Biosynthesis of reverse transcriptase from Rauscher murine leukemia virus by synthesis and cleavage of a gag-pol read-through viral precursor polyprotein

Abstract

Reverse transcriptase (RT; RNA-dependent DNA nucleotidyltransferase) from Rauscher leukemia virus is synthesized in infected cells by way of a read-through poly- rotein of 200,000 molecular weight. This polyprotein (Pr200(gag-pol)) was precipitated by antiserum to RT; in a previous study all the monospecific antisera to gag proteins recognized Pr200(gag-pol). Pr200(gag-pol) contains both p30 and RT peptide sequences. Intermediate RT-related precursors of 145,000 (Pr145(pol)), 135,000 (Pr135(pol)), and 125,000 (Pr125(pol)) molecular weights were specifically recognized by precipitation from infected cell extracts by antiserum to RT. These proteins shared methionine-containing tryptic peptide sequences with a virion polypeptide of 80,000 molecular weight (p80(pol)) precipitate by antiserum to RT. Purification of active RT enzyme from virions labeled with [(3)H]methionine showed that p80(pol) was the major component, based on analysis by gel electrophoresis and tryptic peptide mapping experiments. A polypeptide (Pr80(pol)), similar in size to mature viral p80(pol), was also precipitated from infected cells by antiserum to RT. Its peptide map was nearly identical to that of virion p80(pol). Pulse-chase studies showed that Pr80(pol), Pr125(pol), and Pr135(pol) were stable polypeptides, whereas Pr200(gag-pol) and Pr145(pol) were unstable precursors. Pulse-chase studies with the protein synthesis inhibitor, cycloheximide, showed that the processing of Pr200(gag-pol) occurred for a short time in the absence of protein synthesis.