Formation of the Semliki Forest virus membrane glycoprotein complexes in the infected cell

In Semliki Forest virus (SFV)-infected cells, all structural proteins are translated from a 26S mRNA using a single initiation site. The capsid protein which is made first is released into the cytoplasm whereas the two membrane proteins, p62 (the precursor for E2 and E3) and E1, are inserted into the rough endoplasmic reticulum membrane. Based on gradient centrifugation and cross-linking studies, it can be seen that the p62 and E1 polypeptides form a complex immediately after synthesis and migrate to the plasma membrane in the form of a p62-E1 complex. The processing of p62 to E2 and E3 is first seen 25 to 30 min after a 10 min pulse of radioactive amino acids. This cleavage can be inhibited by addition of antisera specific for E1 and E3, thus supporting the view that, as in the case of the related Sindbis virus, this cleavage occurs on the external face of the plasma membrane. Proteolytic digestion of crude vesicle preparations derived from plasma membranes, combined with peptide mapping, indicate that the carboxy-terminal end of E2 spans the cell plasma membrane, there being a portion of mol. wt about 3000 located towards the cytosol.