Are all Hsp70 chaperones created equally?

Hsp70 chaperones are essential proteins that have a pivotal role in protein folding, refolding and disaggregation. The Hsp70s activity is regulated by Hsp40 co-chaperones, also known as J-domain proteins, which deliver clients to Hsp70s and catalyze their ATP hydrolysis. Hsp40 proteins share a highly conserved J-domain that binds Hsp70s and is sufficient for their activation, but vary in their diverse client binding sites. Together, Hsp70 and Hsp40 form a chaperon system common in all kingdoms of life.

In humans, there are several different Hsp70 genes, and over 50 Hsp40 genes which encode for different isoforms; some are constitutively expressed, while others are stress-induced. Just the cytosol contains the constitutively expressed HSPA8 and three distinct stress-induced Hsp70s (HSPA1A, HSPA1B and HSPA6). However, it is still unclear whether they are redundant or have distinct functions under stress. As the specificity of Hsp70 is predetermined by their interaction with the Hsp40 co-chaperones, we characterize the Hsp40-Hsp70 complexes affinity and activity focusing on two stress-induced Hsp70s (HSPA1A, HSPA6), compared to the constitutively expressed HSPA8.

We characterized the binding of various Hsp40s to the chosen Hsp70s, and found that the binding sites with Hsp40 J-domains are highly conserved. However, the affinities of these interactions varied, suggesting that even within the highly conserved Hsp70 family there is functional differentiation. The most striking differences were observed for HSPA6, which interacted strongly only with a small subset of Hsp40s (DNAJB1, DNAJB4). Thus, HSPA6 may have evolved to maintain specific critical functions under severe stress conditions.