A protein phosphorylation module patterns theBacillus subtilisspore outer coat

Abstract

AbstractAssembly of theBacillus subtilisspore coat involves over 80 protein components, which self-organize into a basal layer, a lamellar inner coat, a striated electrondense outer coat and a more external crust. CotB is an abundant component of the outer coat. Its C-terminal moiety contains a region, termed SKRB, formed by a series of serine-rich repeats, which we show is phosphorylated by the coat-associated Ser/Thr kinase CotH at multiple Ser residues. Another coat protein, CotG, which contains a central repeat region, SKRG, interacts with the C-terminal moiety of CotB and promotes its phosphorylation by CotHin vivoand in a heterologous system. CotG itself is phosphorylated by CotH but phosphorylation is enhanced in the absence of CotB. Spores of acotHD288Astrain, producing an inactive form of the kinase, like those formed by acotGdeletion mutant, lack the characteristic pattern of electrondense outer coat striations, while retaining the crust. Specifically, in the absence of CotG or CotH activity, most of the outer coat proteins are assembled but form a layer of amorphous material that peels-off the spore if crust formation is genetically ablated. In contrast, deletion of the SKRBregion, has no major impact on the structure of the outer coat. Thus, phosphorylation of CotG by CotH is the principal factor establishing the structural pattern of the spore outer coat. The presence of thecotB/cotH/cotGcluster in several species closely related toB. subtilisand ofcotG-like proteins in nearly all spore-formers that also code for a CotH homologue hints at the importance of this protein phosphorylation module in the morphogenesis of the spore outer layers.