3D meshwork architecture of the outer coat protein CotE: implications for bacterial endospore sporulation and germination

Abstract

Bacillus cereus, a Gram-positive aerobic bacterium commonly found in soil, food, and water, forms endospores that can withstand harsh environmental conditions. The endospores are encased in a protective spore coat consisting of multiple layers of proteins, among which, CotE serves as a crucial morphogenetic protein within the outer coat. In this study, we observed that the homotrimeric CotE protein underwent further oligomerization induced by Ca2+ and was subsequently dissociated by dipicolinic acid, a compound released from the spore core during germination. Through cryo-electron microscopy and tomography analyses of the Ca2+-induced CotE oligomer, combined with structural predictions and biochemical studies, we propose a three-dimensional meshwork organization facilitated by tryptophan-based interactions between CotE trimers. The resulting meshwork was organized in a defective diamond-like tetrahedral configuration. These insights enhance our understanding of how CotE contributes to endospore morphogenesis and germination through the rapid disassembly of these layers.