How SpoVM Interacts with Lipid Bilayers and Bacterial Cell Membranes

Abstract

SpoVM, a small protein with 26 residues, is essential for spore formation in Bacillus subtili. SpoVM is produced in the mother cell chamber of the sporangium during the process of sporulation and is recruited to the polar septum after the sporangium undergoes asymmetric division. There are reports suggesting that SpoVM localizes to the surface of the forespore by sensing membrane curvature. Besides its transformation from random coils to α-helices when binding to a lipid bilayer, the molecular mechanism of how SpoVM interacts with membranes has not been clarified. For example, we found that the curvature dependence of the binding affinity is difficult to detect. By using single giant unilamellar vesicle method, we found that SpoVM binds to the membrane and expands the surface area of membrane at low concentration with no molecular leakage from the vesicle. However, above the critical concentration, SpoVM causes leakage of the content dye from the GUV. The critical concentrations for different lipid compositions including phosphoethanolamine, phosphatidylglycerol, phosphatidylcholine and cardiolipin were further investigated. Surprisingly, the critical concentration for GUVs with 30% PE is more than six times higher than for other lipid compositions without PE. To understand how SpoVM interact with bacterial membranes, fluorescent microscopy was used to directly observe the effect of SpoVM on Escherichia coli in real time. The result showed that SpoVM makes both the outer membrane and the cytoplasmic membrane of Escherichia coli permeable to Sytox Green above a critical concentration. Oriented circular dichroism results show that SpoVM changes the orientation of its helical axis from parallel to perpendicular with respect to the plane of bilayers. Our results suggest that the SpoVM-membrane interaction follows the same pattern as many other helical antimicrobial peptides, such as magainin, melittin and islet amyloid polypeptides.