Collagen-like Motifs of SasG: A Novel Fold for Protein Mechanical Strength

The Staphylococcus aureus surface protein G (SasG) is associated with host colonisation and biofilm for-mation. As colonisation occurs at the liquid-substrate interface bacteria are subject to a myriad of external forces and, presumably as a consequence, SasG displays extreme mechanical strength. This mechanical phenotype arises from the B-domain; a repetitive region composed of alternating E and G5 subdomains. These subdomains have an unusual structure comprising collagen-like regions capped by triple-stranded fl-sheets. To identify the determinants of SasG mechanical strength, we characterised the mechanical phenotype and thermodynamic stability of 18 single substitution variants of a pseudo-wildtype protein. Visualising the mechanically-induced transition state at a residue-level by /-value analysis reveals that the main force-bearing regions are the N-and C-terminal 'Mechanical Clamps' and their side-chain inter-actions. This is tailored by contacts at the pseudo-hydrophobic core interface. We also describe a novel mechanical motif - the collagen-like region and show that glycine to alanine substitutions, analogous to those found in Osteogenesis Imperfecta (brittle bone disease), result in a significantly reduced mechanical strength.(c) 2023 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecom-mons.org/licenses/by/4.0/).

Automated summary

The Staphylococcus aureus surface protein G (SasG) is associated with host colonization and biofilm formation. It shows extreme mechanical strength due to its unique structure and interactions. The main force-bearing regions of the protein are the N- and C-terminal 'Mechanical Clamps' and their side-chain interactions, as revealed by residue-level analysis.