Signal recognition particle receptor protein. Binds GTP specifically. The GTPase activity is inhibited by the N-terminus of the protein until binding to the thylakoid membrane. Activates the GTPase activity of FFC/cpSRP54 when bound to the cpSRP complex. Required for light-harvesting chlorophyll a/b-binding protein (LHCP) integration into thylakoids. Might be also functionally linked to the Sec translocation machinery. Monomer. Interacts with FFC/cpSRP54, a component of the cpSRP complex, composed of a FFC/cpSRP54 monomer and a CAO/cpSRP43 dimer. The complex with FFC/cpSRP54 is formed when both proteins are bound with GTP. The membrane-binding domain is capable of partial insertion into the lipid bilayer. Expressed in green tissues. Low levels in roots and seeds. Peak of expression 20 days after germination. Not induced by light or iron. The N-terminal domain (39-56) is necessary and sufficient for thylakoid binding. Seedling lethal. Unlike eukaryotic or prokaryotic signal recognition particle (SRP), the chloroplast SRP from higher plants lacks an SRP-RNA component. It targets both chloroplast-encoded and nucleus-encoded substrates to the thylakoid membrane, post-translationally for the nucleus-encoded proteins and co-translationally for the chloroplast-encoded proteins. Belongs to the GTP-binding SRP family.