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Research Report: VAM10 (YOR068C) — Vacuolar Morphogenesis Protein 10 in Saccharomyces cerevisiae

1. Gene and Protein Identity

VAM10 (UniProt: Q08474) is encoded by the open reading frame YOR068C (also known as YOR29-19) on chromosome XV of Saccharomyces cerevisiae (strain ATCC 204508 / S288c). The gene encodes a protein designated Vacuolar Morphogenesis Protein 10 (Vam10p). Notably, the VAM10 gene is genomically embedded in proximity to VPS5 (YOR069W), which encodes a sorting nexin component of the Retromer complex (chen2025molecularbasisfor pages 2-3, chen2025molecularbasisfor pages 1-2). Vam10p is a relatively small protein without well-characterized structural domains in standard protein databases, and it does not belong to a recognized protein family. Despite this, the protein plays a functionally important role in the vacuole fusion pathway.

2. Discovery and Identification

Vam10 was identified in a genome-wide screen for vacuolar morphology mutants conducted by Seeley, Kato, Wickner, and Eitzen (2002). In this screen, 4,828 nonessential yeast gene deletions were scored for vacuole morphology using the fluorescent vital dye FM4-64 (seeley2002genomicanalysisof pages 1-2). The original VAM (vacuolar morphology) genes had been identified earlier by Wada et al. (1992), and six of the nine original VAM genes encode subunits of the HOPS tethering complex (Vam1p = Vps11p, Vam2p = Vps41p, Vam5p = Vps33p, Vam6p = Vps39p, Vam8p = Vps18p, Vam9p = Vps16p), with Vam3p and Vam7p encoding vacuolar SNAREs and Vam4p encoding the Rab GTPase Ypt7p (seeley2002genomicanalysisof pages 1-2). However, the initial vam screen was not saturated, and the genomic approach by Seeley et al. identified 137 candidate VAM genes, including VAM10 (YOR068C), which displayed a prominent class B vacuole fragmentation phenotype in 90% of cells upon deletion (seeley2002genomicanalysisof pages 9-11).

3. Primary Function: Role in Vacuole Fusion Priming

3.1 Vam10 Defines a Sec18p-Independent Priming Step

The primary functional characterization of Vam10p was reported by Kato and Wickner (2003, Proceedings of the National Academy of Sciences, 100:6398–6403), who demonstrated that Vam10p defines a Sec18p-independent step of priming that allows yeast vacuole tethering (ostrowicz2009dynamicsandarchitecture pages 140-143). This is a critical finding, because it establishes that Vam10p operates in a priming pathway that is mechanistically distinct from the well-characterized Sec18p/NSF-dependent cis-SNARE disassembly reaction.

In the established model of homotypic vacuole fusion, the reaction proceeds through ordered stages: (1) priming, in which vacuoles are prepared for their first productive contact; (2) tethering, the reversible bridging of vacuoles mediated by the HOPS complex and the Rab GTPase Ypt7p; (3) docking, involving trans-SNARE complex formation between opposing membranes; and (4) fusion/bilayer mixing (ostrowicz2008yeastvacuolefusion pages 3-5, seeley2002genomicanalysisof pages 1-2). The canonical priming reaction is initiated by ATP hydrolysis by the AAA-ATPase Sec18p (the yeast NSF homolog), which disassembles cis-SNARE complexes with the help of its co-factor Sec17p (yeast α-SNAP), leading to the release of Sec17p from vacuoles, dissociation of the soluble SNARE Vam7p, and activation of SNAREs for subsequent trans-pairing (ostrowicz2008yeastvacuolefusion pages 3-5, seeley2002genomicanalysisof pages 1-2).

Vam10p function has been connected to the priming stage of vacuole fusion alongside other priming-associated processes such as Vac8 palmitoylation (ostrowicz2008yeastvacuolefusion pages 5-5). Importantly, Vam10p acts in a parallel or complementary pathway to Sec18p-dependent SNARE disassembly, indicating that priming involves multiple, coordinated preparatory events beyond SNARE recycling alone (ostrowicz2009dynamicsandarchitecture pages 140-143).

3.2 Distinction from HOPS Complex

It is critical to note that Vam10p is NOT a subunit of the HOPS tethering complex. The HOPS complex is a well-defined hexameric complex consisting of four class C core subunits (Vps11p, Vps16p, Vps18p, Vps33p) and two HOPS-specific subunits (Vps39p/Vam6p and Vps41p/Vam2p) (ostrowicz2008yeastvacuolefusion pages 6-7). Vam10p is instead classified among "additional proteins involved in fusion," a category that also includes calmodulin, protein phosphatase 1, Bem1, and others (ostrowicz2008yeastvacuolefusion pages 6-7, ostrowicz2008yeastvacuolefusion pages 7-8). Deletion or antibody inhibition of these additional factors strongly affects vacuole fusion, indicating an intimate connection to the fusion cascade, but their precise molecular mechanisms have not been as thoroughly dissected as those of the core HOPS-SNARE-Rab machinery (ostrowicz2008yeastvacuolefusion pages 7-8).

Recent cryo-EM structural studies of the HOPS complex have resolved its architecture as a highly extended, rigid structure (~430 Å × 130 Å) resembling a "baseball pitcher," with core subunits Vps11p and Vps18p forming the central body through antiparallel α-solenoid interactions and the HOPS-specific subunits Vps39p and Vps41p anchored at the periphery for Rab GTPase (Ypt7p) binding (diao2024molecularstructuresand pages 4-6). Vam10p is absent from this structural complex, confirming its role as an accessory factor rather than a core tethering complex component.

4. Subcellular Localization

Vam10p functions at the yeast vacuole, the fungal equivalent of the mammalian lysosome. The vacuole is a large, dynamic organelle that serves as the major degradative compartment and ion/metabolite storage organelle in yeast cells (ostrowicz2008yeastvacuolefusion pages 1-3). Wild-type yeast cells typically possess one to three large vacuoles, and defects in vacuole fusion result in class B (multiple small vacuoles) or class C (highly fragmented vacuoles) phenotypes (seeley2002genomicanalysisof pages 1-2). The vam10Δ deletion strain displays a class B phenotype with 90% of cells showing multiple small vacuoles, consistent with a defect in homotypic vacuole fusion rather than complete pathway disruption (seeley2002genomicanalysisof pages 9-11).

Vam10p is associated with the vacuolar membrane, where it participates in the priming stage that prepares vacuoles for the tethering event mediated by HOPS and Ypt7p. Its function is required to allow vacuole tethering to proceed efficiently (ostrowicz2009dynamicsandarchitecture pages 140-143).

5. Biochemical and Signaling Pathways

5.1 Integration into the Vacuole Fusion Cascade

The vacuole fusion reaction represents one of the best-characterized membrane fusion events in eukaryotic cell biology, providing a model system for understanding the general principles of endomembrane dynamics (ostrowicz2008yeastvacuolefusion pages 1-3, ostrowicz2008yeastvacuolefusion pages 3-5). The cascade involves:

• Sec18p/Sec17p-dependent priming: Disassembly of cis-SNARE complexes, release of Sec17p, Vam7p, and the HOPS complex from membranes (ostrowicz2008yeastvacuolefusion pages 3-5, ostrowicz2008yeastvacuolefusion pages 5-5).
• Sec18p-independent priming (Vam10p pathway): A parallel priming step defined by Vam10p function, which is necessary for downstream tethering to occur (ostrowicz2009dynamicsandarchitecture pages 140-143).
• Tethering: Mediated by the HOPS complex binding to Ypt7p-GTP on both vacuolar membranes (ostrowicz2008yeastvacuolefusion pages 5-5, ostrowicz2008yeastvacuolefusion pages 6-7).
• Docking: Trans-SNARE complex formation involving the Q-SNAREs Vam3p, Vam7p, Vti1p and the R-SNARE Nyv1p (ostrowicz2008yeastvacuolefusion pages 5-6, ostrowicz2008yeastvacuolefusion pages 5-5).
• Fusion: Ca²⁺ release, calmodulin signaling, protein phosphatase 1 action, and lipid bilayer mixing (seeley2002genomicanalysisof pages 1-2, ostrowicz2008yeastvacuolefusion pages 7-8).

Vam10p acts upstream of HOPS-mediated tethering but in a pathway that does not depend on the Sec18p ATPase activity. This positions it as a factor that may regulate the readiness of the vacuolar membrane or associated proteins for the initial bridging contact.

5.2 Relationship to Other Priming Factors

Additional reactions classified as priming events include Vac8p palmitoylation (which couples the armadillo repeat protein Vac8p to the membrane for its role in docking and fusion) and ergosterol-dependent processes (ostrowicz2008yeastvacuolefusion pages 5-5, seeley2002genomicanalysisof pages 1-2). The Vam10p-dependent step is distinct from ergosterol-mediated priming, which was shown to be required for Sec18p-mediated Sec17p release (seeley2002genomicanalysisof pages 7-9). This further supports the model of multiple parallel priming pathways that collectively prepare vacuoles for productive fusion.

6. Genomic Context and Phenotypic Evidence

The VAM10/YOR068C locus is notable for its genomic proximity to VPS5/YOR069W (seeley2002genomicanalysisof pages 9-11, chen2025molecularbasisfor pages 2-3). The Seeley et al. (2002) study specifically noted that "Three genes (YLR320w, YNL281w, and YOR068c) are adjacent to genes that are also required for normal vacuole morphology. Further analysis will be required to determine whether these deletions act by altering the expression level of an adjacent gene" (seeley2002genomicanalysisof pages 9-11). VPS5 encodes a sorting nexin subunit of the Retromer complex involved in endosome-to-Golgi retrieval, and a recent structural study noted that "a copy of Vam10 (the VAM10 gene is embedded" in the VPS5 genomic region (chen2025molecularbasisfor pages 2-3). This raises the question of whether the vam10Δ phenotype could partly reflect perturbation of VPS5, although the functional characterization by Kato and Wickner (2003) provided direct biochemical evidence for Vam10p function in the in vitro vacuole fusion assay.

The vam10Δ strain does not show a moderate or strong VPS (vacuolar protein sorting) phenotype for CPY secretion, suggesting that Vam10p is specifically involved in homotypic vacuole fusion rather than biosynthetic trafficking to the vacuole (seeley2002genomicanalysisof pages 7-9, seeley2002genomicanalysisof pages 9-11). This is consistent with the distinction between VAM genes (involved in fusion and morphology) and VPS genes (involved in protein sorting), which have substantial but incomplete overlap (seeley2002genomicanalysisof pages 7-9).

7. Summary Table of Vacuole Fusion Machinery

The following table summarizes the relationship of Vam10p to the core vacuole fusion machinery:

Table: This table summarizes the major yeast vacuole fusion machinery components relevant to VAM10 annotation, including HOPS, Rab, SNAREs, and additional fusion factors. It highlights that VAM10/YOR068C is distinct from HOPS and is instead an accessory factor acting in a Sec18-independent priming step.

8. Current Status and Limitations

VAM10 remains a relatively understudied gene. The primary functional characterization paper (Kato & Wickner, 2003) established its role in Sec18-independent priming, but the precise molecular mechanism by which Vam10p promotes vacuole tethering remains unknown. It is not known whether Vam10p acts as an enzyme, a structural scaffold, or a regulatory factor. No crystal or cryo-EM structure of Vam10p has been reported, and its protein interaction partners have not been comprehensively mapped. The genomic proximity to VPS5 introduces a caveat regarding potential effects on neighboring gene expression, though the in vitro biochemical data support a direct role for Vam10p in the fusion reaction.

In the context of the broader vacuole fusion field, recent years have seen substantial advances in structural understanding of the HOPS and CORVET tethering complexes (diao2024molecularstructuresand pages 4-6) and in reconstitution of the minimal fusion machinery. However, VAM10 itself has not been a focus of these recent studies, and its mechanism represents an open question in the field of endomembrane fusion.

References (Key citations used)

• Seeley, E.S., Kato, M., Margolis, N., Wickner, W., & Eitzen, G. (2002). Genomic analysis of homotypic vacuole fusion. Molecular Biology of the Cell, 13(3), 782–794.
• Kato, M. & Wickner, W. (2003). Vam10p defines a Sec18p-independent step of priming that allows yeast vacuole tethering. PNAS, 100(11), 6398–6403.
• Ostrowicz, C.W., Meiringer, C.T.A., & Ungermann, C. (2008). Yeast vacuole fusion: A model system for eukaryotic endomembrane dynamics. Autophagy, 4(1), 5–19.
• Diao, J., Yip, C.K., & Zhong, Q. (2024). Molecular structures and function of the autophagosome-lysosome fusion machinery. Autophagy Reports, 3(1).
• Chen, K.-E. et al. (2025). Molecular basis for the assembly of the Vps5-Vps17 SNX-BAR proteins with Retromer. Nature Communications, 16, 3568.

References

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15. (ostrowicz2009dynamicsandarchitecture pages 48-51): Clemens Werner Ostrowicz. Dynamics and architecture of the hops tethering complex in yeast vacuole fusion. Text, Jan 2009. URL: https://doi.org/10.11588/heidok.00009655, doi:10.11588/heidok.00009655. This article has 0 citations and is from a peer-reviewed journal.

16. (seeley2002genomicanalysisof pages 3-5): E. Scott Seeley, Masashi Kato, Nathan Margolis, William Wickner, and Gary Eitzen. Genomic analysis of homotypic vacuole fusion. Molecular biology of the cell, 13 3:782-94, Mar 2002. URL: https://doi.org/10.1091/mbc.01-10-0512, doi:10.1091/mbc.01-10-0512. This article has 209 citations and is from a domain leading peer-reviewed journal.

Artifacts

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Citations

1. seeley2002genomicanalysisof pages 1-2
2. seeley2002genomicanalysisof pages 9-11
3. ostrowicz2009dynamicsandarchitecture pages 140-143
4. ostrowicz2008yeastvacuolefusion pages 5-5
5. ostrowicz2008yeastvacuolefusion pages 6-7
6. ostrowicz2008yeastvacuolefusion pages 7-8
7. diao2024molecularstructuresand pages 4-6
8. ostrowicz2008yeastvacuolefusion pages 1-3
9. seeley2002genomicanalysisof pages 7-9
10. chen2025molecularbasisfor pages 2-3
11. seeley2002genomicanalysisof pages 2-3
12. seeley2002genomicanalysisof pages 3-5
13. chen2025molecularbasisfor pages 1-2
14. ostrowicz2008yeastvacuolefusion pages 3-5
15. ostrowicz2008yeastvacuolefusion pages 5-6
16. ostrowicz2009dynamicsandarchitecture pages 48-51
17. https://doi.org/10.1038/s41467-025-58846-8,
18. https://doi.org/10.1091/mbc.01-10-0512,
19. https://doi.org/10.11588/heidok.00009655,
20. https://doi.org/10.4161/auto.5054,
21. https://doi.org/10.1080/27694127.2024.2305594,