Sequential translation of nonstructural proteins in cells infected with a Semliki Forest virus mutant.

Four nonstructural proteins with apparent molecular weights of 70,000 (ns-70), 86,000 (ns-86), 78,000 (ns-78), and 60,000 (ns-60) were translated in cells infected with Semliki Forest virus ts-1 mutant and maintained at the restrictive temperature. After synchronization of the initiation of protein synthesis these proteins were synthesized in the above order, suggesting that they are translated as a polyprotein starting from one initiation site. Two short-lived intermediates with apparent molecular weights of 155,000 and 135,000 were regularly detected. The former is presumably the precursor of proteins ns-70 and ns-86 and the latter of ns-78 and ns-60. The sequence of the structural proteins in their polyprotein was confirmed to be capsid-envelope E-2 (and E-3)-envelope E-1, beginning from the NH2-terminal end. We conclude that in Semliki-Forest-virus-infected cells two polyproteins are synthesized, one for the structural (130,000 daltons), the other for the nonstructural proteins (close to 300,000 daltons).