Heat shock proteins (HSPs) are conserved, ubiquitous molecular chaperones that play various roles in preserving proteostasis, particularly in response to cellular stressors (e.g., heat, oxidation). In addition, their dysregulation is linked to various diseases, including neurodegeneration, cancer, and fungal infections. HSPs are generally grouped by molecular weight into families with related functions. Members of the 70-kDa family (Hsp70s) are central, energy-driven chaperones that bind and release clients in a cyclical manner dependent on ATP hydrolysis, facilitated by multiple co-chaperones such as nucleotide exchange factors (NEFs). In addition to solubilizing protein aggregates, members of the 110-kDa family (Hsp110s) constitute the primary eukaryotic NEFs in the cytosol and nucleus, accelerating ADP release to promote refolding. Because most mechanistic models derive from fungal proteins, we biochemically and structurally analyzed homologous human and fungal Hsp70-Hsp110 machineries to identify points of divergence. We confirmed that complex formation and accelerated nucleotide exchange for the fungal pair is ATP-dependent. By contrast, the human pair displays distinct nucleotide requirements and is uniquely stabilized upon assembly. Cross-pairing (human-fungal mismatches) suggests that the nucleotide preference is not intrinsic to either subunit but likely emerges from the matched human Hsp70-Hsp110 interface. Furthermore, substrate binding biases Hsp70 away from self-association toward heterodimer-competent states with Hsp110, but effects differ across native and mismatched pairs. Structural analyses of the human and fungal complexes support the biochemical observations and provide insight into human-specific aspects of Hsp110 NEF activity. Taken together, this data support species-dependent mechanisms for Hsp70-Hsp110 machineries and motivates strategies to selectively target fungal proteostasis for antifungal development.
Kidd et al. (Sun,) studied this question.