Novel Insights into the Human Hsc70 Structure by Cross-Linking Mass Spectrometry and Molecular Modeling.

Heat shock cognate protein 70 (Hsc70) is a 71 kDa molecular chaperone belonging to the Hsp70 family of heat shock proteins. These proteins act as ATP-dependent molecular machines that assist protein folding under both physiological and stress conditions such as hypoxia, heat shock, and pH fluctuations. In addition to general chaperone functions, Hsc70 performs specialized roles, including uncoating clathrin-coated vesicles, facilitating protein transport into organelles, and targeting proteins for lysosomal degradation. Members of the Hsp70 family are known to form dimers and higher oligomers, but the structural organization and functional relevance of these assemblies remain poorly understood. Earlier studies also suggested that J-domain proteins (JDPs) can promote Hsp70 dimerization. In this study, we used chemical cross-linking, high-resolution Fourier transform mass spectrometry (FTMS), 15N isotopic labeling, and advanced data analysis to investigate the structural organization of Hsc70 dimers. Cross-link-derived distance restraints enabled structural modeling of Hsc70 monomers and dimers using AlphaLink2. Our results reveal distinct ATP- and ADP-state dimer conformations that coexist in equilibrium. In the presence of the cochaperone DnaJB1, we observed a shift in the dimer-monomer equilibrium, accompanied by enhanced ATP hydrolysis and formation of intermediate species. These findings demonstrate that the Hsc70 dimer population is structurally heterogeneous and depends on nucleotide state and cochaperone interactions.