Involved in cotranslational and post-translational sorting of thylakoid proteins. Binds GTP specifically. Activates the GTPase activity of CPFTSY when bound together. Required for light-harvesting chlorophyll a/b-binding protein (LHCP) integration into thylakoids. Component of the cpSRP complex, composed of a FFC/cpSRP54 monomer and a CAO/cpSRP43 dimer. Interacts (via C-terminus) with CAO/cpSRP43 and with CPFTSY. Interacts with the third transmembrane domain of LHCP. Interacts with the PSBA/D1 nascent chain, but only when it is attached to the 70S ribosome and if it is shorter than 189 amino acid residues. Most abundant in green shoot tissue and lower levels seen in the roots and etiolated buds. The M-domain stimulates the interaction between CPFTSY and the chloroplast signal recognition particle (cpSRP). The C-terminal residues (539-564) are required for interaction with CAO/cpSRP43. Pale green with delayed development. 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. SRP54 subfamily.