Intramolecular amide bonds stabilize pili on the surface of bacilli

摘要

Gram-positive bacteria elaborate pili and do so without the participation of folding chaperones or disulfide bond catalysts. Sor-tases, enzymes that cut pilin precursors, form covalent bonds that link pilin subunits and assemble pili on the bacterial surface. We determined the x-ray structure of BcpA, the major pilin subunit of Bacillus cereus. The BcpA precursor encompasses 2 Ig folds (CNA_2 and CNA_3) and one jelly-roll domain (XNA) each of which synthesizes a single intramolecular amide bond. A fourth amide bond, derived from the Ig fold of CNA_1, is formed only after pilin subunits have been incorporated into pili. We report that the domains of pilin precursors have evolved to synthesize a discrete sequence of intramolecular amide bonds, thereby conferring structural stability and protease resistance to pili.