Sequential translation of capsid and membrane protein genes of alphaviruses

INVESTIGATIONS of the multiplication of alphaviruses such as Semliki Forest virus (SPY) or Sindbis virus have suggested that the three glycoproteins 1 (E1, E2 and E3) located in the viral envelope are initially synthesised as a large precursor poly-peptide which contains the amino acid sequences of them all 2–7 . This protein, NVP97 (ref. 5), is cleaved to give E1 and another precursor polypeptide, NVP63, which is in turn cleaved to give E2 and E3 (refs 6 and 7). By analogy with the picorna-virus system, it has been widely suggested that all the viral structural proteins are derived by cleavage of a common precursor and, indeed, proteins of a size sufficient to encompass the viral capsid protein, as well as the envelope proteins, have been found in infected cells treated with amino acid analogues and inhibitors of proteolytic enzymes 10,11 , and in cells infected with certain classes of temperature-sensitive mutant 2,3,12,13 . Doubts have been cast on the validity of the analogy, however, by reports 8,9,16–19 that cell-free translation of SFV or Sindbis virus mRNA in several systems resulted in the production of discrete viral proteins. This situation is in marked contrast to that of translation of picornavirus RNA, in which a spectrum of polypeptides of various lengths is formed by premature termination at many points along the messenger; since no post-translational cleavage seems to take place, no polypeptides found in the mature virus or in infected cells are formed 20,21 . The possibility arose, therefore, that the alphavirus capsid protein and envelope proteins are synthesised independently.