The signal sequence of the p62 protein of Semliki Forest virus is involved in initiation but not in completing chain translocation

The importance of our results in this work lies in the fact that they rule out translocation models in which the signal sequence would have a role throughout the whole process of chain translocation. For instance, if the translocation site is represented by a multisubunit protein complex forming an aqueous channel across the membrane for chain transfer (see signal hypothesis, Blobel and Dobberstein, 1975 ; amphiphatic tunnel hypothesis, Rapoport, 1985; translocator protein hypothesis, Singer et al., 1987a,b) , then the signal sequence could be involved in its assembly or "opening" but apparently not for keeping it together or open until chain transfer is completed (as suggested in Singer et al., 1987a) . Similarly, when considering models in which the chain transfer occurs directly through a lipid membrane (see the helical hairpin hypothesis, Engelrnan and Steitz, 1981; direct transfer model, von Heijne and Blomberg, 1979; phospholipid channel hypothesis, Nesmayanova, 1982) the interaction of the signal sequence with the lipid bilayer could be of importance only at the stage of translocation initiation but not at the actual chain transfer step.