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

To obtain information about the location of the three proteolytic fragments in the linear structure of the Ii chain, the same digestion was performed on [35S]cysteine-labeled Ii chain. A single cysteine residue is known to occur in Ii chains and this residue is located in the NHz-terminal portion (11, 31, 42) . Fig. 2 , lanes 5-7 shows the results obtained with [35S] cysteine-labeled Ii chains before (lane 5) or after the digestion with 5 ug/ml (lane 6) or 50 ug/ml (lane 7) of V8 protease. Ii chain and the fragments A and C are radioactively labeled with cysteine, but not fragment B. Thus, Staphylococcus aureus V8 protease cuts the Ii chain essentially twice giving rise to three polypeptides labeled A, B, and C in Fig. 2 .