Project description:Gram-positive bacteria elaborate pili and do so without the participation of folding chaperones or disulfide bond catalysts. Sortases, 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.
Project description:Pilin precursors are the building blocks of pili on the surface of Gram-positive bacteria; however, the assembly mechanisms of these adhesive fibers are unknown. Here, we describe the chemical bonds that assemble BcpA pilin subunits on the surface of Bacillus cereus. Sortase D cleaves BcpA precursor between the threonine (T) and the glycine (G) residues of its LPXTG sorting signal and catalyzes formation of an amide bond between threonine (T) of the sorting signal and lysine (K) in the YPKN motif of another BcpA subunit. Three CNA B domains of BcpA generate intramolecular amide bonds, and one of these contributes also to pilus formation. Conservation of catalysts and structural elements in pilin precursors in Gram-positive bacteria suggests a universal mechanism of fiber assembly.
Project description:Pili on the surface of Streptococcus pyogenes play a crucial role in adhesion to and colonization in human cells. The major pilin subunit, Spy0128, features intramolecular covalent isopeptide bonds that autocatalytically form between the side chains of lysine and asparagine residues and are regarded as important factors in conveying structural stability. In support of this notion, single-molecule force spectroscopy experiments with Spy0128 recently demonstrated the inextensibility of these bonds under mechanical load. However, the molecular determinants of their apparent absolute durability remain unknown. Here, we studied the impact of the isopeptide bond in the Spy0128 C-terminal domain on the mechanical properties of this subunit using force-probe molecular dynamics simulations and force distribution analysis. Even in the presence of the covalent cross-link, the pili ?-sandwich domain undergoes partial unfolding, albeit at ?50% higher rupture forces and with the ability to rapidly refold on the nanosecond timescale. We find that the isopeptide bond is located right at the point of stress concentration in the protein, leading to relative, yet not absolute, mechanical stabilization by the additional cross-link. Our findings indicate how the isopeptide bond enhances the mechanical stability and refolding capability at the molecular level, ensuring that the domain remains predominantly in a potentially adhesive conformation.