Substrate binding site flexibility of the small heat shock protein molecular chaperones

Small heat shock proteins (sHSPs) serve as a first line of defense against stress-induced cell damage by binding and maintaining denaturing proteins in a folding-competent state. In contrast to the well-defined substrate binding regions of ATP-dependent chaperones, interactions between sHSPs and substrates are poorly understood. Defining substrate-binding sites of sHSPs is key to understanding their cellular functions and to harnessing their aggregation-prevention properties for controlling damage due to stress and disease. We incorporated a photoactivatable cross-linker at 32 positions throughout a well-characterized sHSP, dodecameric PsHsp18.1 from pea, and identified direct interaction sites between sHSPs and substrates. Model substrates firefly luciferase and malate dehydrogenase form strong contacts with multiple residues in the sHSP N-terminal arm, demonstrating the importance of this flexible and evolutionary variable region in substrate binding. Within the conserved alpha-crystallin domain both substrates also bind the beta-strand (beta 7) where mutations in human homologs result in inherited disease. Notably, these binding sites are poorly accessible in the sHSP atomic structure, consistent with major structural rearrangements being required for substrate binding. Detectable differences in the pattern of cross-linking intensity of the two substrates and the fact that substrates make contacts throughout the sHSP indicate that there is not a discrete substrate binding surface. Our results support a model in which the intrinsically-disordered N-terminal arm can present diverse geometries of interaction sites, which is likely critical for the ability of sHSPs to protect efficiently many different substrates.