Yet more intramolecular cross-links in Gram-positive surface proteins

Abstract

The surface of Gram-positive bacteria comprises a single membrane and, typically, a thick layer of cross-linked peptidoglycan that imparts strength and rigidity. Anchored to this cell wall are protein assemblies, such as pili, and surface proteins, such as microbial surface components recognizing adhesive matrix molecules (MSCRAMMs) that act as surface adhesins. These proteins are critical not only for bacterial binding to host surfaces in the early stages of infection but also for biofilm formation and immune evasion. In a series of studies published since 2007, these cell surface proteins and protein assemblies have been shown to contain isopeptide and thioester bonds, highly unusual intramolecular covalent linkages between amino acid side chains. These cross-links have either a structure-stabilizing role or may be directly involved in adhesion. Using an elegant combination of structural biology and mass spectrometry, Kwon et al. (1) reveal in PNAS an ester bond as yet another covalent linkage in a putative MSCRAMM from the Gram-positive pathogen Clostridium perfringens . Remarkably, this ester bond, joining the side chains of a Thr and Gln residue, is equivalent to an unresolved acyl-enzyme intermediate, formed on an autocatalytic pathway that resembles the mechanism of serine proteases. Despite fundamental differences in the way they are encoded in bacterial genomes, following delivery/assembly at the cell surface, pili and many MSCRAMMs share important similarities. In such proteins, a series of repetitive domains (commonly referred to as stalk regions) are responsible for the length of the structure and allow an N-terminal region to be positioned away from the cell surface. At the C terminus, both pili and MSCRAMMs are anchored to the cell wall by sortases (2). Although many of the building blocks of Gram-positive surface proteins and pili bear some resemblance to their Gram-negative … [↵][1]1To whom correspondence should be addressed. E-mail: Mark.Banfield{at}jic.ac.uk. [1]: #xref-corresp-1-1