THE OLIGOMERIZATION REACTION OF THE SEMLIKI FOREST VIRUS MEMBRANE-PROTEIN SUBUNITS

The Semliki Forest virus (SFV) spike is composed of three copies of a membrane protein heterodimer. The two subunits of this heterodimer (p6 2 and E1) are synthesized sequentially from a common mRNA together wit h the capsid (C) in the order C-p62-E1. In this work heterodimerizatio n of the spike proteins has been studied in BHK 21 cells. The results indicate that: (a) the polyprotein is cotranslationally cleaved into i ndividual chains; (b) the two membrane protein subunits are initially not associated with each other in the endoplasmic reticulum (ER); (c) heterodimerization occurs predominantly between subunits that originat e from the same translation product (heterodimerization in cis); (d) t he kinetics of subunit association-are very fast (t(1/2) = 4 min); and (e) this heterodimerization is highly efficient. To explain the cis-d irected heterodimerization reaction we suggest that the p62 protein, w hich is made before E1 during 26S mRNA translation, is retained at its translocation site until also the E1 chain has been synthesized and t ranslocated at this same site. The mechanism for p62 retention could e ither be that the p62 anchor sequence cannot diffuse out from an ''act ive'' translocation site or that the p62 protein is complexed with a p rotein folding facilitating machinery that is physically linked to the translocation apparatus.