SRP was first identified in mammalian cells in the early 1980s as a soluble complex of six polypeptides and a 300 nucleotide RNA that targets proteins to the endoplasmic reticulum (ER) 2.

Before the translocation of the pre-protein across the ER membrane, a ribonucleoprotein called signal recognition particle (SRP) binds to the signal peptide emerging from the ribosome. Then the ...