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

Several lines of evidence suggest that Ii chain spans the V8---~] membrane and exposes the NH2-terminal end on the cytoplasmic and the COOH-terminal end on the lumenal side of the membrane of the ER. (a) Sequence data derived from a cDNA for Ii chain locate a single stretch of 26 hydrophobic or uncharged amino acid residues close to the NH2-terminal >-~ [-" A end (1 l, 31, 42) . Hydrophobic sequences are known to occur 4 in regions of proteins that span the membrane. (b) When V8 protease is used to digest the cytoplasmically exposed portion Proteins that span the membrane once can expose either their COOH-terminal end (type 1 membrane proteins) or their NH2-terminal end (type 2 membrane proteins) on the cytoplasmic side. Like secretory proteins, type 1 membrane proteins are usually synthesized with cleavable signal sequences. Their membrane insertion proceeds co-translationally and requires SRP and DP. The same requirements were found for B 'C Figure 7 . Schematic representation ofli chain, li chain spans the ER membrane and exposes the NH2-terminal end on the cytoplasmic side and the COOH-terminal end on the lumenal side. Proposed locations of cleavage sites for Staphylococcus aureus V8 protease and the fragments A, B, and C are indicated. In-I antibody recognizes that part of fragment C that is exposed on the cytoplasmic side. membrane insertion ofli chain for a typical type 2 membrane protein.