Client Specificity of an ATP-independent Chaperone is Regulated by a Temperature Sensitive Switch.

The chloroplast signal recognition particle 43 (cpSRP43) is an ATP-independent chaperone that delivers the light-harvesting chlorophyll-binding proteins (LHCPs) to the thylakoid membrane while also maintaining the levels of tetrapyrrole biosynthesis (TBS) proteins. How cpSRP43 coordinates these roles to protect two very different classes of clients has remained unclear. We show that cpSRP43 samples two distinct conformations: a well-folded closed state that protects unfolded membrane protein LHCPs through contacts with its substrate binding domain (SBD) and an open state with a partially disordered SBD that is less active towards LHCP but remains effective at protecting TBS proteins from heat-induced aggregation. Binding of the additional cpSRP54 subunit stimulates cpSRP43's chaperone activity towards LHCPs by stabilizing the closed state but inhibits its chaperone activity towards TBS proteins. Strikingly, the cpSRP43-cpSRP54 binding affinity is greatly weakened at high temperatures. Taken together, this shows that temperature can direct cpSRP43 towards one of these two client pathways. Under normal conditions, most cpSRP43 is tightly bound to cpSRP54 in the closed state and is primarily dedicated to the protection and delivery of LHCPs. However during heat stress, as chloroplast proteins become increasingly susceptible to heat-induced aggregation, cpSRP43 dissociates from cpSRP54 and is rapidly repurposed into a stress-responsive chaperone for TBS proteins.