Signal recognition particle: a ribonucleoprotein required for cotranslational translocation of proteins, isolation and properties.

Microsomal membranes (RM) l lose their ability to translocate nascent secretory, 2,3 lysosomal, 4 and membrane s proteins upon salt extraction if they are assayed in the wheat germ cell-free translation system. From the salt extract, a ribonucleoprotein [termed signal recognition particle (SRP)] was purified to homogeneity and shown to be composed of six different polypeptide chains 3 and one 7 S RNA molecule.6 Addition of the crude salt extract or purified SRP to salt-extracted RM (K-RM) restored their ability not only to catalyze translocation of secretory 2,3 and lysosomal 4 proteins across the microsomal membrane, but also to catalyze the asymmetric integration of transmembrane proteins s into the membrane. The function of SRP in the translocation process was shown to involve the recognition of the signal sequence of these proteins in their nascent state, 7 the specific binding of polysomes synthesizing these proteins to RM vesicles, 8 and the initiation of the translocation event. In the absence