Phosphorylation-dependent regulation of Hsp70 chaperones stimulates client recruitment

Molecular chaperones of the Hsp70 family play essential roles in maintaining proteostasis, particularly under conditions of cellular stress. Posttranslational modifications of Hsp70, collectively termed the chaperone code, are emerging as critical regulators of chaperone function, yet their mechanistic contributions remain incompletely understood. Here, we investigate the functional significance of a conserved phosphorylation site in Hsp70, corresponding to serine 326 in yeast Ssa1 and serine 329 in human HSPA8. We demonstrate that DNA double-strand break stress increases cellular reliance on Hsp70 activity in yeast, highlighting a role for chaperones in the DNA damage response. Loss of Ssa1 serine 326 phosphorylation impairs multiple Hsp70-dependent functions, including prion sequestration and glucocorticoid receptor maturation, indicating that this modification is required for optimal chaperone activity. Extending these findings to human cells, we show that the homologous HSPA8 serine 329 residue is necessary for efficient clearance of polyglutamine aggregates. Mechanistically, a phospho-deficient HSPA8-S329A mutant exhibits reduced client-binding capacity. Together, our findings identify a conserved phosphorylation event that enhances Hsp70 function by promoting client engagement, providing new insight into how the chaperone code regulates proteostasis across species.