Precursor Polypeptides to Structural Proteins of Visna Virus

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Abstract
Visna virus is a retrovirus which replicates in fibroblast-like cells of the sheep choroid plexus through a lytic cycle. Visna virions contain 3 major low MW proteins (p30, p16 and p14) which, together with the genomic RNA and several molecules of reverse transcriptase, constitute the core structure of the virions. The core is surrounded by an envelope containing a major glycoprotein (gp135). By analogy with the oncoviruses, these 3 groups of structural proteins (i.e., the internal proteins, the envelope glycoprotein, and the reverse transcriptase) are probably encoded by the gag, env and pol genes, respectively. To elucidate the genetic organization of the visna virus genome and its expression. The synthesis of viral proteins in infected sheep choroid plexus cells was studied. Intracellular viral proteins were detected by immunoprecipitation of pulse-labeled cell extracts with monospecific [rabbit] sera raised against p30, p16 and gp135 and resolution of the proteins by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Immunoprecipitation with anti-p30 and anti-p16 sera allowed the characterization of the 55,000-dalton polypeptide precursor to internal virion proteins p30, p16 and p14 (Pr55gag). Tryptic peptide mapping confirmed the precursor-product relationship between Pr55gag and the 3 internal proteins. A gag-related polypeptide of 150,000 daltons was also detected. This polypeptide, which was less abundant than Pr55gag is a likely precursor to the viral reverse transcriptase (Pr50gag-pol). Pr55gag and Pr150gag-pol are not glycosylated. The precursor related to major envelope protein gp135 is a glycosylated polypeptide with an average MW of 150,000 (gPr150env). Pulse-chase experiments indicated that gPr150env matures into glycoprotein gp135 intracellularly; gp135 was never preponderant in cell extracts. The non-glycosylated form of gPr150env, which accumulated in the presence of 2-deoxy-D-glucose, appeared as a polypeptide of .apprx. 100,000 daltons. Visna virus apparently codes for the largest non-glycosylated env-related precursor among all of the retroviruses and therefore probably contains the largest env gene.