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

Mouse CH 1.1. cells, which express relatively large amounts of Ii chain, were labeled for 1 h with [3SS]methionine and Ii chains were precipitated with In-1 antibody. Immunoprecipitated polypeptides were characterized by SDS PAGE, either directly (Fig. 2, lanes 1 and 4) or after treatment with 5 ug/ ml (Fig. 2, lane 2) or 50 ug/ml (Fig. 2, lane 3) 78 protease. As expected, Ii chain and a 41-kD protein were precipitated with the In-1 antibody (26, 52, 53) . The 41-kD protein is an Ii gene product generated most likely by differential splicing (52) . With increasing concentrations of the V8 protease, Ii chain was digested into three major polypeptides, labeled A, B, and C (Fig. 2) . Polypeptide A has a molecular weight of ~29 kD, B o f~2 0 kD, and C of ~10 kD. It appears that the cleavage of A generates fragments B and C adding up to the molecular weight of 29 kD.