FIG. 8.

Model of B. anthracis spore maturation. BclA and ExsK are deposited to the spore late in sporulation, with BclA deposited prior to ExsK. Both proteins assemble first to the mother-cell-proximal pole and then around the spore surface in an ExsFA/BxpB-dependent manner. Due to the partial occlusion of ExsK by BclA, it is likely that these two proteins are adjacent to each other on the spore surface. Once deposited to the spore surface, ExsK assembles into high-molecular-mass complexes in a BclA-dependent manner. Because ExsK assembles to the exosporium and underlying spore compartments after the formation of a complete basal layer, ExsK may be synthesized by ribosomes in the mother cell as well as by ribosomes trapped within the interspace. In mature spores, ExsK remains asymmetrically distributed and is more concentrated at the mother-cell-proximal pole (depicted as dark-red triangles at the pole and light-red triangles around the spore). Gray ovals, BclA; gray-striped arrows, assembly of BclA around the spore surface.