Ser28, Ser29 in Ser33p; Ser3 may have analogous positions) do not disrupt the enzyme’s catalytic domains but may create docking sites or conformational changes affecting enzyme function or protein-protein interactions (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). No enzymatic active-site phosphorylation has been noted, implying regulation by phosphorylation is likely modulatory rather than directly inhibitory to substrate binding. Beyond phosphorylation, no other covalent modifications (such as acetylation or ubiquitination) are well documented for Ser3p, but large-scale proteomic surveys could reveal additional sites.
In summary, Ser3p’s structural layout consists of: an N-terminal cofactor-binding domain, a central catalytic domain, and a C-terminal regulatory domain. This architecture is consistent with a role for allosteric regulation (binding of effector molecules like serine) and oligomerization. The presence of conserved domains and motifs underscores the evolutionary conservation of mechanism – yeast Ser3p shares the structural features necessary for its dehydrogenase activity and likely for feedback control, akin to its bacterial and mammalian counterparts (pmc.ncbi.nlm.nih.gov).
Expression of SER3 is tightly regulated, reflecting the cell’s need to produce serine only when required. Transcriptional Regulation: SER3 is subject to a unique form of transcriptional repression via a cryptic upstream non-coding RNA. Under serine-replete conditions, a noncoding RNA called SRG1 (Serine Regulatory Gene 1) is transcribed from the intergenic region upstream of SER3. Transcription of SRG1 across the SER3 promoter interferes with SER3 transcription, effectively silencing SER3 when exogenous serine is abundant (pmc.ncbi.nlm.nih.gov). This mechanism, known as transcriptional interference, was discovered as one of the first examples of a regulatory long noncoding RNA in yeast. The expression of SRG1 (and hence repression of SER3) is itself controlled by serine levels: when cells have plenty of serine, SRG1 is highly expressed; when serine is scarce, SRG1 transcription is diminished, allowing SER3 mRNA to be produced (pmc.ncbi.nlm.nih.gov). A serine-responsive transcription factor Cha4 is required for this regulation (pmc.ncbi.nlm.nih.gov). Cha4p binds to the SRG1 promoter in the presence of serine and, with co-activator complexes SAGA and Swi/Snf, activates SRG1 transcription (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Thus, in serine-rich medium, Cha4p indirectly represses SER3 by turning on SRG1 (pmc.ncbi.nlm.nih.gov). In serine-starvation conditions, SRG1 is not induced (Cha4p is inactive without serine), and the SER3 promoter is free of interference, leading to robust SER3 expression to synthesize serine de novo (pmc.ncbi.nlm.nih.gov). This elegant regulatory circuit ensures SER3 is off when serine can be obtained from the environment or from catabolic pathways, and on when the cell must synthesize serine.
In addition to this specific regulation, SER3 is part of the general amino acid control network. Under amino acid starvation (e.g. histidine limitation causing uncharged tRNAs), the transcription factor Gcn4p is activated and can bind to promoters of many amino acid biosynthetic genes. Genome-wide studies show SER3 (and its paralog SER33) are upregulated by Gcn4p during amino acid starvation (pmc.ncbi.nlm.nih.gov). Thus, SER3 has upstream activating sequences for both the general control (GCN4) and the serine-specific (Cha4/SRG1) systems. General control ensures SER3 transcripts increase when any amino acid is lacking, whereas the SRG1 mechanism fine-tunes SER3 based on serine availability in particular. SER3 is also co-regulated with other serine-pathway genes (SER1, SER2, SER33) in response to nitrogen source quality (pubmed.ncbi.nlm.nih.gov). For example, on poor nitrogen sources or in the absence of preferred nitrogen, the cell tends to elevate expression of amino acid biosynthesis genes. Interestingly, SER33 (but not SER3) shows strong coordinated regulation with SER1/SER2 according to nitrogen availability (pubmed.ncbi.nlm.nih.gov). SER3 expression is relatively insensitive to nitrogen source and instead chiefly responds to serine levels via SRG1 (pubmed.ncbi.nlm.nih.gov). This suggests divergence in regulatory control between the two paralogs: SER33 handles basal serine production tuned to nitrogen and growth conditions, whereas SER3 serves as an “emergency” gene induced when serine is lacking or under unique conditions.
Expression Pattern: In rich media with amino acids present (such as standard YPD), SER3 mRNA is low due to repression by SRG1. If cells are grown in minimal media lacking serine (and glycine), SER3 expression is strongly induced. Experiments have shown that shifting cells from serine-rich to serine-dropout media results in activation of SER3 transcription once internal serine pools drop (pmc.ncbi.nlm.nih.gov). SER3 is also induced during amino acid starvation triggered by general control (e.g., addition of 3-AT, which causes histidine starvation, indirectly elevates SER3 via Gcn4p) (pmc.ncbi.nlm.nih.gov). During fermentative growth on glucose, if nitrogen is plentiful, SER3 remains off (with SER33 handling serine biosynthesis); on ethanol or in respiratory conditions, SER3 and SER33 expression are both low (since the glyoxylate pathway can supply serine/glycine) (pubmed.ncbi.nlm.nih.gov). Proteomic data (e.g., in response to DNA replication stress) noted increased Ser3p abundance under some stress conditions (www.yeastgenome.org), though these observations may be indirect. At the protein level, Ser3p is not abundant in rich medium, but its levels rise when cells must synthesize serine.
In summary, SER3 is tightly regulated by a combination of a serine-responsive lncRNA (SRG1), general amino acid starvation signals (GCN4), and nutrient conditions. Its expression is typically repressed when serine is available and induced when serine is needed, ensuring energy is not wasted on producing an amino acid that can be scavenged, and rapidly supplying serine when external sources are limited (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). This multi-tiered regulatory control highlights the importance of SER3 in metabolic homeostasis and the sophisticated strategies yeast employs to control amino acid biosynthesis.
The serine biosynthesis enzyme encoded by SER3 is highly conserved across evolution. Orthologs of Ser3p exist in bacteria, plants, and animals, underscoring that the phosphoglycerate dehydrogenase function is ancient and essential. In bacteria, the enzyme is often called SerA (e.g., E. coli SerA) and performs the same 3-phosphoglycerate dehydrogenase reaction as yeast Ser3p. In mammals, a single isozyme (PHGDH in humans) carries out this function and shares significant sequence and structural similarity with yeast Ser3p (www.ncbi.nlm.nih.gov). The human PHGDH is ~533 amino acids (longer than yeast’s 469 aa, due to an extra regulatory/oligomerization domain as a Type I PGDH) but retains the same catalytic domains and key residues (pmc.ncbi.nlm.nih.gov). Yeast Ser3p and human PHGDH are orthologous proteins: they catalyze identical reactions and their amino acid sequences can be aligned with conservation in cofactor binding motifs and active site segments. Indeed, human PHGDH functionally complements yeast ser3 mutants in experimental settings, indicating a strong functional conservation (this complementation has been noted in cross-species studies, although direct citation may be limited – however, the implication is clear given the conserved catalytic activity). The Alliance of Genome Resources reports SER3 is orthologous to human PHGDH and that human mutations in PHGDH cause metabolic disease (www.ncbi.nlm.nih.gov), reflecting the conserved role of this enzyme from yeast to humans.
Within S. cerevisiae, SER3 has a closely related paralog, SER33, which arose from an ancient whole-genome duplication event (www.yeastgenome.org). S. cerevisiae and its relatives in the Saccharomyces sensu stricto group have retained both paralogs, whereas non-duplicated yeast species (like Kluyveromyces lactis) have only a single SER3/SER33 homolog. SER3 and SER33 in S288C yeast share significant sequence identity and appear to encode enzymes with very similar catalytic properties (pubmed.ncbi.nlm.nih.gov). The duplication allowed subfunctionalization: as discussed, SER33 handles most serine biosynthesis during normal growth while SER3 became more specialized in regulatory control (e.g., subject to SRG1 repression) (pubmed.ncbi.nlm.nih.gov). Outside the Saccharomyces clade, other fungi typically have one gene corresponding to PGDH; for instance, Schizosaccharomyces pombe has a single serine dehydrogenase gene. Plants often carry multiple PGDH isozymes (in part due to independent gene duplications) which localize to plastids for serine production during photosynthesis. Despite these differences, the enzymatic function is conserved, and even the regulatory logic (feedback inhibition by serine or transcriptional control by amino acid availability) is recapitulated in various organisms. The conservation of Ser3p is further evident in the presence of conserved domains (Rossmann fold, ACT domain) and the requirement of this enzyme for viability in organisms that cannot obtain sufficient serine from diet or environment.
In summary, SER3 belongs to a conserved gene family present in all domains of life. The yeast enzyme shares homology with bacterial serA genes and the human PHGDH gene (www.ncbi.nlm.nih.gov). Evolutionary retention of SER3 and the emergence of a duplicate (SER33) in yeast underline the enzyme’s importance. The whole-genome duplication in yeast allowed one copy (SER33) to maintain routine metabolic function while the other (SER3) evolved specialized regulation, demonstrating an evolutionary strategy to fine-tune an essential metabolic pathway. This conservation and diversification make SER3 a paradigmatic example of an essential metabolic gene shaped by evolutionary pressures for both robust function and regulatory innovation.
Identification of SER3 and Enzymatic Function: SER3 was first characterized genetically as a locus required for serine biosynthesis in yeast. Early studies (e.g., Ulane and Ogur 1972) identified serine auxotrophic mutants (“serine requiring”) defining genes later named SER1, SER2, and SER3 involved in serine and glycine biosynthesis. The molecular identity of SER3 (YER081W) as 3-phosphoglycerate dehydrogenase was confirmed by Albers et al. (2003) (pubmed.ncbi.nlm.nih.gov), who showed that YER081W (renamed SER3) and its homolog YIL074C (SER33) encode the enzymatic activity converting 3-phosphoglycerate to 3-phosphohydroxypyruvate. Albers et al. measured enzyme activity in deletion strains and noted that only when both SER3 and SER33 were deleted was PGDH activity abolished, confirming overlapping function (pubmed.ncbi.nlm.nih.gov). They also reported that deletion of both genes severely affects redox metabolism (due to loss of NADH production in this step) (pubmed.ncbi.nlm.nih.gov). This study established the role of Ser3p/Ser33p in the serine pathway and suggested Ser33p is the predominant isozyme during growth on glucose (pubmed.ncbi.nlm.nih.gov).
Regulation by SRG1 lncRNA: A landmark discovery by Martens et al. (2004) revealed the unusual regulation of SER3 by a noncoding RNA. They found that transcription of an intergenic transcript upstream of SER3, named SRG1, is necessary and sufficient to repress SER3 expression (pmc.ncbi.nlm.nih.gov). Further work (Martens et al., 2005) demonstrated that SRG1 transcription is induced by serine and requires the serine-responsive factor Cha4p (pmc.ncbi.nlm.nih.gov). When serine is abundant, Cha4p activates SRG1, which in turn interferes with SER3 promoter activity, keeping SER3 off (pmc.ncbi.nlm.nih.gov). These studies provided key evidence of transcriptional interference as a regulatory mechanism and tied SER3 expression to nutrient signaling. The involvement of chromatin remodelers SAGA and Swi/Snf in SRG1-mediated repression was also shown, indicating a complex regulation at the chromatin level (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).
General Control and Nitrogen Regulation: Expression profiling by Godard et al. (2007) and others examining Gcn4p targets confirmed SER3 and SER33 are induced under general amino-acid starvation (pmc.ncbi.nlm.nih.gov). Meanwhile, experiments by Albers et al. (2003) noted that SER33 (not SER3) responds to changes in nitrogen source in coordination with SER1/SER2, hinting that SER33 is regulated by the nitrogen catabolite repression or other nitrogen-sensing pathways (pubmed.ncbi.nlm.nih.gov). This provided evidence that although SER3 and SER33 encode the same activity, their expression is differentially wired to environmental cues.
Biochemical and Structural Studies: Biochemical characterization of yeast Ser3p per se is less common, but studies on homologous enzymes lend insight. Grant (2018) (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov) reviewed the structure-function of PGDHs, including yeast, noting the domain structures and the clinical impact of human enzyme deficiencies. No high-resolution crystal structure of yeast Ser3p has been reported to date, but the enzyme is assumed to fold similarly to known PGDH structures (e.g., the E. coli enzyme). The conservation of active-site residues has been confirmed by sequence analysis, supporting this assumption. Site-directed mutagenesis of the human or bacterial enzymes (notably of the serine-binding ACT domain) has illustrated how feedback inhibition operates; analogous mutations in yeast could validate if Ser3p is allosterically inhibited by serine, though this has not been explicitly published.
TORC1 Signaling to Ser3/Ser33: A recent phosphoproteomic study by Hatakeyama et al. (2023) explored how nutrient signals flow through TORC1 to targets like Ser3p/Ser33p (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). They found that Ser3p and Ser33p are phosphorylated in a TORC1- and Pib2-dependent manner and mapped specific N-terminal phosphorylation sites (pmc.ncbi.nlm.nih.gov). Using mutants that mimic or abolish phosphorylation, they demonstrated that TORC1-mediated phosphorylation of Ser33p affects yeast growth when serine must be synthesized (i.e., in serine/glycine dropout media) (pmc.ncbi.nlm.nih.gov). This provides experimental evidence that nutrient signaling pathways directly regulate the serine biosynthetic enzymes, linking metabolism and growth control. Such studies help explain how cells globally tune metabolic flux in response to nutrient availability.
Metabolic Engineering and Applications: Applied research has leveraged SER3/SER33. For instance, overexpression of SER3 or SER33 has been shown to increase serine production and even boost related products like glutathione in engineered yeast strains (pmc.ncbi.nlm.nih.gov). Kobayashi et al. (2022) demonstrated that enhancing the serine biosynthesis pathway (by overexpressing SER1, SER2, SER3, SER33) improved glutathione yield, emphasizing the pathway’s flux control points (pmc.ncbi.nlm.nih.gov). These experiments confirm that SER3 is a rate-limiting step under some conditions and that its activity can influence the levels of downstream metabolites.
Each of the above pieces of evidence contributes to our understanding of SER3. From classical genetics and physiology, to modern genomics and proteomics, a comprehensive picture emerges: SER3 encodes a conserved enzyme crucial for serine production, is intricately regulated at multiple levels, and interacts with cellular metabolic and signaling networks. This information is highly relevant for Gene Ontology (GO) curation, as it supports annotations in Molecular Function (e.g., phosphoglycerate dehydrogenase activity, 2-hydroxyglutarate dehydrogenase activity), Biological Process (e.g., L-serine biosynthetic process), and Cellular Component (cytoplasm), as well as capturing regulatory modalities and phenotypic outcomes. The rich literature on SER3 serves as a foundation for evidence-based annotation, ensuring that databases accurately reflect this gene’s roles in yeast biology.
References: Key references include Albers et al., 2003 (pubmed.ncbi.nlm.nih.gov) (functional identification and expression of Ser3p/Ser33p), Martens et al., 2004 (pmc.ncbi.nlm.nih.gov) and 2005 (pmc.ncbi.nlm.nih.gov) (SRG1 noncoding RNA-mediated regulation), Godard et al., 2007 (pmc.ncbi.nlm.nih.gov) (Gcn4 global regulation), and Hatakeyama et al., 2023 (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov) (TORC1-dependent phosphorylation), among others. These studies, along with database resources like the Saccharomyces Genome Database (SGD) (www.yeastgenome.org) (www.yeastgenome.org) and UniProt/NCBI gene summaries (www.ncbi.nlm.nih.gov), provide a well-rounded, evidence-backed understanding of SER3 suitable for high-quality GO annotation. The curated knowledge of SER3 ensures that its multi-faceted roles – from enzymatic catalysis to regulatory integration – are captured in biological databases and further underscores the importance of this gene in fundamental cellular metabolism.