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

The inability for In-1 antibody to precipitate Ii chain after protease digestion of intact microsomes could indicate that the antibody recognizes the part of the polypeptide chain that is exposed on the cytoplasmic side of the membrane. To detect processed Ii chain, we used Con A-Sepharose. Ii chain is known to be glycosylated and should therefore bind to Con A (43) . It furthermore is the major 35S-labeled glycoprotein synthesized by spleen cells and should therefore be readily detectable among the spleen cell glycoproteins. Fig. 1, lane 6 shows that this is indeed the case and that proteinase K treatment of microsomal vesicles reduces the size of Ii chain by ~3 kD. This reduction in size indicates that mouse Ii chain, as its human counterpart, spans the membrane and exposes ~30 amino acid residues on the cytoplasmic side of the membrane. This result also demonstrates that the cytoplasmic portion is essential for the recognition of Ii chain by In-1 antibody.