Characterization of the exosporium basal layer protein BxpB of Bacillus anthracis

Bacillus anthracis spores, the cause of anthrax, are enclosed by a prominent loose-fitting structure called the exosporium. The exosporium is composed of a basal layer and an external hair-like nap. The filaments of the hair-like nap are apparently formed by a single collagen-like glycoprotein called BclA, whereas several different proteins form or are tightly associated with the basal layer. In this study, we used immunogold electron microscopy to demonstrate that BxpB (also called ExsF) is a component of the exosporium basal layer. Binding to the basal layer by an anti-BxpB monoclonal antibody was greatly increased by the loss of BclA. We found that BxpB and BctA are part of a stable complex that appears to include the putative basal layer protein ExsY and possibly other proteins. Previous results suggested that BxpB was glycosylated; however, our results indicate that it is not a glycoprotein. We showed that Delta bxpB spores, which lack BxpB, contain an exosporium devoid of hair-like nap even though the Delta bxpB strain produces normal levels of BclA. These results indicated that BxpB is required for the attachment of BclA to the exosporium. Finally, we found that the efficiency of production of Delta bxpB spores and their resistance properties were similar to those of wild-type spores. However, Delta bxpB spores germinate faster than wild-type spores, indicating that BxpB suppresses germination. This effect did not appear to be related to the absence from Delta bxpB spores of a hair-like nap or of enzymes that degrade germinants.