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

The fact that the 40 residue fragment of the NH2-terminal region of the p62 protein is able to translocate two different reporter molecules into microsomes constitutes in our mind convincing evidence for signal sequence activity in this protein fragment. A more precise location of the p62 signal sequence within the 40 residue p62 fragment can be done with the aid of the known consensus features of a signal sequence. The most typical characteristic of a signal sequence is a stretch of 10-12 uncharged residues, mostly hydrophobic ones (von Heijne, 1985) . This part of the signal sequence probably forms an alpha helix in the ER membrane (Emr and Silhavy, 1983; Briggs et al., 1985 Briggs et al., , 1986 Kendall et al., 1986; Batenburg et al., 1988) . The only possible candidate region within the 40 residue p62 fragment having these features is the 13 residue segment between Pro 3 and Pro 17 (see box in uppermost sequence in Fig. 7) . The Pro-rich region in the middle of the 40 residue fragment would not form an alpha-helix, and the COOH-terminal part of the p62 segment contains a high number of charged residues. As shown in Fig. 7 these features are conserved in all those alphaviruses where the p62 protein has been sequenced. Thus, we find the experimental results, together with the structural considerations discussed above, highly indicative that the 16 NH2terminal residues of p62 constitute its signal sequence.