Comparative structural and functional analysis of the glycine-rich regions of Class A and B J-domain protein cochaperones of Hsp70.

J-domain proteins are critical Hsp70 co-chaperones. A and B types have a poorly understood glycine-rich region (G rich ) adjacent to their N-terminal J-domain (J dom ). We analyzed the ability of J dom /G rich segments of yeast Class B Sis1 and a suppressor variant of Class A, Ydj1, to rescue the inviability of sis1-∆. In each, we identified a cluster of G rich residues required for rescue. Both contain conserved hydrophobic and acidic residues and are predicted to form helices. While, as expected, the Sis1 segment docks on its J-domain, that of Ydj1 does not. However, data suggest both interact with Hsp70. We speculate that the G rich -Hsp70 interaction of Classes A and B J-domain proteins can fine tune the activity of Hsp70, thus being particularly important for the function of Class B.