Characterization of mutations in small heat shock proteins that influence substrate binding

The small heat shock proteins (sHsp), ubiquitous cellular homeostasis machinery, are oligomeric chaperone proteins. sHsps play an important role in maintaining cell function and survival under stress conditions such as high temperatures. By binding to non‐native proteins in an ATP‐independent manner, sHsps effectively prevent harmful aggregation of denatured proteins, promote proper protein folding, or facilitate protein degradation. Defects in sHsps can result in abnormal accumulation of proteins or aberrant protein folding, which lead to many pathologies such as cataract, tumor and neurodegenerative diseases including Parkinson's disease and Alzheimer's disease. Structural studies have demonstrated that sHsps contain a highly conserved α‐crystallin domain flanked by variable N‐terminal and C‐terminal regions. They usually exist in large oligomers and are capable of adopting different oligomeric states in response to different conditions. Therefore, because of their dynamic structures, the specific mechanisms of how sHsp bind to substrate and function are still not well understood. Nonetheless, it is suggested that the variable N‐terminal region of the protein contributes to substrate binding and oligomerization. We have made several mutations in the sHsps to characterize regions that influence oligomerization and make contacts with substrate proteins. Our studies suggest that the hydrophobic regions of the N‐terminal domain are important for substrate‐binding. Additionally, mutations in the a‐crystallin domain also contribute to the dynamics of protein oligomerization and sHsp‐client interactions.