The L18 Domain of Light-harvesting Chlorophyll Proteins Binds to Chloroplast Signal Recognition Particle 43

Chloroplast signal recognition particle (cpSRP) is a novel type of SRP that contains a homolog of SRP54 and a 43-kDa subunit absent from all cytoplasmic SRPs but lacks RNA. It is also distinctive in its ability to post-translationally interact with light-harvesting chlorophyll proteins (LHCP), hydrophobic proteins synthesized in the cytoplasm and targeted to the thylakoid via the stroma. LHCP integration into thylakoid membranes requires the two subunits of cpSRP, cpFtsY, GTP, and the membrane protein ALB3. It had previously been shown that the L18 domain, an 18-amino acid peptide between the second and third transmembrane domains, and a hydrophobic domain are required for interaction with cpSRP. In the present study we used a pull-down assay, with cpSRP43 or cpSRP54 fused to glutathione-transferase, to study interactions between cpSRP43, cpSRP54, LHCP, and cpFtsY. cpFtsY was not observed to form significant interactions with any of the proteins even in the presence of nonhydrolyzable GTP analogs. Our data indicate that cpSRP43 binds to the L18 domain, that cpSRP54 binds to the hydrophobic domain, and that LHCP and cpSRP54 independently bind to cpSRP43. These data confirm that the novel post-translational interaction between LHCP and cpSRP is mediated through binding to cpSRP43.