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. Part of the signal recognition particle protein translocation system, which is composed of SRP and 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. Belongs to the GTP-binding SRP family. SRP54 subfamily.