Roles of the Bacillus anthracis Spore Protein ExsK in Exosporium Maturation and Germination

The Bacillus anthracis spore is the causative agent of the disease anthrax. The outermost structure of the B. anthracis spore, the exosporium, is a shell composed of approximately 20 proteins. The function of the exosporium remains poorly understood and is an area of active investigation. In this study, we analyzed the previously identified but uncharacterized exosporium protein ExsK. In contrast to other exosporium proteins, we found that ExsK is present in at least two distinct locations the spore surface as well as a more interior location underneath the exosporium. In spores that lack the exosporium basal layer protein ExsFABxpB. In spores lacking the exosporium-surface protein Bcla, ExsK fails to mature into high molecular weight species observed in wild-type spores. These data suggest that the assembly and maturation of ExsK within the exosporium are dependent on ExsFABxpB and Bcla. We also found that ExsK is not required for virulence but it does inhibit germination. Based on these data, we propose a revised model of exosporium maturation and assembly, and suggest a novel role for the exosporium in germination.