Subcellular localization of a small sporulation protein in Bacillus subtilis.

SpoVM is an unusually small (26-residue-long) protein that is produced in the mother cell chamber of the sporangium during the process of sporulation in Bacillus subtilis. We investigated the subcellular localization of SpoVM, which is believed to be an amphipathic alpha-helix, by using a fusion of the sporulation protein to the green fluorescence protein (GFP). We found that SpoVM-GFP is recruited to the polar septum shortly after the sporangium undergoes asymmetric division and that the fusion protein localizes to the mother cell membrane that surrounds the forespore during the subsequent process of engulfment. We identified a patch of three residues near the N terminus of the proposed alpha-helix that is needed both for proper subcellular localization and for SpoVM function. We also identified a patch of residues on the opposite face of the helix and residues near both ends of the protein that are needed for SpoVM function but not for subcellular localization. Subcellular localization of SpoVM-GFP was found to require an unknown gene(s) under the control of the mother cell transcription factor sigmaE. We propose that the N-terminal patch binds to an unknown anchoring protein that is produced under the control of sigmaE and that other residues important in SpoVM function to recruit an unknown sporulation protein(s) to the mother cell membrane that surrounds the forespore. Our results provide evidence that SpoVM function depends on proper subcellular localization.