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this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.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.
The experimentally characterized S. pombe spo2 gene encodes a small, 133-amino-acid, approximately 15-kDa sporulation-specific protein. Its primary role is best described as a meiotic spindle-pole-body (SPB) assembly/adaptor function: Spo2 is recruited to the cytoplasmic face of the SPB by Spo15 and is required to recruit Spo13, modify the meiosis-II SPB outer plaque, and initiate forespore-membrane (FSM) formation. No enzyme reaction, transported substrate, or direct lipid-transfer activity has been demonstrated. (nakase2008meioticspindlepole pages 9-10, nakase2008meioticspindlepole pages 8-9, nakase2008meioticspindlepole pages 1-2, nakase2008meioticspindlepole pages 4-5)
A critical annotation conflict was identified. The primary study showed that spo2 is independently transcribed from a neighboring or overlapping locus annotated as vps1302, which encodes an approximately 354-kDa VPS13-family protein. Consequently, the supplied UniProt statement that C6Y4C2/Spo2 “belongs to the VPS13 family” does not align with the experimentally defined 133-aa Spo2 product and probably reflects locus/model conflation. VPS13 lipid-transporter activity should not be assigned to Spo2 without renewed sequence and transcript-model curation. (nakase2008meioticspindlepole pages 4-5, nakase2008meioticspindlepole pages 3-4, ching2024coolcontactscryoelectronmicroscopy pages 4-5)
The organism-specific literature concerns Schizosaccharomyces pombe and the sporulation-defective locus originally named spo18, subsequently established by complementation and mapping as spo2. The locus maps to the chromosome-II SPBC16C6 region corresponding to the user-specified SPBC16C6.14 context. The original spo2-B317 mutation is a one-nucleotide insertion at codon 32 that causes a frameshift; deletion of the independently defined gene reproduces the asporogenous phenotype. (nakase2008meioticspindlepole pages 4-5, nakase2008meioticspindlepole pages 3-4)
Full-length cDNA analysis established an independent spo2 transcript encoding 133 aa. The 2008 investigators explicitly distinguished it from the adjacent/overlapping vps1302 annotation. This is decisive because bona fide VPS13 proteins are hundreds of kilodaltons, whereas the experimental Spo2 protein is only about 15 kDa and was reported to have no recognizable motifs or significant database homology. (nakase2008meioticspindlepole pages 4-5, nakase2008meioticspindlepole pages 3-4)
Thus, the symbol is potentially ambiguous at the database-annotation level, but the relevant biological entity is clear: the small fission-yeast sporulation protein Spo2, not an unrelated organism’s SPO2 and not the large Vps1302 protein.
Spo2 is not currently supported as an enzyme, transporter, receptor, or signaling catalyst. The strongest functional model is that it serves as a small structural/adaptor component of the sporulation-specific SPB outer plaque. In the recruitment hierarchy, the constitutive large coiled-coil SPB protein Spo15 acts upstream, Spo2 is recruited next, and Spo13 is recruited efficiently only after Spo2 is present. Ectopically expressed Spo2 can localize to vegetative SPBs, but this targeting still requires Spo15; coexpression of Spo2 promotes Spo13 recruitment. (nakase2008meioticspindlepole pages 9-10, nakase2008meioticspindlepole pages 8-9)
Yeast two-hybrid assays detected Spo2 interactions with both Spo15 and Spo13, whereas a Spo13–Spo15 interaction was not detected. Together with the localization dependencies, this supports—but does not definitively prove—a model in which Spo2 physically bridges or organizes Spo15 and Spo13 at the modified outer SPB. Two-hybrid evidence should be regarded as evidence of binary interaction potential rather than proof of direct binding in the native meiotic complex. (nakase2008meioticspindlepole pages 8-9)
The resulting complex enables the SPB to acquire competence for FSM initiation. The FSM is the newly generated double membrane that begins near the cytoplasmic face of each meiosis-II SPB and subsequently encloses a haploid nucleus. Spo2 therefore acts at the transition from meiotic centrosome/SPB remodeling to de novo daughter-cell membrane biogenesis. (nakase2008meioticspindlepole pages 7-8, nakase2008meioticspindlepole pages 1-2, nakase2008meioticspindlepole pages 2-3)
spo2 is a meiosis-induced gene. Its transcript is absent or nearly undetectable in vegetative cells, rises after nitrogen starvation, and is largely controlled by the forkhead transcription factor Mei4: mei4 disruption strongly reduces spo2 expression, while ectopic Mei4 expression induces it. Spo2 protein becomes detectable shortly before meiosis I, reaches maximum abundance during meiosis II—the interval in which FSM assembly starts—and declines afterward. (nakase2008meioticspindlepole pages 5-7, nakase2008meioticspindlepole pages 4-5)
This timing is consistent with a dedicated sporulation function rather than a constitutive role in SPB duplication or mitosis. The experimentally supported pathway can be summarized as:
nitrogen starvation/meiotic program → Mei4-dependent spo2 induction → Spo15-dependent SPB recruitment of Spo2 → Spo13 recruitment → meiosis-II SPB outer-plaque remodeling → FSM initiation. (nakase2008meioticspindlepole pages 9-10, nakase2008meioticspindlepole pages 5-7, nakase2008meioticspindlepole pages 8-9)
A functional Spo2–GFP fusion appears as discrete foci at both ends of meiotic spindles and colocalizes with Sad1, a constitutive SPB marker. Positional analysis places Spo2 on the cytoplasmic side of the meiotic SPB, overlapping Spo13 and adjacent to the site at which nascent FSM material is assembled. Its functional site is therefore intracellular and highly localized: the modified outer surface of meiosis-II SPBs, not the nucleus, plasma membrane, vacuole, or extracellular space. (nakase2008meioticspindlepole pages 7-8, nakase2008meioticspindlepole pages 5-7, nakase2008meioticspindlepole pages 1-2)
Localization is conditional rather than intrinsic: Spo15 is required for efficient Spo2 recruitment. In turn, loss of Spo2 leaves Spo15 preferentially associated with SPBs but causes Spo13–GFP to become weak at SPBs and predominantly nuclear, demonstrating that Spo2 occupies an intermediate position in the assembly pathway. (nakase2008meioticspindlepole pages 8-9)
Both the frameshift allele and gene deletion permit normal vegetative growth and broadly normal meiotic nuclear-division kinetics, showing that Spo2 is dispensable for mitosis and for execution of meiosis itself. Nevertheless, spo2Δ cells produce virtually no spores. Their meiosis-II SPBs fail to undergo normal sporulation-specific structural modification, and the FSM markers GFP-Psy1 and GFP-Spo3 fail to form normal membranes. (nakase2008meioticspindlepole pages 7-8, nakase2008meioticspindlepole pages 2-3, nakase2008meioticspindlepole pages 4-5)
The key quantitative molecular observations are a 133-aa predicted product of approximately 15 kDa, contrasted with the neighboring approximately 354-kDa Vps1302 annotation. These values are central to resolving the database conflict. Exact percentages for spore formation and sample sizes were not recoverable from the available text, so “virtually no spores” is retained as the study’s qualitative result rather than converted into an unsupported numerical estimate. (nakase2008meioticspindlepole pages 7-8, nakase2008meioticspindlepole pages 4-5, nakase2008meioticspindlepole pages 3-4)
| Claim | Evidence type | Finding | Confidence | Key source/date |
|---|---|---|---|---|
| Target identity | Genetic complementation, locus mapping, full-length cDNA sequencing, allele sequencing | S. pombe spo2—formerly spo18—maps to the SPBC16C6 region and encodes an independently transcribed 133-aa, approximately 15-kDa protein distinct from the neighboring or overlapping approximately 354-kDa vps1302 annotation. The original spo2-B317 allele has a codon-32 frameshift. | High | Nakase et al., June 2008 (nakase2008meioticspindlepole pages 4-5, nakase2008meioticspindlepole pages 3-4) |
| Meiotic expression | Synchronous meiosis, transcript analysis, immunoblotting, mei4 perturbation | spo2 transcription is absent or negligible in vegetative cells, induced after nitrogen starvation during meiosis, and largely dependent on the Mei4 forkhead transcription factor. Spo2 protein appears shortly before meiosis I, peaks during meiosis II, and then declines. | High | Nakase et al., June 2008 (nakase2008meioticspindlepole pages 5-7, nakase2008meioticspindlepole pages 4-5) |
| Cellular localization | Functional Spo2–GFP imaging, Sad1 colocalization, SPB-position analysis | Spo2 localizes as dots at meiotic spindle ends, colocalizes with the constitutive SPB marker Sad1, overlaps with Spo13, and resides on the cytoplasmic side of the meiotic spindle pole body near the site where forespore-membrane assembly begins. | High | Nakase et al., June 2008 (nakase2008meioticspindlepole pages 7-8, nakase2008meioticspindlepole pages 5-7, nakase2008meioticspindlepole pages 1-2) |
| Recruitment hierarchy and interactions | Mutant-dependency analysis, ectopic coexpression, yeast two-hybrid assays | Recruitment follows the hierarchy Spo15 → Spo2 → Spo13: constitutive SPB scaffold Spo15 is required for Spo2 targeting, whereas Spo2 enables efficient Spo13 recruitment. Spo2 interacts with both Spo15 and Spo13 in two-hybrid assays; no direct Spo13–Spo15 interaction was detected. | High for recruitment; moderate for direct physical association | Nakase et al., June 2008 (nakase2008meioticspindlepole pages 9-10, nakase2008meioticspindlepole pages 8-9) |
| Loss-of-function phenotype | Gene deletion, meiotic time course, spore quantification, fluorescence microscopy and SPB ultrastructural analysis | spo2Δ cells grow vegetatively and execute meiotic nuclear divisions with essentially normal kinetics, but form virtually no spores. Sporulation-specific SPB modification is severely defective, and GFP-Psy1/GFP-Spo3 imaging shows failure to initiate the forespore membrane. | High | Nakase et al., June 2008 (nakase2008meioticspindlepole pages 7-8, nakase2008meioticspindlepole pages 2-3, nakase2008meioticspindlepole pages 4-5) |
| Primary functional annotation | Integrated genetics, localization, interaction and phenotype evidence | Spo2 is best annotated as a small, sporulation-specific SPB assembly/adaptor factor that helps convert the cytoplasmic face of the meiosis-II SPB into a platform competent to initiate forespore-membrane formation. No enzymatic reaction, transporter substrate or catalytic activity has been demonstrated. | High for structural/adaptor role; unknown biochemical mechanism | Nakase et al., June 2008 (nakase2008meioticspindlepole pages 9-10, nakase2008meioticspindlepole pages 8-9, nakase2008meioticspindlepole pages 1-2) |
| VPS13-family annotation conflict | Comparative assessment of locus/transcript evidence against modern VPS13 structural biochemistry | A VPS13 lipid-transporter function should not be assigned to the 133-aa Spo2 product. Bona fide VPS13 proteins are exceptionally large repeating-β-groove bridge proteins; experimentally studied fragments alone span approximately 1,350–1,390 residues and bind or transfer multiple lipids. Those properties fit the separate approximately 354-kDa Vps1302 annotation, not independently transcribed Spo2. | High that direct VPS13 annotation is unsupported; database curation needed | Nakase et al., June 2008; Ching et al., January 2024; Neiman, September 2024 (nakase2008meioticspindlepole pages 3-4, ching2024coolcontactscryoelectronmicroscopy pages 4-5, neiman2024membraneandorganelle pages 9-11) |
Table: Evidence grading distinguishes experimentally characterized 133-aa Spo2 from the separate large Vps1302 protein. It summarizes the strongest support for Spo2’s meiosis-specific spindle-pole-body role and identifies the VPS13 assignment as a likely locus-level annotation conflict.
Modern work defines VPS13 proteins as exceptionally large bridge-like lipid-transfer proteins. Extended repeating β-groove structures contain inward-facing hydrophobic residues and form channels capable of accommodating multiple lipids. Studied VPS13 fragments alone span approximately 1,350–1,390 residues; a roughly 1,350-residue fragment copurified with about 10 lipid molecules and promoted lipid transfer between vesicles in vitro. (ching2024coolcontactscryoelectronmicroscopy pages 4-5)
A 2024 review of ascospore membrane biology describes budding-yeast Vps13 as a bridge-like transporter recruited to prospore membranes by Spo71 and regulated by Spo73, with a proposed role in moving bulk lipid from the ER into the growing prospore membrane. That mechanism is biologically relevant to fungal spore-membrane expansion, but it concerns bona fide Vps13—not S. pombe Spo2. (neiman2024membraneandorganelle pages 9-11)
The family assignment therefore fails three consistency checks:
Accordingly, lipid-transfer activity, membrane-contact-site bridging, and lipid-substrate specificity must not be annotated for Spo2. The most likely explanation is that a protein-family annotation belonging to Vps1302 was propagated to C6Y4C2/SPBC16C6.14 because of overlapping gene models. This should be verified directly against the current UniProt sequence and PomBase transcript coordinates before automated annotation transfer.
No 2023–2024 primary study was found that directly re-examined S. pombe Spo2. The most recent authoritative context is the September 2024 ascospore-membrane review, which emphasizes that de novo spore membranes encapsulate meiotic nuclei and that VPS13-family bridge transport can support membrane expansion in budding yeast; it does not revise the experimentally established identity or mechanism of fission-yeast Spo2. DOI: https://doi.org/10.1128/mmbr.00013-24. (neiman2024membraneandorganelle pages 9-11)
The definitive target-specific study remains Nakase et al., published June 2008 in Molecular Biology of the Cell. DOI: https://doi.org/10.1091/mbc.e08-02-0118. Its convergence of complementation, transcript analysis, functional GFP localization, deletion phenotypes, recruitment dependencies, and interaction assays provides strong support for the SPB-adaptor model. (nakase2008meioticspindlepole pages 7-8, nakase2008meioticspindlepole pages 9-10, nakase2008meioticspindlepole pages 5-7, nakase2008meioticspindlepole pages 8-9, nakase2008meioticspindlepole pages 3-4)
Current real-world use is principally as a research model rather than a clinical or industrial target. Spo2 loss provides a genetically precise means to uncouple meiotic nuclear division from spore-membrane initiation; Spo2–GFP can report assembly of the sporulation-specific SPB plaque; and the Spo15–Spo2–Spo13 module offers a tractable system for studying how centrosome-like organelles acquire membrane-organizing capacity. (nakase2008meioticspindlepole pages 7-8, nakase2008meioticspindlepole pages 5-7, nakase2008meioticspindlepole pages 2-3)
There is no demonstrated Spo2 enzymatic activity, transporter substrate, high-resolution structure, lipid-binding activity, or direct biochemical reconstitution. Even the adaptor/bridge description is a cell-biological model based on recruitment and two-hybrid data rather than a purified ternary complex. Conservation outside S. pombe was not established in the foundational study. (nakase2008meioticspindlepole pages 8-9, nakase2008meioticspindlepole pages 4-5)
A defensible functional annotation is:
Sporulation-specific, meiosis-induced spindle-pole-body component required for outer-plaque remodeling and initiation of forespore-membrane assembly; recruited by Spo15 and required for efficient Spo13 recruitment.
Recommended qualifiers are: nonenzymatic structural/adaptor role; cytoplasmic face of meiotic SPBs; Mei4-regulated expression; no established substrate; no validated VPS13-family lipid-transfer activity. Resolving C6Y4C2 fully will require database reconciliation of the 133-aa spo2 transcript with the overlapping/adjacent large vps1302 gene model.
References
(nakase2008meioticspindlepole pages 9-10): Yukiko Nakase, Michiko Nakamura-Kubo, Yanfang Ye, Aiko Hirata, Chikashi Shimoda, and Taro Nakamura. Meiotic spindle pole bodies acquire the ability to assemble the spore plasma membrane by sequential recruitment of sporulation-specific components in fission yeast. Molecular biology of the cell, 19 6:2476-87, Jun 2008. URL: https://doi.org/10.1091/mbc.e08-02-0118, doi:10.1091/mbc.e08-02-0118. This article has 34 citations and is from a domain leading peer-reviewed journal.
(nakase2008meioticspindlepole pages 8-9): Yukiko Nakase, Michiko Nakamura-Kubo, Yanfang Ye, Aiko Hirata, Chikashi Shimoda, and Taro Nakamura. Meiotic spindle pole bodies acquire the ability to assemble the spore plasma membrane by sequential recruitment of sporulation-specific components in fission yeast. Molecular biology of the cell, 19 6:2476-87, Jun 2008. URL: https://doi.org/10.1091/mbc.e08-02-0118, doi:10.1091/mbc.e08-02-0118. This article has 34 citations and is from a domain leading peer-reviewed journal.
(nakase2008meioticspindlepole pages 1-2): Yukiko Nakase, Michiko Nakamura-Kubo, Yanfang Ye, Aiko Hirata, Chikashi Shimoda, and Taro Nakamura. Meiotic spindle pole bodies acquire the ability to assemble the spore plasma membrane by sequential recruitment of sporulation-specific components in fission yeast. Molecular biology of the cell, 19 6:2476-87, Jun 2008. URL: https://doi.org/10.1091/mbc.e08-02-0118, doi:10.1091/mbc.e08-02-0118. This article has 34 citations and is from a domain leading peer-reviewed journal.
(nakase2008meioticspindlepole pages 4-5): Yukiko Nakase, Michiko Nakamura-Kubo, Yanfang Ye, Aiko Hirata, Chikashi Shimoda, and Taro Nakamura. Meiotic spindle pole bodies acquire the ability to assemble the spore plasma membrane by sequential recruitment of sporulation-specific components in fission yeast. Molecular biology of the cell, 19 6:2476-87, Jun 2008. URL: https://doi.org/10.1091/mbc.e08-02-0118, doi:10.1091/mbc.e08-02-0118. This article has 34 citations and is from a domain leading peer-reviewed journal.
(nakase2008meioticspindlepole pages 3-4): Yukiko Nakase, Michiko Nakamura-Kubo, Yanfang Ye, Aiko Hirata, Chikashi Shimoda, and Taro Nakamura. Meiotic spindle pole bodies acquire the ability to assemble the spore plasma membrane by sequential recruitment of sporulation-specific components in fission yeast. Molecular biology of the cell, 19 6:2476-87, Jun 2008. URL: https://doi.org/10.1091/mbc.e08-02-0118, doi:10.1091/mbc.e08-02-0118. This article has 34 citations and is from a domain leading peer-reviewed journal.
(ching2024coolcontactscryoelectronmicroscopy pages 4-5): Cyan Ching, Julien Maufront, Aurélie di Cicco, Daniel Lévy, and Manuela Dezi. Cool-contacts: cryo-electron microscopy of membrane contact sites and their components. Contact, Jan 2024. URL: https://doi.org/10.1177/25152564241231364, doi:10.1177/25152564241231364. This article has 13 citations.
(nakase2008meioticspindlepole pages 7-8): Yukiko Nakase, Michiko Nakamura-Kubo, Yanfang Ye, Aiko Hirata, Chikashi Shimoda, and Taro Nakamura. Meiotic spindle pole bodies acquire the ability to assemble the spore plasma membrane by sequential recruitment of sporulation-specific components in fission yeast. Molecular biology of the cell, 19 6:2476-87, Jun 2008. URL: https://doi.org/10.1091/mbc.e08-02-0118, doi:10.1091/mbc.e08-02-0118. This article has 34 citations and is from a domain leading peer-reviewed journal.
(nakase2008meioticspindlepole pages 2-3): Yukiko Nakase, Michiko Nakamura-Kubo, Yanfang Ye, Aiko Hirata, Chikashi Shimoda, and Taro Nakamura. Meiotic spindle pole bodies acquire the ability to assemble the spore plasma membrane by sequential recruitment of sporulation-specific components in fission yeast. Molecular biology of the cell, 19 6:2476-87, Jun 2008. URL: https://doi.org/10.1091/mbc.e08-02-0118, doi:10.1091/mbc.e08-02-0118. This article has 34 citations and is from a domain leading peer-reviewed journal.
(nakase2008meioticspindlepole pages 5-7): Yukiko Nakase, Michiko Nakamura-Kubo, Yanfang Ye, Aiko Hirata, Chikashi Shimoda, and Taro Nakamura. Meiotic spindle pole bodies acquire the ability to assemble the spore plasma membrane by sequential recruitment of sporulation-specific components in fission yeast. Molecular biology of the cell, 19 6:2476-87, Jun 2008. URL: https://doi.org/10.1091/mbc.e08-02-0118, doi:10.1091/mbc.e08-02-0118. This article has 34 citations and is from a domain leading peer-reviewed journal.
(neiman2024membraneandorganelle pages 9-11): Aaron M. Neiman. Membrane and organelle rearrangement during ascospore formation in budding yeast. Sep 2024. URL: https://doi.org/10.1128/mmbr.00013-24, doi:10.1128/mmbr.00013-24. This article has 15 citations and is from a domain leading peer-reviewed journal.