Interaction of SpoVM and FtsH

A putative morphogenetic protein, SpoVM

We have identified a developmental gene, spoVM, which encodes a small polypeptide of less than 3kD (MKFYTIKLPKFLGGIVRAMLGSFRKD) with an ORF of just 26 codons (Levin et al., 1993). spoVM expression occurs in the mother cell chamber and is controlled by sigma E, the predecessor of sigma K. A spoVM null mutant consisting of a transposon insertion within the spoVM ORF produces a strong stage IV-V block in spore formation with the production of heat-sensitive spores. These defective spores contain a spore coat but lack a cortex. In addition, the spoVM mutant causes a noticeable effect on sigma K directed gene expression (Levin et al., 1993). Most apparent is the almost complete block in expression of the cotA gene and a 50% reduction in gerE expression. However, expression of the sigma K-controlled cotA gene was unaffected in the spoVM mutant suggesting that SpoVM is likely to play a morphogenetic rather than a regulatory role in sporulation. The small size of this protein precludes an enzymatic function while its phenotype suggests involvement in sigma K-controlled gene expression while not acting directly as a transcription factor.

SpoVM is amphipathic and interacts with phospholipid membranes

Projection of the SpoVM sequence onto a helical wheel shows a striking partitioning of charged residues. We have examined the binding and secondary structure of a synthetic SpoVM protein exposed to water-lipid interfaces. Our results suggest that 1) SpoVM has no secondary structure in aqueous solution but becomes significantly helical when exposed to lipid vesicles. 2) SpoVM is highly surface active and can target and associate with lipid mono/bilayers. In the absence of structural NMR data or X-ray crystallography, we can assume, only, that an alpha-helical conformation for SpoVM is favoured in a lipid environment.

Helical wheel projection of the 26 amino acid SpoVM polypeptide

SpoVM interacts with the membrane-bound FtsH protein

We have shown that SpoVM interacts with the 7OkD FtsH protein (Cutting et al., 1997). FtsH is involved in a variety of ATP-dependent protein-membrane interactions, including secretion, protein assembly, membrane fusion and proteolysis (Confalonieri and Duguet, 1995). The ATP-binding domain is highly homologous with that of the 'AAA-protein family' which includes a number of important eukaryotic proteins (e.g., SUG1, Paslp, Secl8p, NSF and VCP) which are involved in membrane-associated events (e.g., vesicle and membrane fusion, protein assembly and secretion). FtsH is membrane bound and SpoVM most likely uses its extreme amphipathicity to target the membranes enabling it to interact with FtsH. We have recently characterized the B. subtilis ftsH gene and shown that it is vegetatively expressed, essential for cell growth, and required for the initiation of spore formation (Lysenko et al., 1997). Our most recent studies (Cutting et al., 1997) have shown that SpoVM antagonizes the action of FtsH and, together, these two proteins are involved in controlling critical morphogenetic stages of spore development.

For more information on the AAA protein family visit the AAA-server.

References

Confalonieri, F. and Duguet, M. (1995). A 200-amino acid ATPase module in search of a basic function. BioEssays. 17: 639-650.

Cutting, S., Anderson, M., Lysenko, E., Page, A., Tomoyasu, T., Tatematsu, K., Tatsuta, T., Kroos, L. and Ogura, T. (1997). SpoVM, a small protein essential to development in Bacillus subtilis, interacts with the ATPdependent protease, FtsH. J. Bacteriol., 179: 5534-5542

Levin, P. A., Fan, N., Ricca, E., Driks, A., Losick, R. and Cutting, S. (1993). An unusually small gene required for sporulation by Bacillus subtilis. Mol Microbiol. 9: 761-771.

Lysenko, E., Ogura, T. and Cutting, S. (1997). Characterization of the ftsH gene of Bacillus subtilis. Microbiol. 143: 971-978.

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