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).