Characterization of the Relationship between the Chaperone and Lipid-Binding Functions of the 70-Kda Heat-Shock Protein, HspA1A

HspA1A is a molecular chaperone that plays indispensable roles in cellular survival. HspA1A also translocates to the plasma membrane (PM) of stressed and cancer cells. This translocation results in the cell-surface presentation of HspA1A rendering these tumors radiation insensitive. Thus, a putative therapeutic would be to inhibit HspA1A’s PM translocation. However, to specifically stop the PM translocation of HspA1A, which is lipid-driven, it is imperative to characterize the lipid-binding regions of HspA1A and the relationship between the chaperone and lipid-binding functions of HspA1A, which remain unknown. To elucidate this relationship, we determined the effect of binding to phosphatidylserine (PS) on the secondary structure and chaperone functions of HspA1A. Circular dichroism revealed that binding to PS had minimal alterations on HspA1A’s secondary structure. Measuring the release of inorganic phosphate revealed that PS-binding had no effect on the ATPase activity of HspA1A. In contrast, PS-binding showed subtle but consistent increases in the refolding activities of HspA1A. These observations strongly support the notion that the chaperone and lipid-binding activities of HspA1A are dependent but the regions mediating these functions do not overlap. These findings provide the basis for future interventions to inhibit HspA1A’s PM-translocation in tumor cells, making them sensitive to radiation therapy.