Structural and Functional Studies of the Streptococcal Fibrillar Adhesin CshA

Abstract

Adherence of bacteria to biotic or abiotic surfaces is a prerequisite for host colonization and represents an important step in microbial pathogenicity. This attachment is facilitated by bacterial adhesins at the cell surface. Due to their size and often elaborate multi‐domain architectures, these polypeptides represent challenging targets for detailed structural and functional characterization. The multifunctional fibrillar adhesin CshA, which mediates binding to both host molecules and other microorganisms, is an important determinant of colonization by Streptococcus gordonii, an oral commensal and opportunistic human pathogen. CshA has a low sequence identity to other proteins, indicating potentially novel structural characteristics.Our studies indicate that the N‐terminal region of CshA forms a functional head of the fibril, with a disordered portion at the tip. The C‐terminal region comprises 17 repeat blocks, each of ~99 aa residues (three incomplete), which form a long, flexible stalk, presenting the potential binding domain distal from the bacterial cell surface. In this study we present a structural analysis of CshA, derived from a combination of X‐ray crystallography, NMR, SAXS, and complementary biophysical methods. The data suggest that CshA has a novel structure not previously reported for bacterial cell surface adhesins.CshA binds the high‐molecular‐weight glycoprotein fibronectin (Fn) via the N‐terminal non‐repetitive region, and this protein‐protein interaction has been proposed to promote S. gordonii colonization at multiple sites within the host. However, the molecular details of how these two proteins interact had not previously been investigated. In vitro binding studies support a hitherto unreported two‐state ‘catch‐clamp’ mechanism of Fn binding by CshA. The disordered N‐terminal domain of CshA acts to ‘catch’ Fn, via formation of a rapidly assembled but also readily dissociable pre‐complex, enabling its neighbouring ligand binding domain to tightly ‘clamp’ the two polypeptides together. This study presents a new paradigm for target binding by a bacterial adhesin, the identification of which will inform future efforts towards the development of anti‐adhesive agents that target S. gordonii and related streptococci.Support or Funding InformationThis work was supported in part by National Institutes of Health grants DE016690 (to H. F. J. and R. J. L.) and DE012505 (to R. J. L.), Biotechnology and Biological Sciences Research Council Grant BB/I006478/1 (to P. R. R.), and Royal Society University Research Fellowship Award UF080534 (to P. R. R.). The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.