Assembly of the BclB glycoprotein into the exosporium and evidence for its role in the formation of the exosporium cap' structure in Bacillus anthracis

The outermost layer of the Bacillus anthracis spore consists of an exosporium comprised of an outer hair-like nap layer and an internal basal layer. A major component of the hair-like nap is the glycosylated collagen-like protein BclA. A second collagen-like protein, BclB, is also present in the exosporium. BclB possesses an N-terminal sequence that targets it to the exosporium and is similar in sequence to a cognate targeting region in BclA. BclB lacks, however, sequence similarity to the region of BclA thought to mediate attachment to the basal layer via covalent interactions with the basal layer protein BxpB. Here we demonstrate that BxpB is critical for correct localization of BclB during spore formation and that the N-terminal domains of the BclA and BclB proteins compete for BxpB-controlled assembly sites. We found that BclB is located principally in a region of the exosporium that excludes a short arc on one side of the exosporium (the so-called bottle-cap region). We also found that in bclB mutant spores, the distribution of exosporium proteins CotY and BxpB is altered, suggesting that BclB has roles in exosporium assembly. In bclB mutant spores, the distance between the exosporium and the coat, the interspace, is reduced.