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

It is specifically this last question we have addressed in the present work. We describe the characteristics and behavior of the uncleaved signal sequence of the p62 protein of Semliki Forest virus (SFV) l upon translocation across the ER membrane in vitro. The p62 protein is one subunit of the heterodimeric spike protein of the SFV membrane (reviewed in Garoff et al., 1982) . It is made as a precursor protein together with the other structural proteins of SFV, i.e., the nucleocapsid protein, C, and the other spike subunit, El. The three proteins are synthesized from a 4.1-kb long mRNA in the order C, p62, and El, and separated by cleavage of the growing precursor chain. During synthesis of the p62 polypeptide at the ER all but a 31 residue COOH-terminal portion and the membrane anchor is translocated across the membrane. The p62 signal sequence has so far been only roughly localized to the NH2-terminal third of the polypeptide chain (Garoffet al., 1978; Bonatti et al., 1984) . We show here that the signal sequence of p62 consists of a 16 residue peptide at its NH2-terminal region. This region includes one out of four glycosylation sites (Asn~3) for N-linked oligosaccharide on the p62 chain. We also demonstrate that the glycosylation of the p62 signal sequence occurs early during chain translocation. As this modification of the signal region most likely correlates with its release into the lumen of ER it follows that the signal sequence of p62 is probably only needed for an initial step in chain translocation and not to