Molecular tiling on the surface of a bacterial spore – the exosporium of the Bacillus anthracis/cereus/thuringiensis group

Bacteria of the genera Bacillus and Clostridium form highly resistant spores, which in the case of some pathogens act as the infectious agents. An exosporium forms the outermost layer of some spores; it plays roles in protection, adhesion, dissemination, host targeting in pathogens and germination control. The exosporium of the Bacillus cereus group, including the anthrax pathogen, contains a 2D-crystalline basal layer, overlaid by a hairy nap. BclA and related proteins form the hairy nap, and require ExsFA (BxpB) for their localization on the basal layer. Until now, the identity of the main structural protein components of the basal layer was unknown. We demonstrate here that ExsY forms one of the essential components. Through heterologous expression in Escherichia coli, we also demonstrate that ExsY can self-assemble into ordered 2D arrays that mimic the structure of the exosporium basal layer. Self-assembly is likely to play an important role in the construction of the exosporium. The ExsY array is stable to heat and chemical denaturants, forming a robust layer that would contribute to overall spore resistance. Our structural analysis also provides novel insight into the location of other molecular components anchored onto the exosporium, such as BclA and ExsFA.