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

The possibility that our results about p62 protein translocation would be unique to the viral system .and different from the general translocation process in the ER we find most unlikely. Several results from this and earlier works suggest that the signal sequence of the p62 protein functions much in the same way as cleavable ones do. Firstly, studies with a temperature-sensitive mutant of SFV, ts3, have shown that the signal sequence of p62 requires a free NH2-terminal end for function (Hashimoto et al., 1981) . At the nonpermissive temperature the ts3 mutant is defective in cleavage between the C and the p62 protein region of the protein precursor because of a mutation that inactivates the autoproteolytic activity of C. This defect results in a translocation negative phenotype for the p62 protein. Secondly, the p62 signal sequence has been shown to be SRP dependent. If the mRNA for the structural proteins of SFV is translated in vitro in a wheat germ-derived system that is supplemented with saltwashed (and SRP-deprived) membranes then p62 translocation is observed only in the presence of exogenous SRP (Bonatti et al., 1984) ~ If SRP is supplemented without membranes then p62 translation is arrested. Thirdly, our time course study about p62 synthesis and glycosylation in this work clearly demonstrates that the p62 chain is translocated cotranslationally across the ER membrane. This was also suggested by earlier studies in vitro (Garoff et al., 1978; Bonatti et al., 1984) . In these studies it was shown that both microsomal membranes as well as SRP have to be added to the synthesis mixture before extensive lengths (,,o100 amino acid residues) of the p62 chains have been translated.