It exists in cytoplasm and is a slender protein containing RNA. The signal recognition particle SRP consists of six polypeptide chains (SRP54, SRP 19, SRP68, SRP72, SRP9, SRP 14) and 7RNA, with more than 300 bases. The two opposite ends of SRP can be divided into two main functional units, namely, S area and Alu area. S region includes SRP 19, SRP54, SRP68 and SRP72, and its function is to identify signal sequences and interact with SR. Alu region includes heterodimers composed of SRP9 and SRP 14, and its main functions are translation and extended arrest. P54 includes three functional domains, which are called N, G and M respectively. At its C-terminal, it is rich in methionine residues and has a signal sequence binding site. P68 and P72 play an important role in protein translocation, while P9 and P 14 are located at the other end of SRP, which can prevent the newly synthesized peptide chain from extending. In the middle of SRP, near P68, RNA has the site of ribonuclease digestion. The structural composition of SRP is the molecular basis of its function.