Localization and Assembly of the Novel Exosporium Protein BetA of Bacillus anthracis

Abstract

The exosporium of Bacillus anthracis is comprised of two distinct layers: a basal layer and a hair-like nap that covers the basal layer. The hair-like nap contains the glycoproteins BclA and, most likely, BclB. BclA and BclB are directed to assemble into the exosporium by motifs in their N-terminal domains. Here, we identify a previously uncharacterized putative gene encoding this motif, which we have named betA (Bacillus exosporium-targeted protein of B. anthracis). Like bclA, betA encodes a putative collagenlike repeat region. betA is present in several genomes of exosporium-producing Bacillus species but, so far, not in any others. Using fluorescence microscopic localization of a BetA-enhanced green fluorescent protein (eGFP) fusion protein and immunofluorescence microscopy with antiBetA antibodies, we showed that BetA resides in the exosporium basal layer, likely underneath BclA. BetA assembles at the spore surface at around hour 5 of sporulation and under the control of BxpB, similar to the control of deposition of BclA. We suggest a model in which BclA and BetA are incorporated into the exosporium by a mechanism that depends on their similar N termini. These data suggest that BetA is a member of a growing family of exosporium proteins that assemble under the control of targeting sequences in their N termini.

The Exosporium Layer of Bacterial Spores: a Connection to the Environment and the Infected Host

“…BetA resides in the exosporium basal layer, likely underneath BclA. BetA assembles at the spore surface at around hour 5 of sporulation, and its assembly is BxpB dependent, a feature common to the spore collagen-like glycoproteins with the N-terminal exosporium targeting motif (57,58,91). A Bacillus megaterium protein (for example, the sequence under GenBank accession number WP_047934722) displays 28% identity and 51% similarity over a 139-residue sequence with the B. anthracis BetA protein.…”

The Bacillus anthracis Exosporium: What’s the Big “Hairy” Deal?

“…The BetA protein was recently identified and found to contain a small collagen-like repeat region which exhibits strain-associated variation in the number of repeats. It is believed that BetA is located within the basal layer of the exosporium underneath the BclA protein (33). It contains a conserved amino acid sequence in the N-terminal domain needed for targeting the protein to the exosporium (33).…”

“…It is believed that BetA is located within the basal layer of the exosporium underneath the BclA protein (33). It contains a conserved amino acid sequence in the N-terminal domain needed for targeting the protein to the exosporium (33). Currently, the role of the BetA has not been defined.…”

Endospores, Sporulation and Germination

“…The exosporium is composed of glycoproteins specific to this layer and it is believed to be an expansion of the outermost coat layer [46]. In B. anthracis, the exosporium may have a role in the interactions between the endospore and the host [47]. In B. anthracis, the exosporium may have a role in the interactions between the endospore and the host [47].…”