Identification of a protein covalently linked to the 5' terminus of tobacco vein mottling virus RNA

The nature of the 5' terminus of tobacco vein mottling virus (TVMV) RNA, a member of the potyvirus group, was investigated. Digestion of viral RNA with ribonuclease led to the appearance of a new polypeptide band of apparent molecular weight 24,000 (24 kDa) after electrophoresis on denaturing polyacrylamide gels. Viral RNA was subjected to radioiodination with the protein-specific Bolton-Hunter reagent. Radioactivity cosedimented on sucrose gradients with intact TVMV RNA. Treatment of the 125I-labeled product with the proteinase Pronase resulted in substantial loss of trichloroacetic acid-precipitable radioactivity. A radioactive species of 24 kDa was released when the(125)I-labeled product was subjected to ribonuclease digestion. To localize the point of attachment, the 125I-labeled TVMV RNA was partially hydrolyzed and used as a hybridization probe against four previously described recombinant plasmids containing TVMV cDNA and a fifth plasmid containing additional 5'-terminal sequences. Hybridization was strongest to plasmids containing the most 5'-terminal sequences. Antisera against the five known proteins encoded by TVMV RNA failed to precipitate significant amounts of the new protein. This genome-linked viral protein (VPg) thus constitutes the sixth polypeptide to be associated with TVMV. It also has the largest apparent molecular weight of any RNA-linked VPg reported to date.     

Specific in vitro cleavage of Mason-Pfizer monkey virus capsid protein: evidence for a potential role of retroviral protease in early stages of infection

Processing of Gag polyproteins by viral protease (PR) leads to reorganization of immature retroviral particles and formation of a ribonucleoprotein core. In some retroviruses, such as HIV and RSV, cleavage of a spacer peptide separating capsid and nucleocapsid proteins is essential for the core formation. We show here that no similar spacer peptide is present in the capsid-nucleocapsid (CA-NC) region of Mason-Pfizer monkey virus (M-PMV) and that the CA protein is cleaved in vitro by the PR within the major homology region (MHR) and the NC protein in several sites at the N-terminus. The CA cleavage product was also identified shortly after penetration of M-PMV into COS cells, suggesting that the protease-catalyzed cleavage is involved in core disintegration.