The True Tales of the Flexible Tails - Interaction of J-domain Proteins with Hsp70 Chaperones

Hsp70s are ubiquitous molecular chaperones tasked with safeguarding proteins throughout their entire lifecycle, from synthesis to degradation, and are thus critical for maintaining protein homeostasis. The ATPase cycle of Hsp70s is regulated by a large set of dedicated co-chaperones consisting of nucleotide exchange factors and of J-domain proteins (JDPs). Humans contain multiple such JDPs, all of which interact through their highly conserved J-domain with Hsp70s in an ATP dependent manner. The most abundant JDPs in the cell typically belong to the very structurally-similar class-A and class-B families. Despite their many structural similarities, however, class A and B JDPs still exhibit significant functional differences.

Here we used NMR to uncover a new autoinhibitory mechanism, present only in class B JDPs, that prevents these proteins from premature activation of the Hsp70 chaperones. We have further found that this mechanism is regulated by the interaction of Hsp70 with an additional site located in the C-terminal substrate binding domain I (CTDI) of DnaJB1. This binding of the Hsp70 C-terminal disordered region to DnaJB1 CTDI causes a conformational change that exposes the DnaJB1 J-domain for Hsp70 binding, with only the second interaction leading to activation of Hsp70 catalytic activity. This, then points to a new regulatory interaction between class-B DnaJ chaperones and Hsp70, that is absent in class A DnaJs. We further show that this regulation is essential for DnaJB1-Hsp70 function in disaggregation of amyloid fibers, but not in protein refolding, pointing to a new mechanism which dictates the functional specificities of the JDP co-chaperones.