Translation of bottom component RNA of cowpea mosaic virus in reticulocyte lysate: faithful proteolytic processing of the primary translation product

A regulatory role for the 32K protein in proteolytic processing of cowpea mosaic virus polyproteins

We have studied the regulation of proteolytic processing of the polyproteins encoded by cowpea mosaic virus M-RNA and B-RNA. For that purpose mutations were introduced in full-length cDNA clones of these RNAs. RNA transcripts were translated in rabbit reticulocyte lysate and the effect of mutations on the processing was analysed. These studies revealed that the 32K protein is released from the 200K B-polyprotein by an intramolecular cleavage and remains associated with the 170K protein, probably by interaction with the 58K domain of the 170K protein. In this complex the conformation of the 170K protein is such that further cleavages are very slow. This complex carries out the processing of the Gln/Met site in the M-polyprotein. The 170K protein produced by a B-RNA mutant that lacks the 32K coding region was efficiently processed into 110K, 87K, 84K, 60K, 58K and 24K cleavage products. Thus, the 32K protein regulates the B-polyprotein processing by slowing it down and, on the other hand, enhances trans cleavage of M-polyproteins at a Gln/Met site.

Processing of VPg-containing polyproteins encoded by the B-RNA from cowpea mosaic virus

To study the processing of putative VPg precursors the expression of specific mutant transcripts derived from a full-length cDNA clone of cowpea mosaic virus (CPMV) B-RNA was examined in a rabbit reticulocyte lysate system. This study revealed that the 170K protein produced by a B-RNA mutant that lacks the 32K coding region was efficiently processed by mainly intramolecular cleavages at three different sites into three sets of proteins of 60K + 110K, 84K + 87K, and 58K + 112K. Further cleavage of the 60K protein into 58K and VPg has not been observed in this in vitro system. The 84K protein can be further processed by an intramolecular cleavage reaction via two alternative pathways, either into 26K (VPg + 24K) and 58K proteins or into 24K and 60K proteins. VPg can be released from the 112K (VPg + 110K) precursor either directly or via the 26K intermediate. Immunoblot analysis showed that the 112K protein is present in CPMV-infected plant cells indicating that the in vitro observations may hold true in vivo.

Sequence upstream of the 24K protease enhances cleavage of the cowpea mosaic virus B RNA-encoded polyprotein at the junction between the 24K and 87K proteins

To investigate cleavage at the junction between the cowpea mosaic virus (CPMV) 24K and 87K proteins, plasmids were constructed containing the sequence of bottom-component (B) RNA encoding the 110K protein plus a variable length of upstream coding sequence. Transcripts derived from these clones were translated in rabbit reticulocyte lysate and the appearance of the 87K protein was used to assess the efficiency of cleavage at the 24K-87K junction. The results show that the 110K protein, containing the contiguous sequence of the 24K and 87K proteins, is stable and that efficient cleavage at 24K-87K junction requires the presence of amino acids upstream of the 24K protease. These observations show that the 170K protein rather than the 110K protein is the precursor of the 87K protein and suggest a mechanism whereby both the B RNA-encoded 110K and 87K proteins can accumulate during infection.

Mutational analysis of the putative catalytic triad of the cowpea mosaic virus 24K protease

To investigate the mechanism of action of the cowpea mosaic virus (CPMV) 24K protease, a full-length cDNA clone of bottom component (B) RNA has been constructed from which RNA can be transcribed in vitro using T7 RNA polymerase. Translation of the resulting RNA in rabbit reticulocyte lysate leads to the synthesis of a 200 kDa product (the 200K protein) which cleaves itself in a manner identical to that of the product translated from B RNA isolated from virions. Site-directed mutagenesis of the full-length clone was used to examine the effects of altering individual amino acids in the 24K protease on its activity. The results obtained are consistent with the prediction that the 24K protease is structurally similar to the trypsin-like family of serine proteases and suggest that His40, Glu76, and Cys166 comprise the active site. Substitution of Cys166 by a serine residue results in an enzyme with reduced catalytic activity.

Inhibition of proteolytic processing of the polyproteins of cowpea mosaic virus by hemin

Cleavages of the polyproteins synthesized from cowpea mosaic virus (CPMV) B RNA and M RNA in rabbit reticulocyte lysates are inhibited by hemin. Cleavage of the CPMV B RNA-encoded 200K polyprotein and of the M RNA-encoded 60K intermediary precursor protein were most sensitive to hemin inhibition, while cleavages of other precursor proteins were less sensitive. A significant observation was that at a hemin concentration of 25 microM, but not at higher concentrations, the 60K protein was cleaved to yield the two viral capsid proteins. This cleavage reaction has not been observed in vitro previously.

Identification of putative polymerase gene product in cells infected with murine coronavirus A59

The virion RNA of mouse hepatitis virus, strain A59 (MHV-A59) is believed to be the mRNA for the viral RNA-dependent RNA polymerase. The cell-free translation of virion RNA results in the synthesis of two predominant products p220 and p28 (M. R. Denison and S. Perlman, 1986, J. Virol. 60, 12-18). p28 is a basic protein and is readily detected by two-dimensional gel electrophoresis. When infected cells and isolated virions were assayed for this protein by two-dimensional gel electrophoresis, p28 could be detected in infected cells labeled at late times after infection, but not at early times or in purified virions. p28 represents the first protein product of the putative coronavirus polymerase gene to be identified in infected cells.

Detection of a Novel Protein Encoded by the Bottom-Component RNA of Cowpea Mosaic Virus, Using Antibodies Raised against a Synthetic Peptide

A peptide was synthesized that corresponded to a sequence in the cowpea mosaic virus bottom-component RNA-encoded 200-kilodalton polyprotein showing homology to the picornaviral 3C proteases. By injecting a rabbit with this peptide, antibodies were obtained that allowed the detection of a novel viral protein derived from the 200-kilodalton polyprotein. This protein, which had a size of 24 kilodaltons was found in both infected cowpea leaves and cowpea protoplasts.

Determination of the Proteolytic Processing Sites in the Polyprotein Encoded by the Bottom-Component RNA of Cowpea Mosaic Virus

The bottom-component RNA (B-RNA) of cowpea mosaic virus is expressed by the production of a ∼200,000-dalton polyprotein (200K polyprotein), from which the functional proteins are formed by specific proteolytic cleavages. Partial amino-terminal sequences of the various B-RNA-encoded proteins have now been determined. Comparison of the information obtained with the B-RNA sequence allowed the localization of the coding regions for these proteins on B-RNA, the calculation of their precise molecular weights, and the determination of the cleavage sites at which they are released from the polyprotein precursor. Sequence analysis of the 32K protein, which is derived from the amino-terminal end of the 200K polyprotein, indicated that the AUG codon at nucleotide position 207 of the RNA sequence is the translation initiation codon. Sequence analysis of the 170K, 110K, 87K, 84K, 60K, and 58K proteins revealed the existence of three types of cleavage site in the 200K polyprotein: glutamine-serine (two sites), glutamine-methionine (one site), and glutamine-glycine (one site) amino acid pairs. The nature of these cleavage sites suggested that two different viral proteases are involved in the processing of the B-RNA-encoded polyprotein.