Heterogeneous binding of the SH3 client protein to the DnaK molecular chaperone

The molecular chaperone heat shock protein 70 (Hsp70) plays a vital role in cellular processes, including protein folding and assembly, and helps prevent aggregation under physiological and stressrelated conditions. Although the structural changes undergone by full-length client proteins upon interaction with DnaK (i.e., Escherichia coli Hsp70) are fundamental to understand chaperone-mediated protein folding, these changes are still largely unexplored. Here, we show that multiple conformations of the SRC homology 3 domain (SH3) client protein interact with the ADP-bound form of the DnaK chaperone. Chaperone-bound SH3 is largely unstructured yet distinct from the unfolded state in the absence of DnaK. The bound client protein shares a highly flexible N terminus and multiple slowly interconverting conformations in different parts of the sequence. In all, there is significant structural and dynamical heterogeneity in the DnaK-bound client protein, revealing that proteins may undergo some conformational sampling while chaperonebound. This result is important because it shows that the surface of the Hsp70 chaperone provides an aggregation-free environment able to support part of the search for the native state.