The signal recognition particle (SRP) is a protein-RNA complex that recognizes and transports specific proteins to the endoplasmic reticulum in eukaryotes and the plasma membrane in prokaryotes. The eukaryotic SRP is composed of six distinct polypeptides bound to an RNA molecule (the 7SL RNA), with GTPase activity. In the GTP-bound state, it recognizes an endoplasmic reticulum signal sequence of eight or more nonpolar amino acid residues at its center. It transiently binds to the endoplasmic reticulum signal sequence in a nascent protein, to the large ribosomal unit, and to the SRP receptor. The core of the SRP is universal, being conserved in all three kingdoms.
The SRP is found in the cytosol. In eukaryotes, it binds to the endoplasmic reticulum signal sequence found in an emerging secretory protein and subsequently delivers the protein along with the ribosome creating it to the endoplasmic reticulum membrane. When SRP binds the endoplasmic reticulum signal sequence, it also binds the ribosome and thereby pauses translation until the entire SRP-bound translation complex is transported to the endoplasmic reticulum, where translation resumes.
SRP and its receptor initiate the transfer of the nascent chain across the endoplasmic reticulum membrane. Once the chain reaches the translocon, SRP and its receptor both hydrolyze their bound GTP to GDP, then dislocate allowing translation to restart
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