ASSEMBLY OF THE 68-KD AND 72-KD PROTEINS OF SIGNAL RECOGNITION PARTICLE WITH 7S RNA

Signal recognition particle (SRP), the cytoplasmic ribonucleoprotein p article that mediates the targeting of proteins to the ER, consists of a 7S RNA and six different proteins. The 68- (SRP68) and 72- (SRP72) kD proteins of SRP are bound to the 7S RNA of SRP as a heterodimeric c omplex (SRP68/72). Here we describe the primary structure of SRP72 and the assembly of SRP68, SRP72 and 7S RNA into a ribonucleoprotein part icle. The amino acid sequence deduced from the cDNA of SRP72 reveals a basic protein of 671 amino acids which shares no sequence similarity with any protein in the sequence data libraries. Assembly of SRP72 int o a ribonucleoprotein particle required the presence of 7S RNA and SRP 68. In contrast, SRP68 alone specifically bound to 7S RNA. SRP68 conta cts the 7S RNA via its NH2-terminal half while COOH-terminal portions of SRP68 and SRP72 are in contact with each other in SRP. SRP68 thus s erves as a link between 7S RNA and SRP72. As a large NH2-terminal doma in of SRP72 is exposed on SRP it may be a site of contact to other mol ecules involved in the SRP cycle between the ribosome and the ER membr ane.