OLIGOMERIZATION-DEPENDENT FOLDING OF THE MEMBRANE-FUSION PROTEIN OF SEMLIKI-FOREST-VIRUS

The spikes of alphaviruses are composed of three copies of an E2-E1 he terodimer. The E1 protein possesses membrane fusion activity, and the E2 protein, or its precursor form, p62 (sometimes called PE2), control s this function. Both proteins are, together with the viral capsid pro tein, translated from a common C-p62-E1 coding unit. In an earlier stu dy, we showed that the p62 protein of Semliki Forest virus (SFV) dimer izes rapidly and efficiently in the endoplasmic reticulum (ER) with th e E1 protein originating from the same translation product (so-called heterodimerization in cis) (B.-U. Barth, J. M. Wahlberg, and H. Garoff , J. Cell Biol. 128:283-291, 1995). In the present work, we analyzed t he ER translocation and folding efficiencies of the p62 and E1 protein s of SFV expressed from separate coding units versus a common one. We found that the separately expressed p62 protein translocated and folde d almost as efficiently as when it was expressed from a common coding unit, whereas the independently expressed E1 protein was inefficient i n both processes. In particular, we found that the majority of the tra nslocated E1 chains were engaged in disulfide-linked aggre gates. This result suggests that the E1 protein needs to form a complex with p62 to avoid aggregation. Further analyses of the E1 aggregation showed th at it occurred very rapidly after E1 synthesis and could not be avoide d significantly by the coexpression of an excess of p62 from a separat e coding unit. These latter results suggest that the p62-E1 heterodime rization has to occur very soon after E1 synthesis and that this is po ssible only in a cis-directed reaction which follows the synthesis of p62 and E1 from a common coding unit. We propose that the p62 protein, whose synthesis precedes that of the E1 protein, remains in the trans locon of the ER and awaits the completion of E1. This strategy enables the p62 protein to complex with the E1 protein immediately after the latter has been made and thereby to control (suppress) its fusion acti vity.