InterPro domain: IPR009018

The signal recognition particle (SRP) is a multimeric protein, which along with its conjugate receptor (SR), is involved in targeting secretory proteins to the rough endoplasmic reticulum (RER) membrane in eukaryotes, or to the plasma membrane in prokaryotes [ 1 , 2 ]. SRP recognises the signal sequence of the nascent polypeptide on the ribosome. In eukaryotes this retards its elongation until SRP docks the ribosome-polypeptide complex to the RER membrane via the SR receptor [ 3 ]. Eukaryotic SRP consists of six polypeptides (SRP9, SRP14, SRP19, SRP54, SRP68 and SRP72) and a single 300 nucleotide 7S RNA molecule. The RNA component catalyses the interaction of SRP with its SR receptor [ 4 ]. In higher eukaryotes, the SRP complex consists of the Alu domain and the S domain linked by the SRP RNA. The Alu domain consists of a heterodimer of SRP9 and SRP14 bound to the 5' and 3' terminal sequences of SRP RNA. This domain is necessary for retarding the elongation of the nascent polypeptide chain, which gives SRP time to dock the ribosome-polypeptide complex to the RER membrane. In archaea, the SRP complex contains 7S RNA like its eukaryotic counterpart, yet only includes two of the six protein subunits found in the eukarytic complex: SRP19 and SRP54 [ 5 ].

This superfamily represents both the 9kDa SRP9 and the 14kDa SRP14 components. Both SRP9 and SRP14 have the same (beta)-alpha-beta(3)-alpha fold. The heterodimer has pseudo two-fold symmetry and is saddle-like, consisting of a curved six-stranded beta-sheet that has four helices packed on the convex side and an exposed concave surface lined with positively charged residues. The SRP9/SRP14 heterodimer is essential for SRP RNA binding, mediating the pausing of synthesis of ribosome associated nascent polypeptides that have been engaged by the targeting domain of SRP [ 6 ].

1. X-ray structures of the signal recognition particle receptor reveal targeting cycle intermediates. PLoS ONE 2, e607
2. Human autoantibodies against the 54 kDa protein of the signal recognition particle block function at multiple stages. Arthritis Res. Ther. 8, R39
3. Signal recognition particle-dependent protein targeting, universal to all kingdoms of life. Rev. Physiol. Biochem. Pharmacol. 146, 55-94
4. The signal recognition particle (SRP) RNA links conformational changes in the SRP to protein targeting. Mol. Biol. Cell 18, 2728-34
5. Reconstitution of the signal recognition particle of the halophilic archaeon Haloferax volcanii. Nucleic Acids Res. 30, 4166-75
6. Human signal recognition particle (SRP) Alu-associated protein also binds Alu interspersed repeat sequence RNAs. Characterization of human SRP9. J. Biol. Chem. 270, 10179-86