The signal recognition particle (SRP) binds to the signal peptide of proteins as they are being translated. The binding of the SRP halts translation and the complex is then transported to the endoplasmic reticulum's cytoplasmic surface. The SRP then aids translocation of the protein through the ER membrane. The SRP is a ribonucleoprotein that is composed of a small RNA and several proteins. One of these proteins is the SRP19 protein  (Sec65 in yeast [2,3]).
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 [PUBMED:17622352, PUBMED:16469117]. 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 [PUBMED:12605305]. 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 [PUBMED:17507650]. 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 [PUBMED:12364595].
Lingelbach K, Zwieb C, Webb JR, Marshallsay C, Hoben PJ, Walter P, Dobberstein B; , Nucleic Acids Res 1988;16:9431-9442.: Isolation and characterization of a cDNA clone encoding the 19 kDa protein of signal recognition particle (SRP): expression and binding to 7SL RNA. PUBMED:2460823 EPMC:2460823 .