Processing, assembly and localization of a Bacillus anthracis spore protein

All Bacillus spores are encased in macromolecular shells. One of these is a proteinacious shell called the coat that, in Bacillus subtilis, provides critical protective functions. The Bacillus anthracis spore is the infectious particle for the disease anthrax. Therefore, the coat is of particular interest because it may provide essential protective functions required for the appearance of anthrax. Here, we analyse a protein component of the spore outer layers that was previously designated BxpA. Our data indicate that a significant amount of BxpA is located below the spore coat and associated with the cortex. By SDS-PAGE, BxpA migrates as a 9 kDa species when extracted from Sterne strain spores, and as 11 and 14 kDa species from Ames strain spores, even though it has predicted masses of 27 and 29 kDa, respectively, in these two strains. We investigated the possibility that BxpA is subject to post-translational processing as previously suggested. In B. subtilis, a subset of coat proteins is proteolysed or cross-linked by the spore proteins YabG or TgI, respectively. To investigate the possibility that similar processing occurs in B. anthracis, we generated mutations in the yabG or tgl genes in the Sterne and Ames strains and analysed the consequences for BxpA assembly by SDS-PAGE. We found that in a tgl mutant of B. anthracis, the apparent mass of BxpA increased. This is consistent with the possibility that TgI directs the cross-linking of BxpA into a form that normally does not enter the gel. Unexpectedly, the apparent mass of BxpA also increased in a yabG mutant, suggesting a relatively complex role for proteolysis in spore protein maturation in B. anthracis. These data reveal a previously unobserved event in spore protein maturation in B. anthracis. We speculate that proteolysis and cross-linking are ubiquitous spore assembly mechanisms throughout the genus Bacillus.