Single-molecule spectroscopy reveals chaperone-mediated expansion of substrate protein

Significance Molecular chaperones are a group of proteins that are essential for avoiding the aggregation of other proteins in the crowded cellular environment. Chaperones function by interacting with these substrate proteins in different ways. However, it has remained a challenge to measure the changes that occur in the substrate proteins and understand how these changes prevent misfolding or aggregation. Here we investigate a chaperone system that keeps the substrate protein denatured by clamping the polypeptide chain. We observe an expansion of the substrate protein chain up to 30-fold in volume owing to steric repulsion between multiple copies of the chaperone bound to a single substrate protein. In this way, unwanted interactions within or between substrate proteins can be prevented.