Impaired cleavage of the joint gag-pol polyprotein precursor and virion assembly in a temperature-sensitive mutant of rous sarcoma virus

A temperature-sensitive coordinate mutant tsLA83 of Prague (PR-B) strain of Rous sarcoma virus at the nonpermissive temperature (41°) produces noninfectious virus particles (NI-LA83) which contained only 3% of the reverse-transcriptase activity present in infectious virions. Analyses of [³⁵S]methionine-labeled NI-LA83 showed the presence of all of the viral proteins except reverse transcriptase. Pulse-chase analyses of the virus-specified proteins in cells infected with LA83 or PR-B showed that the gag and glycoprotein precursors, Pr76^gag and gPr95^env, respectively, were processed at both 35 and 41°. The reverse-transcriptase precursor, Prl80P^gag-pol however, was not processed in LA83-infected cells at 41°. In contrast, cells infected with LA83 or PR-B at 35° as well as with PR-B at 41° showed normal cleavage of Pr180^gag-pol. A shiftdown of LA83-infected cells at 41° to the permissive temperature 35° resulted in the normal processing of Pr180^gag-pol, and production of infectious virus containing reverse transcriptase. Electron microscopic analysis showed that at 41° cells infected with LA83 showed a large number of budding structures but fewer released particles. A shiftdown from 41 to 35° resulted in an increase of virus particles with a concomitant decrease in budding structures suggesting that the processing of reverse-transcriptase precursor is related to virion assembly.