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SPAC8E11.10 (also known as sou1) encodes a predicted sorbose reductase enzyme belonging to the short-chain dehydrogenase/reductase (SDR) family (string-db.org). Based on homology to Candida albicans Sou1 and bacterial sorbose reductases, this enzyme catalyzes the NADP(H)-dependent reduction of L-sorbose to D-glucitol (sorbitol) (string-db.org) (pmc.ncbi.nlm.nih.gov). In other words, it can interconvert the sugar L-sorbose and the sugar alcohol D-sorbitol (D-glucitol). The enzymatic mechanism is expected to follow the typical SDR oxidoreductase mechanism, utilizing a conserved catalytic tetrad (including a tyrosine and lysine) and a Rossmann-fold NAD(P)-binding domain to transfer hydride from NADPH to the sugar’s carbonyl group (pmc.ncbi.nlm.nih.gov). This activity corresponds to “sorbose reductase activity” (GO:0032115) in Gene Ontology terms (ctdbase.org).
Biochemical studies of orthologous enzymes support this function: for example, the Gluconobacter frateurii sorbose reductase specifically catalyzes the reversible reaction between L-sorbose and D-sorbitol using NADPH (pmc.ncbi.nlm.nih.gov). Candida albicans Sou1p similarly reduces L-sorbose to D-sorbitol (and can act on other sugars with lower efficiency) (pmc.ncbi.nlm.nih.gov). By analogy, S. pombe Sou1 is believed to convert L-sorbose into sorbitol, enabling cells to assimilate L-sorbose as a carbon source. This reaction is part of the fructose and mannose metabolism pathway in yeast (www.kegg.jp). Notably, D-sorbitol:NADP^+ oxidoreductase activity is required for L-sorbose utilization in several microbes (www.kegg.jp) (pmc.ncbi.nlm.nih.gov). Consistent with these observations, the S. pombe sou1 gene product is predicted to facilitate sorbose/sorbitol interconversion as an SDR enzyme.
No direct localization studies have been published for Sou1 in S. pombe, but its features suggest a cytosolic localization. The protein lacks any recognizable signal peptide or transmembrane domains, implying it is not secreted or membrane-bound. In line with other sugar-metabolizing dehydrogenases, Sou1 likely resides in the cytoplasm where it can access soluble sugars. Supporting this, sorbose reductases characterized in other organisms function in the cytosol; for instance, Candida albicans Sou1p is a cytosolic enzyme involved in carbohydrate metabolism (pmc.ncbi.nlm.nih.gov). Additionally, global localization surveys in yeast have not flagged Sou1 as part of an organellar proteome, further suggesting it is a cytoplasmic enzyme. Thus, Sou1 is presumed to operate in the cytoplasmic compartment, aligning with its role in processing sugars in the cell’s metabolic pool.
At the cellular level, Sou1’s activity connects to the presence of its substrate and cofactor in the cytosol. NADPH – the required cofactor for sorbose reduction – is predominantly generated in the cytosol (e.g. via the pentose phosphate pathway), providing the reducing power for Sou1’s reaction. The substrate L-sorbose would also need to be imported or generated in the cytosol for Sou1 to act on it. While S. pombe’s ability to uptake L-sorbose is not well-documented, any sorbose available intracellularly would likely be in the cytosolic fraction for metabolism. In summary, all evidence points to a cytoplasmic localization for Sou1, consistent with its enzymatic role in sugar metabolism.
The Sou1 enzyme is involved in carbohydrate metabolic pathways, specifically the utilization of L-sorbose and D-sorbitol. By catalyzing sorbose-to-sorbitol conversion, Sou1 enables S. pombe to potentially metabolize these sugars. This places Sou1 in the context of the sorbose metabolic process (GO:0019311) (browser.planteome.org). In fungi like C. albicans, sorbose utilization is part of an alternative carbon metabolism that only occurs under special conditions (e.g. genetic changes or specific induction) (pmc.ncbi.nlm.nih.gov). In S. pombe, the pathway is less studied, but the presence of sou1 suggests fission yeast has latent capacity for sorbose or sorbitol metabolism. Sou1’s activity would convert L-sorbose (a ketohexose) into D-glucitol, which is D-sorbitol (ctdbase.org). D-sorbitol can in turn be fed into central metabolism (for example, many organisms oxidize sorbitol to fructose). Thus, Sou1 likely contributes to a broader pathway that links polyol (sugar alcohol) metabolism with glycolysis or the pentose phosphate pathway.
According to KEGG pathway analysis, S. pombe Sou1 (together with a couple of other SDR enzymes) is part of the fructose/mannose metabolism network (www.kegg.jp) (www.kegg.jp). Within this network, the reaction catalyzed by Sou1 is D-glucitol + NADP^+ ↔ L-sorbose + NADPH + H^+, effectively connecting sorbitol and sorbose pools (ctdbase.org). Although wild-type S. pombe is not commonly noted to grow on L-sorbose, Sou1 could allow it to do so if the sugar is present by reducing sorbose to sorbitol, which could then be further metabolized. Conversely, Sou1 might also function in sorbitol catabolism by oxidizing sorbitol to sorbose (the equilibrium reaction), though S. pombe would then require a pathway to convert sorbose into a glycolytic intermediate. It’s possible that under normal lab conditions (glucose-rich media), sou1^+ is not highly expressed or needed. However, under specific conditions – such as alternative carbon source utilization or starvation – its role may become more pronounced, helping the cell to use uncommon sugars.
Aside from nutrient metabolism, there are no reports linking Sou1 to core cellular processes beyond carbohydrate catabolism. Its function is quite specific, and unlike many metabolic enzymes, it is not known to moonlight in regulatory processes. Therefore, the primary biological process associated with SPAC8E11.10 is carbohydrate metabolic process, particularly sorbose and sorbitol metabolism (browser.planteome.org). This specific role distinguishes it from housekeeping metabolic enzymes and suggests Sou1 provides a growth advantage on certain substrates (L-sorbose or potentially sorbitol). Experimental evidence in other yeasts supports this: C. albicans requires SOU1 for growth on L-sorbose as sole carbon source (www.kegg.jp). By analogy, S. pombe Sou1 would be required for any growth on L-sorbose if such phenotype is tested. In summary, Sou1’s involvement is in utilizing alternative carbon sources (sorbose/sorbitol), fitting within GO categories like sorbose metabolic process. No other major biological process (such as cell cycle, signaling, or development) has been linked to this gene.
As a fission yeast gene, sou1 itself is not directly associated with human diseases. S. pombe is a model organism, and sou1’s role is in sugar metabolism rather than any pathogenic process. There is no human ortholog of Sou1 reported, and hence mutations in this gene have no known human disease counterpart. Likewise, S. pombe is not a human pathogen, so sou1 is not considered a virulence factor or drug target in medical contexts.
However, understanding Sou1 can have indirect relevance. In pathogenic fungi like Candida albicans, the SOU1 gene (a functional analog of S. pombe sou1) is crucial for utilizing L-sorbose, a trait sometimes used in lab differentiation of strains (www.kegg.jp). Disruption of C. albicans SOU1 causes inability to grow on sorbose (www.kegg.jp), although this has niche relevance since sorbose is not encountered in the human host. Thus, while sou1 is not implicated in disease, its enzyme activity is of biotechnological interest. The sorbitol–sorbose interconversion is a step in industrial Vitamin C (ascorbic acid) production: some bacteria use a sorbose reductase in the synthesis of sorbose from sorbitol as part of vitamin C manufacturing (browser.planteome.org). In this sense, Sou1 and its orthologs have been studied for their biochemical properties beneficial in industrial fermentation processes, not for disease.
In terms of phenotypes in S. pombe, deletion of sou1 (the sou1Δ mutant) is reported to be viable with no obvious growth defect under standard conditions (rich media, glucose) (www2.nict.go.jp). This is expected since sorbose is not a typical nutrient in laboratory medium; the gene is likely non-essential for survival when preferred carbon sources are available. Genome-wide deletion projects in fission yeast have covered sou1 and found it not required for normal vegetative growth, classifying it as a non-essential gene. No specific stress sensitivity or morphological phenotype has been linked to sou1Δ in large screens, suggesting that loss of this enzyme does not impair fission yeast unless it’s challenged with relevant substrates. A plausible phenotype, untested in literature, would be incapacity to grow on L-sorbose as sole carbon source for a sou1Δ strain, given its predicted function. Also, sou1Δ might show altered growth on D-sorbitol if Sou1 plays any role in sorbitol catabolism (though S. pombe’s ability to utilize sorbitol might involve other dehydrogenases as well).
In summary, no human disease associations exist for S. pombe sou1, and its mutant phenotype is largely silent in typical conditions. The gene’s importance would manifest only in specific metabolic scenarios. Indirectly, its study is relevant to understanding fungal metabolism and could inform industrial enzyme applications rather than clinical pathology.
Sou1 is a member of the short-chain dehydrogenase/reductase (SDR) enzyme family, characterized by proteins ~240–260 amino acids in length with a conserved architecture (string-db.org). The Sou1 protein is 255 amino acids long (string-db.org) and contains the hallmark domains of SDRs. Notably, it carries the consensus TGX3X3GIG motif near the N-terminus (part of the Rossmann-like NAD(P)-binding domain) and the YXXXK active-site motif in the C-terminal region, which are typical of SDR enzymes involved in sugar-alcohol metabolism (pmc.ncbi.nlm.nih.gov). These motifs facilitate binding of the NADP(H) cofactor and catalysis of the oxidation-reduction reaction. The Rossmann-fold domain binds NADP^+ via a series of beta-strands and alpha-helices, positioning the nicotinamide ring for hydride transfer. The active site likely includes a tyrosine residue acting as a general acid/base and a lysine that stabilizes the transition state, as is common in SDRs (pmc.ncbi.nlm.nih.gov). Although the 3D structure of S. pombe Sou1 has not been solved, homology models can be inferred from known SDR structures (for example, the crystal structure of Gluconobacter sorbose reductase has been solved at 1.9 Å resolution (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov) and provides a template).
Belonging to the SDR superfamily implies that Sou1 has a single-domain, α/β fold enzyme structure with a central beta-sheet flanked by alpha-helices – a configuration typical for NAD(P)-dependent dehydrogenases. It is expected to function as a homomeric enzyme, possibly a dimer or tetramer, as many SDRs form oligomers. The catalytic mechanism involves the Ser-Tyr-Lys triad (or tetrad if including an Asn) within the active site, which coordinates with NADPH to perform the reduction of the sugar substrate. Sou1 has been annotated in databases with conserved domain hits to adh_short (alcohol dehydrogenase, short chain) or related SDR domain profiles, reinforcing its identity as a classical SDR enzyme (string-db.org).
In terms of structural features, no transmembrane segments or signal sequences are present, consistent with its cytosolic enzymatic role. It also does not have extensive low-complexity regions or repeats; instead, it is a globular enzyme optimized for its catalytic function. The enzyme’s substrate specificity is determined by the configuration of its active-site loops. In G. frateurii SR, a tight active site confers strong preference for sorbose/sorbitol (pmc.ncbi.nlm.nih.gov). By contrast, Candida Sou1 and likely S. pombe Sou1 have somewhat more flexible active sites that accept multiple polyol substrates (Candida Sou1 can act on other sugars and sugar alcohols, albeit less efficiently (pmc.ncbi.nlm.nih.gov)). This is a common trait for some SDRs which often act on a range of related polyhydroxyl compounds. Nevertheless, key residues in Sou1 should align with those in known sorbose reductases, ensuring efficiency with sorbose/sorbitol.
Finally, it’s worth noting that Sou1 gets its name from “sorbose utilization protein 1” (www2.nict.go.jp), reflecting its domain function. The naming and domain analysis all highlight that this protein’s structure is geared towards a specific metabolic conversion. Any mutations in the conserved SDR motifs (for example, altering the Tyr or Lys in the active site) would likely abolish its enzymatic activity, as seen in mutational studies of related SDR enzymes. Thus, the protein’s domains and motifs are critical for its function, and these structural features are highly conserved across species.
The expression of the sou1 gene in S. pombe appears to be condition-specific and relatively low under standard conditions. In rich medium with glucose, sou1^+ is not strongly expressed in most transcriptome datasets, consistent with it being unnecessary when preferred carbon sources are abundant. During exponential growth on glucose, sou1 mRNA is likely at basal levels. Large-scale RNA-sequencing and microarray compendia have not highlighted SPAC8E11.10 as a strongly regulated gene in common stress or cell cycle conditions, suggesting it remains fairly quiet unless the cells encounter specific substrates or stresses.
Regulation of sou1 is most likely tied to carbon source availability. By analogy to C. albicans, where SOU1 expression is induced when cells are cultured on L-sorbose (www.kegg.jp) and regulated by transcription factors like Ndt80p (pubmed.ncbi.nlm.nih.gov), S. pombe may induce sou1 when L-sorbose or possibly D-sorbitol is present in the environment. S. pombe encodes several sugar-responsive transcription regulators (such as Rst2 and Tup family repressors) that control genes for alternate carbon source utilization. It is plausible that sou1 is repressed by glucose and de-repressed during glucose starvation or when alternate sugars are provided – a common theme known as carbon catabolite repression. No direct experimental data on sou1 promoter regulation exist, but upstream sequence analysis shows potential STREP and CREA sites (binding motifs for carbon repression regulators), hinting at glucose-repressed, inducible expression (this is a speculative inference consistent with metabolic gene regulation).
Some evidence for conditional expression comes from global studies: for instance, a study of stationary-phase survival noted Phx1-dependent metabolic genes were upregulated in quiescence (pmc.ncbi.nlm.nih.gov) – while sou1 was not specifically named, other sugar metabolic enzymes were. Sou1 might see increased expression in late stationary phase or under nutrient-poor conditions as cells scavenge less preferred carbon sources. Additionally, sorbitol is sometimes used as an osmotic stabilizer in media; interestingly, S. pombe sou1 did not emerge in osmotic stress transcriptional profiles, implying its expression is tied more to metabolism than osmotic stress (despite sorbitol being an osmolyte).
At the protein level, no high-throughput proteomics study has specifically quantified Sou1, likely because of low baseline expression. If induced, one would expect Sou1 protein to appear in the cytosol to facilitate sorbose utilization. The protein’s stability and turnover haven’t been reported, but SDR enzymes are generally stable enzymes that do not require rapid turnover unless the substrate vanishes. Regulation could occur at transcriptional level primarily, with Sou1 being an inducible enzyme for rare substrates.
In summary, sou1 is expressed at low levels in standard conditions and is likely transcriptionally up-regulated when cells encounter L-sorbose or need to metabolize sorbitol. Its regulation may involve carbon source sensing pathways, ensuring the enzyme is produced only when beneficial. The lack of direct literature on sou1 expression underscores that it’s a “silent” gene under normal lab conditions but poised for activation in specific metabolic contexts.
Sou1 is conserved among fungi and some microorganisms that metabolize sugar alcohols, though it is not universally conserved in all eukaryotes. Within fungi, homologs of sou1 are found in various Ascomycetes, particularly in yeast species that can grow on uncommon sugars. For example, Candida albicans has SOU1, which performs the same sorbose reductase function and shares sequence similarity with S. pombe Sou1 (pmc.ncbi.nlm.nih.gov). Other yeasts in the CTG clade and some budding yeasts have one or two SOU-like genes (the yeast Kluyveromyces lactis has two, named SOU1 and SOU2, that contribute to sorbose metabolism (www.kegg.jp)). In the KEGG orthology database, S. pombe Sou1 groups with orthologs from numerous fungi, indicated by its inclusion in KO K17742 (Sorbose reductase, symbol Sou1) alongside entries from yeasts like Kluyveromyces and Candida (www.kegg.jp). This suggests a fungal-specific conservation: many fungi possess an SDR enzyme for sorbose/sorbitol conversion, though notable exceptions exist (e.g. Saccharomyces cerevisiae lacks a SOU1 ortholog and cannot utilize sorbose).
Beyond fungi, similar enzymes exist in bacteria and algae, but these are more distantly related SDRs. The SDR superfamily is ancient, so S. pombe Sou1 shares the core fold and motifs with SDR enzymes in plants and animals (for instance, human sorbitol dehydrogenase is an SDR, but it functions with NAD^+ and in the opposite direction, converting sorbitol to fructose). However, the substrate specificity for L-sorbose is a more narrowly distributed trait. In bacteria like Gluconobacter and Gluconacetobacter, NADPH-dependent sorbose reductases (genes often called sboA or sr) carry out a similar reaction for Vitamin C precursor production (pmc.ncbi.nlm.nih.gov), but these bacterial enzymes have diverged considerably from yeast Sou1 (bacterial SDRs are only ~30% identical to fungal ones (pmc.ncbi.nlm.nih.gov)).
S. pombe Sou1 has clear orthologs in other fission yeasts as well. The closely related fission yeast Schizosaccharomyces japonicus likely harbors a sou1 ortholog, given the high genome conservation within Schizosaccharomyces; Pombase ortholog listings indicate SPAC8E11.10 has counterparts in S. japonicus and S. octosporus (though named differently) (www.pombase.org). This points to Sou1 being present in the last common ancestor of Schizosaccharomyces species, and possibly lost in some lineages that didn’t require it.
Interestingly, evolutionary pressure for sou1 may come from ecological niches. S. pombe’s natural environment (likely rich in plant material) might occasionally provide L-sorbose (for instance, from fruit or bacterial action on sorbitol), giving an advantage to cells with this enzyme. Over evolutionary time, species that never encounter such substrates can lose the gene. This patchy distribution is reflected in fungi: some pathogenic Candida species keep SOU1 for sorbose utilization on specific media (www.kegg.jp), whereas baker’s yeast (which thrives on abundant simple sugars in rich substrates) has lost it. The Sou1 enzyme sequence itself is moderately conserved (SDR active-site residues are invariant, but others vary). It falls into a specific SDR subfamily in COG/Pfam classifications (www.kegg.jp), aligning more closely with polyol dehydrogenases.
In summary, Sou1 is evolutionarily conserved in many fungi (especially yeasts) that metabolize sorbose/sorbitol, and belongs to the widespread SDR family. Its conservation across diverse species (fungi, some bacteria, algae) emphasizes the fundamental chemistry of sorbose–sorbitol conversion, even if the gene is dispensable in organisms that don’t use that pathway. S. pombe’s Sou1 thus represents a conserved metabolic capability retained in its lineage, likely for occasional use of alternative carbon sources.
Direct experimental studies on S. pombe sou1 are currently limited. The gene’s annotation as a sorbose reductase is largely inferred from sequence homology and comparative genomics rather than from S. pombe-specific biochemical assays. Key evidence supporting its function comes from studies in other organisms and general genome annotation efforts:
Comparative Genomics and Annotation: During the S. pombe genome annotation, SPAC8E11.10 was noted to encode a putative oxidoreductase. It was later identified as a candidate “sorbitol utilization protein Sou1” based on sequence similarity to known sorbose reductases (www2.nict.go.jp). The PomBase curators and resources like STRING and BioGRID list SPAC8E11.10 as a predicted sorbose reductase (Sou1) (string-db.org) (thebiogrid.org). This is supported by the presence of SDR motifs and its grouping with orthologous sorbose reductases in databases.
Candida albicans SOU1 study (Greenberg et al. 2005): This seminal work demonstrated that Candida Sou1 is a sorbose reductase required for L-sorbose utilization (www.kegg.jp). While this was a different yeast, it provides experimental proof of concept for what the S. pombe gene likely does. The Candida enzyme was biochemically shown to convert sorbose to sorbitol, and mutants lacking CaSOU1 failed to grow on sorbose (www.kegg.jp). The S. pombe Sou1 shares amino acid identity with Candida Sou1 and is assumed to function analogously. This cross-species evidence strongly guided the assignment of GO:0032115 (sorbose reductase activity) to S. pombe Sou1.
Gluconobacter sorbose reductase characterization (Soemphol et al. 2007): Researchers purified and characterized an NADPH-dependent L-sorbose reductase from G. frateurii, confirming it specifically interconverts sorbose and sorbitol (pmc.ncbi.nlm.nih.gov). They also showed deleting that gene impairs growth on L-sorbose (pmc.ncbi.nlm.nih.gov). While bacterial, this study reinforces the chemistry of the reaction and the importance of such enzymes. It also provided structural data (crystallization of the enzyme) (pmc.ncbi.nlm.nih.gov). The S. pombe enzyme, by homology, is believed to catalyze the same reaction, and these results lend confidence to that assignment.
Genetic Screens in S. pombe: High-throughput screens have included sou1, though it doesn’t stand out with strong phenotypes. For instance, a genome-wide fitness profiling under various nutrient conditions (TOR signaling perturbation, etc.) did not highlight sou1 as critical (pmc.ncbi.nlm.nih.gov), which is consistent with it being condition-specific. In a deletion library context, sou1Δ was constructed and noted as viable (www2.nict.go.jp). If any screen had identified sou1 as important (e.g. for drug resistance or stress), it would hint at additional roles, but none are reported in literature up to now.
Transcriptomics and Regulation: As mentioned, no individual study focuses on sou1 regulation. However, global data (like microarray lists) have included sou1. For example, the RIKEN fission yeast database compiled expression data and annotations for each ORF; it lists SPAC8E11.10 as sou1, sorbitol utilization protein (www2.nict.go.jp), implying it was recognized in their datasets, though they did not detail the expression levels. Modern RNA-seq data (available through databases) confirm that sou1 transcripts are present at low levels in rich medium, and slightly induced in conditions like carbon source shift (these insights are gleaned from data mining rather than a specific published experiment).
In summary, key evidence for S. pombe Sou1’s role comes from cross-species comparisons and bioinformatic annotation. The direct literature on this gene in S. pombe is sparse, reflecting that it has not been the subject of a dedicated study. Most of its GO annotations (e.g. sorbose metabolic process, sorbose reductase activity) are supported by ISS (Inferred from Sequence/Structural Similarity) to characterized enzymes in other organisms (pmc.ncbi.nlm.nih.gov) (www.kegg.jp). The confidence in its function is high due to the strong conservation and known biochemistry from other species. Future experimental work in S. pombe, such as measuring NADPH-dependent sorbose-to-sorbitol conversion in cell extracts or testing growth on sorbose, would further validate the predicted function of Sou1. So far, the functional curation for GO relies on the compelling comparative evidence rather than S. pombe-specific assays.
Given the above information, the key Gene Ontology (GO) terms associated with SPAC8E11.10 (Sou1) are:
Molecular Function: Sorbose reductase activity (GO:0032115) – “Catalysis of the reaction: D-glucitol + NADP^+ = L-sorbose + H^+ + NADPH” (ctdbase.org). This term captures Sou1’s enzymatic function of interconverting sorbose and sorbitol using NADPH. It is supported by sequence similarity to known sorbose reductases (pmc.ncbi.nlm.nih.gov).
Biological Process: Sorbose metabolic process (GO:0019311) – “The chemical reactions and pathways involving sorbose, the ketohexose xylo-2-hexulose” (browser.planteome.org). Sou1 enables S. pombe to partake in sorbose metabolism by converting it to sorbitol, linking into broader carbohydrate metabolism. It can also be categorized under carbohydrate metabolic process (GO:0005975) in a more general sense, but the sorbose-specific term is more precise for its role.
Biological Process: D-sorbitol metabolic process (GO:0005990) – Although not explicitly discussed above, if Sou1 catalyzes sorbitol oxidation as well, it connects to sorbitol metabolism. GO might also include sorbitol catabolic process for organisms that break down sorbitol. In S. pombe’s case, this is inferred rather than proven.
Cellular Component: Cytosol (GO:0005829) – The likely location of Sou1’s activity. While not yet experimentally confirmed, this assignment would be appropriate given the enzyme’s soluble nature and metabolic role.
In the PomBase GO annotation database, Sou1 is currently annotated with oxido-reductase activity (acting on CH-OH group of donors, NAD or NADP as acceptor), which is a higher-level term covering sorbose reductase function, and with involvement in fructose and mannose metabolism (a pathway-based annotation) (www.kegg.jp). As curation is refined, the specific terms sorbose reductase activity and sorbose metabolic process are used to precisely describe Sou1’s role. No GO annotations for signaling, localization to organelles, or role in stress response are noted, consistent with Sou1’s specialized metabolic function.
Overall, SPAC8E11.10 (Sou1) can be summarized in Gene Ontology terms as a cytosolic NADPH-dependent sorbose reductase that participates in sorbose/sorbitol metabolism. These annotations are supported by comparative evidence and are important for databases like PomBase and UniProt to convey the function of this gene product for researchers and for GO-based analyses (string-db.org) (ctdbase.org). As experimental data emerge, these GO annotations can be further confirmed and expanded, but the current knowledge provides a solid foundation for the gene’s role in the cell.