Protein Targeting to the Albino3 Insertase

Abstract

The chloroplast thylakoid membrane is the site of photosynthetic light capture and oxygenic electron transport. Nuclear encoded light harvesting chlorophyll‐binding proteins (LHCPs), which bind chlorophyll a/b, provide much of the light absorbing capacity in plants. Following their import into the chloroplast, LHCPs are routed to the thylakoid where they are inserted into the lipid bilayer by the Albino3 (Alb3) insertase. Intra‐chloroplast targeting of LHCPs to Alb3 is accomplished by a chloroplast signal recognition particle (cpSRP), which binds LHCPs in the soluble stroma and directs them to the membrane where they are thought to be passed to Alb3 for post‐translational insertion. CpSRP is composed of a conserved 54 kD subunit (cpSRP54) and a chloroplast‐specific 43 kD subunit (cpSRP43). Data from protein localization and protein interaction assays shows that cpSRP43 binding to Alb3 is critical for LHCP localization. Alb3 is also thought to function in the co‐translational insertion of chloroplast synthesized proteins by a co‐translational targeting/insertion mechanism. This presents the possibility that the post‐ and co‐translational targeting pathways converge at Alb3 for insertion into the membrane. Results of studies presented here may provide an ability to understand how the relative level of post‐ and co‐translational insertion activity by Alb3 is regulated during thylakoid biogenesis.