UNLABELLED: The outermost exosporium layer of spores of Bacillus anthracis, the causative agent of anthrax, is comprised of a basal layer and an external hairlike nap. The nap includes filaments composed of trimers of the collagenlike glycoprotein BclA. Essentially all BclA trimers are tightly attached to the spore in a process requiring the basal layer protein BxpB (also called ExsFA). Both BclA and BxpB are incorporated into stable, high-molecular-mass complexes, suggesting that BclA is attached directly to BxpB. The 38-residue amino-terminal domain of BclA, which is normally proteolytically cleaved between residues 19 and 20, is necessary and sufficient for basal layer attachment. In this study, we demonstrate that BclA attachment occurs through the formation of isopeptide bonds between the free amino group of BclA residue A20 and a side chain carboxyl group of an acidic residue of BxpB. Ten of the 13 acidic residues of BxpB can participate in isopeptide bond formation, and at least three BclA polypeptide chains can be attached to a single molecule of BxpB. We also demonstrate that similar cross-linking occurs in vitro between purified recombinant BclA and BxpB, indicating that the reaction is spontaneous. The mechanism of BclA attachment, specifically, the formation of a reactive amino group by proteolytic cleavage and the promiscuous selection of side chain carboxyl groups of internal acidic residues, appears to be different from other known mechanisms for protein cross-linking through isopeptide bonds. Analogous mechanisms appear to be involved in the cross-linking of other spore proteins and could be found in unrelated organisms. IMPORTANCE: Isopeptide bonds are protein modifications found throughout nature in which amide linkages are formed between functional groups of two amino acids, with at least one of the functional groups provided by an amino acid side chain. Isopeptide bonds generate cross-links within and between proteins that are necessary for proper protein structure and function. In this study, we discovered that BclA, the dominant structural protein of the external nap of Bacillus anthracis spores, is attached to the underlying exosporium basal layer protein BxpB via isopeptide bonds formed through a mechanism fundamentally different from previously described mechanisms of isopeptide bond formation. The most unusual features of this mechanism are the generation of a reactive amino group by proteolytic cleavage and promiscuous selection of acidic side chains. This mechanism, which apparently relies only on short peptide sequences in protein substrates, could be a general mechanism in vivo and adapted for protein cross-linking in vitro.