21003:

A chloroplast homologue of FtsY, cpSRP and GTP are required to reconstitute the soluble phase of LHCP transport into thylakoid membranes.

Authors:	Tu, Chao-Jung(A)Schuenemann, Danja(A)Hoffman, Neil, E.(A)
Affiliations:	(A): Department of Plant Biology, Carnegie Institution of Washington, Stanford, CA 94305 (B): Present Address. Paradigm Genetics, Research Triangle Park, NC 27709-4528
Presenter:	Hoffman, Neil E., nhoffman@paragen.com

Light harvesting chlorophyll proteins (LHCP) are the most abundant proteins of the thylakoid membrane. These hydrophobic proteins are nuclear encoded and are post-translationally targeted to the thylakoid membrane through the cytoplasm, the inter-envelope space, and the stroma. Integration of LHCP into the thylakoid membrane can be reconstituted in vitro by a process that requires proteins in the stroma, the thylakoid membrane, and GTP. We have now determined the soluble factors that are required for the efficient integration of LHCP into the thylakoid membrane. One factor is chloroplast signal recognition particle (cpSRP), which consists of 54 and 43 kD polypeptides. cpSRP is a specialized form of SRP, a cytoplasmic ribonucleoprotein that mediates the co-translational targeting of proteins to membranes. Although both types of SRP contain a 54 kD GTPase homologue, only the cytoplasmic form contains an RNA and only the chloroplast form contains the 43 kD subunit. The 43 kD protein is a dimer that acts as a scaffold to dimerize cpSRP54. Crosslinking studies reveal that cpSRP54 interacts directly with LHCP. As cpSRP54 has little to no ability to interact with LHCP in the absence of cpSRP43, the latter protein appears to enable cpSRP54 to bind LHCP post-translationally. LHCP and cpSRP interact to form a stable targeting intermediate termed transit complex. As transit complex is soluble, undoubtedly one function of cpSRP is to maintain the solubility of LHCP as it is transported through the aqueous phase. Transit complex is not sufficient for LHCP integration. The second factor required is a chloroplast homologue of FtsY. FtsY is a soluble E. coli protein that regulates the GTPase activity of SRP54 and is required for SRP mediated protein targeting. cpFtsY, together with cpSRP and GTP are necessary and sufficient to reconstitute the soluble phase of LHCP transport.