HSPB5 disease-associated mutations have long-range effects on structure and dynamics through networks of quasi-ordered interactions

Small heat shock proteins (sHSPs) are chaperones whose importance in protein homeostasis is exemplified by dozens of missense mutations associated with tissue-specific disease states. Despite decades of studies, the structure, dynamics, and mechanism of chaperone activity remain unclear. Here we show that the human sHSP HSPB5 distinguishes native lens protein γD-crystallin from damaged γD-crystallin even though the mutant/damaged client is folded. The disordered N-terminal region of HSPB5 (NTR) is essential for its chaperone activity, whereas the structured domain (ACD) has no intrinsic activity. Nevertheless, two sHSP mutational hotspots associated with disease, D109 and R120, are located in the ACD. Our studies on wild-type HSPB5 oligomers reveal that distinct regions within the NTR interact with specific grooves presented on the ACD dimer and/or with other NTR sub-regions and that the number of binding partners is greater than the number of binding sites, leading to a large, but finite number of potential combinations of interactions at any given time. The ACD mutations result in increased dynamics and accessibility of the disordered NTR and enhanced chaperone activity in vitro. Our findings reveal that HSPB5 quasi-order is delicately balanced and that perturbations arising from mutations within the structured core cause alterations that contribute to misbalance in eye lens protein homeostasis that lead to cataract formation.