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

Cleavable signal sequences have also been found on proteins that span the membrane once and expose the carboxy (COOH) terminus on the cytoplasmic and the amino (NH2) terminus on the extracytoplasmic side of the membrane. Examples of these types of proteins, which we call here type I membrane proteins, are the G protein of vesicular stomatitis virus (24, 30, 37) and class I and II histocompatibility antigens (12, 29) . The PE2 glycoproteins of Semliki Forest virus and Sindbis virus seem to have an uncleaved signal sequence (5, i 6, 17) . For the PE2 protein of Sindbis virus, an SRP-mediated membrane insertion has been found (5) . Translocation of the NH2-terminal portion of type I membrane proteins is thought to proceed in a manner identical to that of secretory proteins 1. Abbreviations used in this paper." Con A, coneanavalin A; DP, docking protein; ER, endoplasmic reticulum; li, invariant; SRP, signal recognition particle; TCA, trichloroacetic acid. (21, 35, 51) . They become, however, arrested in the membrane by a hydrophobic stop transfer sequence, which is usually located close to the NH2-terminal end of the protein (4, 13, 51) .