Switchable client specificity in a dual functional chaperone coordinates light-harvesting complex biogenesis

The proper assembly of light-harvesting complexes (LHCs) is critical for photosynthesis and requires the biogenesis of light-harvesting chlorophyll a,b-binding proteins (LHCPs) to be coordinated with chlorophyll (Chl) biosynthesis. The mechanism underlying this coordination is not well understood. Here, we show that a conserved molecular chaperone, chloroplast signal recognition particle 43-kDa protein (cpSRP43), provides a molecular thermostat that helps maintain this coordination. cpSRP43 undergoes a conformational rearrangement between a well-folded closed state and a partially disordered open state. Closed cpSRP43 is dedicated to the biogenesis of LHCPs, whereas open cpSRP43 protects multiple Chl biosynthesis enzymes from heat-induced destabilization. Rising temperature shifts cpSRP43 to the open state, enabling it to protect heat-destabilized Chl biosynthesis enzymes. Our results reveal the molecular basis of a posttranslational mechanism for the thermoadaptation of LHC biogenesis. They also demonstrate how an adenosine triphosphate-independent chaperone uses conformational dynamics to switch its activity and client selectivity, thereby adapting to different proteostatic demands under shifting environmental conditions.