Signal recognition particle-dependent membrane insertion of mouse invariant chain: a membrane-spanning protein with a cytoplasmically exposed amino terminus

~ ECRETORY and many membrane-spanning proteins are translocated across or inserted into the membrane of the endoplasmic reticulum (ER) ~ in a co-translational manner (for review see references 3, 21, 35, 50, and 51). Translocation requires a signal sequence on the nascent polypeptide chain and specific receptors, the signal recognition particle (SRP) and the docking protein (DP) or SRP receptor (1, 2, 6, 18, 19, 32, 33, 38, [46] [47] [48] [49] . SRP has been shown to interact with polysomes holding nascent preprolactin chains, and it has been proposed that SRP binds to the signal sequence (46) . When probed in a wheat germ cell free system with prolactin or IgG light chain mRNAs, SRP arrests the peptide elongation after 70-80 amino acids have been polymerized (33, 47, 48) . Elongation proceeds after SRP has bound to the DP, which is located in the ER membranes (18, 19, 33) .