A COMPLEX OF THE SIGNAL SEQUENCE BINDING-PROTEIN AND THE SRP RNA PROMOTES TRANSLOCATION OF NASCENT PROTEINS

Translocation of proteins across the endoplasmic reticulum membrane is initiated by the signal recognition particle (SRP), a cytoplasmic rib onucleoprotein complex consisting of a 7S RNA and six polypeptides, To investigate the functions of the SRP components, we have tested the a ctivities of several SRP subparticles, We show that the SRP GTPase (SR P54) alone binds a signal sequence and discriminates it from a non-sig nal sequence. Although SRP54 alone is unable to promote translocation, SRP54 in a complex with SRP RNA is both necessary and sufficient to p romote translocation of an elongation-arrested nascent protein in a GT P-regulated manner, For co-translational translocation, additional SRP components are required. We discuss the implications of our results f or the function of the Escherichia coli SRP which is homologous to the SRP54/SRP-RNA complex.