Involved in targeting and insertion of nascent membrane proteins into the cytoplasmic membrane. Binds to the hydrophobic signal sequence of the ribosome-nascent chain (RNC) as it emerges from the ribosomes. The SRP-RNC complex is then targeted to the cytoplasmic membrane where it interacts with the SRP receptor FtsY. Interaction with FtsY leads to the transfer of the RNC complex to the Sec translocase for insertion into the membrane, the hydrolysis of GTP by both Ffh and FtsY, and the dissociation of the SRP-FtsY complex into the individual components. Conformation of the Ffh-FtsY complex and regulation of its GTPase activity are modulated by the 4.5S RNA. Formation of the FfH-FtsY complex leads to a mutual stimulation of both GTPases. Part of the signal recognition particle protein translocation system, which is composed of SRP and FtsY. SRP is a ribonucleoprotein composed of Ffh and a 4.5S RNA molecule. Metal ions are essential for the formation and stability of the SRP complex. Interacts with the ribosomes, via ribosomal protein L23. Interacts with FtsY. The SRP-RNC complex is targeted to the cytoplasmic membrane. Composed of three domains: the N-terminal N domain, which is responsible for interactions with the ribosome, the central G domain, which binds GTP, and the C-terminal M domain, which binds the RNA and the signal sequence of the RNC. Essential; deletion experiments lead to loss of inner membrane protein targeting. Also leads to reduced targeting of lipoproteins Lpp and BRP. Belongs to the GTP-binding SRP family. SRP54 subfamily. Truncated N-terminus.