Role of the C-terminal tryptophan residue for the structure-function of the alphavirus capsid protein.

The Semliki Forest virus capsid protein is a multifunctional protein which packages genomic RNA into nucleocapsid structures and binds to viral spike protein during budding. In addition, the capsid protein has an autoproteolytic activity whereby the C-terminal tryptophan is used as the substrate for cotranslational cleavage of the viral structure polyprotein. The autoproteolytic domain of the capsid protein has a chymotrypsin-like fold but has two additional short beta-strands which place the tryptophan into the active site. Here, we have substituted the C-terminal tryptophan of Semliki Forest virus capsid protein for alanine, arginine and phenylalanine and analysed the effects on different functions of the C protein such as nucleocapsid formation, spike binding and autoproteolytic activity. We found that (i) tryptophan is a better substrate for the autoproteolytic activity, (ii) the wild-type tryptophan is the only residue that allows efficient viral growth and (iii) an aromatic residue is important for correct initial folding and stability of the protein. Copyright 1998 Academic Press Limited.