Chloroplast SRP43 and SRP54 independently promote thermostability and membrane binding of light-dependent protochlorophyllide oxidoreductases.

Protochlorophyllide oxidoreductase (POR), which converts protochlorophyllide into chlorophyllide, is the only light-dependent enzyme in chlorophyll biosynthesis. While its catalytic reaction and importance for chloroplast development are well understood, little is known about the post-translational control of PORs. Here, we show that cpSRP43 and cpSRP54, two components of the chloroplast signal recognition particle pathway, play distinct roles in optimizing the function of PORB, the predominant POR isoform in Arabidopsis. The chaperone cpSRP43 stabilizes the enzyme and provides appropriate amounts of PORB during leaf greening and heat shock, whereas cpSRP54 enhances its binding to the thylakoid membrane, thereby ensuring adequate levels of metabolic flux in late chlorophyll biosynthesis. Furthermore, cpSRP43 and the DnaJ-like protein CHAPERONE-LIKE PROTEIN of POR1 concurrently act to stabilize PORB. Overall, these findings enhance our understanding of the coordinating role of cpSPR43 and cpSRP54 in the post-translational control of chlorophyll synthesis and assembly of photosynthetic chlorophyll-binding proteins.