The Folding of SasG: A Long and Remarkably Strong Monomeric Protein Responsible for Biofilm Formation is a Highly Cooperative System

Abstract

SasG has a long repeat region made up of identical repeating E and G5 domains. Although the domains themselves are relatively unstable (indeed E domains on their own are unfolded), the cooperative folding of the domains results in formation of molecules that are long and remarkably mechanically resistant. We have used small angle X-ray scattering and mechanical unfolding methods, combined with simulations, to show that SasG constructs of physiological length are indeed monomeric, highly extended and mechanically strong. Obligate folding cooperativity of the intrinsically disordered E domain couples spatially separate G5 domains both thermodynamically and structurally, creating a superstructure that supersedes the domain architecture. Our findings provide a simple solution for the efficient assembly of mechano-resistant elongated structures of tunable length from a single polypeptide chain and have significant potential for the development of novel biomaterials.