Structure of chloroplast signal recognition particle and its role in chloroplast biogenesis

Abstract

Signal recognition particle (SRP) is a cytoplasmic ribonucleoprotein that facilitates the co-translational insertion of proteins into the endoplasmic reticulum in eukaryotes and the cytoplasmic membrane in prokaryotes. In prokaryotes, the SRP-RNA binds a single 54 kD polypeptide subunit, while in eukaryotes, five additional subunits are bound. A homologue of the 54 kD subunit of SRP was found in chloroplasts; it is 44% and 27% identical to SRP54 from E. coli and dog, respectively (1). This paper describes our progress in determining the structure of cpSRP and its role in the chloroplast. In particular we find that cpSRP, comprised of 43 and 54 kD subunits, plays a role in the biogenesis of light harvesting chlorophyll proteins (LHCP), the major proteins of the thylakoid membrane. LHCP form a large family of related proteins that have three to four transmembrane domains. They are synthesized in the cytoplasm, and are targeted to the thylakoid membrane through the stroma by a post-translational mechanism. Evidence is also presented that cpSRP54 functions independently of cpSRP43 in the co-translational targeting of chloroplast encoded proteins.