A novel signal recognition particle targets light-harvesting proteins to the thylakoid membranes

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 95; no. 17; pp. 10312 - 10316
• Main Authors: Schuenemann, D. (Carnegie Institution of Washington, Stanford, CA.), Gupta, S, Persello-Cartieaux, F, Klimyuk, V.I, Jones, J.D.G, Nussaume, L, Hoffman, N.E
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences of the United States of America 18-08-1998; National Acad Sciences; National Academy of Sciences; The National Academy of Sciences
• Subjects: Antibodies; BINDING; Biological Sciences; Chlorophylls; CHLOROPLASTE; CHLOROPLASTS; Clines; CLOROPLASTO; Cytoplasm; Flowers & plants; Gels; LIGHT; LIGHT HARVESTING COMPLEXES; LUMIERE; LUZ; Membranes; METABOLISME DES PROTEINES; METABOLISMO PROTEICO; NUCLEOPROTEINAS; NUCLEOPROTEINE; NUCLEOPROTEINS; PEPTIDE; PEPTIDES; PEPTIDOS; PISUM SATIVUM; POLYPEPTIDES; PROTEIN METABOLISM; PROTEIN TRANSPORT; PROTEINAS; PROTEINE; PROTEINS; RIBONUCLEOPROTEINS; Ribosomes; RNA; THYLAKOIDE; THYLAKOIDS; TILACOIDES
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.95.17.10312

The mechanisms involved in the posttranslational targeting of membrane proteins are not well understood. The light-harvesting chlorophyll proteins (LHCP) of the thylakoid membrane are a large family of hydrophobic proteins that are targeted in this manner. They are synthesized in the cytoplasm, translocated across the chloroplast envelope membranes into the stroma, hound by a stromal factor to form a soluble intermediate, "transit complex", and then integrated into the thylakoid membrane by a GTP dependent reaction. Signal recognition particle (SRP), a cytoplasmic ribonucleoprotein, is known to mediate the GTP dependent cotranslational targeting of proteins to the endoplasmic reticulum. We show that chloroplasts contain an SRP consisting of, cpSRP54, a homologue of SRP54 and a previously undescribed 43-kDa polypeptide (cpSRP43) instead of an RNA. We demonstrate that both subunits of cpSRP are required for the formation of the transit complex with LHCP. Furthermore, cpSRP54, cpSRP43, and LHCP are sufficient to form a complex that appears to be identical to authentic transit complex. We also show that the complex formed between LHCP and cpSRP, together with an additional soluble factor(s) are required for the proper integration of LHCP into the thylakoid membrane. It appears that the expanded role of cpSRP in posttranslational targeting of LHCP has arisen through the evolution of the 43-kDa protein