Proteolytic processing of the proteins translated from the bottom component RNA of cowpea mosaic virus. The primary and secondary cleavage reactions

Processing of the tobacco etch virus 49K protease requires autoproteolysis

The final products encoded by the tobacco etch virus genome arise by proteolytic cleavage of a single large polyprotein precursor. Processing of the polyprotein at several sites requires the activity of a viral protease of 49,000 molecular weight (49K). We have examined the excision of the 49K protease from polyproteins translated from defined RNA transcripts. Polyproteins containing an intact 49K protein were efficiently processed after synthesis in a rabbit reticulocyte lysate to yield the 49K product. Introduction of a single amino acid substitution (cysteine to alanine) at the putative active site of the 49K protease abolished processing, indicating that the protease was excised from the polyprotein via an autocatalytic mechanism. Release of the 49K protease was determined to require autoproteolysis, since synthetic polyproteins which contained either or both 49K cleavage sites were processed poorly, if at all, in trans reactions. Protein microsequence analysis revealed that processing in vitro occurred between a glutamine-glycine dipeptide to generate the 49K amino terminus.

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.

Cloning and in vitro characterization of the grapevine fanleaf virus proteinase cistron

The region of the genomic RNA-1 from grapevine fanleaf virus isolate F13 (GFLV-F13), containing the proteinase cistron and flanking sequences (nucleotides 3894 to 4789) of the GFLV polyprotein, was modified by PCR mutagenesis to create a start codon and cloned in a transcription vector. The transcripts from the resulting clone (pVP7) produced, upon translation in rabbit reticulocyte lysate, a 37.8-kDa protein which was subsequently cleaved to a stable 28-kDa product. Autocleavage was maximal at pH 7.0-8.5 and at 30 degrees. Inhibition of the activity was greater than 80% when translation was performed in the wheat germ system. In rabbit reticulocyte lysate, inhibition was also obtained with PMSF, EDTA, E-64, Ca+2, Zn+2, and Co+2. The pVP7 translation product acts in cis, in the case of its autocleavage, or in trans in the processing of the viral 122-kDa polyprotein from GFLV RNA-2 into a 66-kDa protein and the 56-kDa coat protein. The carboxy extremity of the complete pVP7 translation product, encoded by nucleotides 4633 to 4789 of RNA-1, was not required for the proteinase activity, at least in trans.

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.

Characterization of the catalytic residues of the tobacco etch virus 49-kDa proteinase

The 49-kDa proteinase of tobacco etch virus (TEV) cleaves the polyprotein derived from the TEV genomic RNA at five locations. Molecular genetic and biochemical analyses of the 49-kDa TEV proteinase were performed to test its homology to the cellular trypsin-like serine proteases. A cDNA fragment, containing the TEV 49-kDa proteinase gene and flanking sequences, was expressed in a cell-free transcription/translation system and resulted in the formation of a polyprotein precursor that underwent rapid self-processing. Site-directed mutagenesis was used to test the effect of altering individual 49-kDa amino acid residues on proteolysis. The data suggest that the catalytic triad of the TEV 49-kDa proteinase could be composed of the His234, Asp269, and Cys339. These findings are consistent with the hypothesis that the TEV 49-kDa proteinase is structurally similar to the trypsin-like family of serine proteinases with the substitution of Cys339 as the active site nucleophile. A structural model of the TEV 49-kDa proteinase proposes other virus-specific differences in the vicinity of the active site triad and substrate-binding pocket. The structure may explain the observed negligible effect of most cellular proteinase inhibitors on the activity of this viral proteinase.

Processing reactions in the later stages of hormone activation

Three tiers of processing have been investigated in the reactions that transform prohormones into their mature end products. Evidence is presented that the proteolytic reactions that convert lipotropin into shortened forms of beta-endorphin take place in individually distinct stages. After these cleavages have occurred, the removal of basic residues by carboxypeptidase H and amidation of the products are effected by independent reactions which do not synergise. Experiments are also described which show that the amidating enzyme can accept certain imino acids as substrates and utilises a mechanism that involves hydroxylation; it is implicit that peptide amidation proceeds by a similar mechanism. These results point to a general concept that pro-hormone processing involves consecutive reactions which take place in a predetermined order.

Proteolytic activity of the cowpea mosaic virus encoded 24K protein synthesized in Escherichia coli

The function of the 24-kilodalton (24K) protein encoded by cowpea mosaic virus (CPMV) has been studied by constructing a bacterial expression plasmid that contained a cloned chimeric segment consisting of partial DNA copies of CPMV M-RNA (including sequences coding for both capsid proteins) and B-RNA (including sequences coding for the 24K protein). Viral sequences were transcribed from the phage T7 promoter phi 10 of plasmid pT7-6 using T7-RNA polymerase expressed from plasmid pGP1-2 present in the same cells. Upon inducing the synthesis of T7-RNA polymerase several new polypeptides that contained CPMV-specific sequences were expressed, as demonstrated by immunoprecipitation and immunoblotting. Furthermore a proteolytic activity was detected in induced cells which cleaved the viral protein sequences specifically at two glutamine-glycine sites. One of the cleavage products represented capsid protein VP23. The proteolytic activity was absent when an 87-bp deletion was introduced in the coding region for the 24K protein, indicating that this protein represented the protease involved in the proteolytic processing at those specific sites.

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.

Cleavage of the cap binding protein complex polypeptide p220 is not effected by the second poliovirus protease 2A

Poliovirus protein 2A contains a short amino acid sequence that also occurs in the putative active site of the known viral proteinase, 3C, previously shown to be responsible for glutamine/glycine cleavages in the poliovirus polyprotein precursor. Experimental evidence indicates that 2A is a second viral proteinase that mediates the cleavage of two tyrosine/glycine cleavages in the generation of virus-specific proteins. Since poliovirus inhibition of host cell protein synthesis correlates with the specific cleavage of the 220,000-Da component of the cap binding protein complex, we have tested whether viral protein 2A contains the p220 cleavage activity. The results show that 2A does not copurify with p220 cleavage activity, partially purified fractions containing high p220 cleavage activity contain no detectable 2A sequences in the form of either mature or precursor protein, and anti-2A serum or IgG does not inhibit p220 cleavage in vitro.