SpoVM localizes by sensing convex membrane surfaces

Understanding the mechanisms that determine how proteins localize within a cell has broad impacts, from biotechnology to human health. The small peptide SpoVM, expressed in Bacillus species during sporulation, localizes via a mechanism previously unobserved in prokaryotes: recognition of membrane curvature. Fluorescence microscopy of Bacillus expressing SpoVM-GFP shows almost exclusive localization to the convex surface of the forespore during sporulation, while a single amino acid mutation, SpoVM(P9A), results in promiscuity to all membrane surfaces. This mislocalization mutant results in a loss of function, and the sporulation efficiency drops by six orders of magnitude compared to the wild type. We have developed an in-vitro system to reconstitute the convex surface of the forespore by coating silica microbeads with a lipid bilayer. We then use flow cytometry to quantify binding of SpoVM-GFP in this in-vitro system. Using this assay, we have characterized the binding saturation and binding kinetics of SpoVM. These findings reveal detail in what may be a general mechanism for localizing proteins to the surfaces of organelles.