| Endospores formed by Bacillus subtilis are encased in a tough protein shell known as the coat, which consists of at least 70 different proteins. We investigated the process of spore coat morphogenesis using a library of about 40 coat proteins fused to GFP and demonstrate that two successive steps can be distinguished in coat assembly. The first step, initial localization of proteins to the spore surface, is dependent on the coat morphogenetic proteins SpoIVA and SpoVM. The second step, spore encasement, requires a third protein, SpoVID. We show that in spoVID mutant cells, most coat proteins assembled into a cap at one side of the developing spore but failed to migrate around and encase it. A domain analysis revealed that the N-terminus of SpoVID is required for encasement and is a structural homolog of a virion protein, whereas the C-terminus interacts directly with SpoIVA. Thus, SpoVM, SpoIVA and SpoVID are recruited to the spore surface in a concerted manner and form a tripartite machine that drives coat formation and spore encasement. In addition, my data indicate that three morphogenetic coat proteins, CotO, CotH and Tgl, have partially redundant functions in spore coat assembly and required for subcellular localization of a subset of proteins. |
