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

Membrane proteins which expose the NH2 terminus on the lumenal side of the ER are usually synthesized with a cleavable signal sequence (35, 51). In contrast, proteins that expose the NH2-terminal end on the cytoplasmic side, like the influenza neuraminidase and the rat and human asialoglycoprotein receptor, are synthesized without a cleavable signal sequence (15, 20, 41) . We therefore asked whether Ii chain is also synthesized without a cleavable signal sequence. In glycoproteins, the presence or absence or a cleavable signal sequence can be determined by comparing the molecular weight of the translation products synthesized in a cell free system in the absence of microsomal membranes with those synthesized in vivo in the presence of tunicamycin. Tunicamycin is known to prevent glycosylation but does not interfere with membrane insertion and cleavage of a signal sequence. As can be seen in Fig. 5 , Ii chain is synthesized in CH 1.1. cells in the presence of tunicamycin as a 25-kD polypeptide chain. The identical molecular weight is found for the Ii chain when it is synthesized in a wheat germ cell free system in the absence of microsomal membranes.