Disease relevance of HC-Pro

• HC-Pro and CI show high identity with those of other potyviruses [1].
• CVYV P1b and P1-HCPro from the potyvirus Plum pox virus showed very similar behaviors in all the assays carried out, suggesting that evolution has found a way to counteract RNA silencing by similar mechanisms using very different proteins in viruses of the same family [2].
• RNA Silencing Suppression by a Second Copy of the P1 Serine Protease of Cucumber Vein Yellowing Ipomovirus, a Member of the Family Potyviridae That Lacks the Cysteine Protease HCPro [2].
• In potato virus X (PVX)/potyviral synergism, increased pathogenicity and accumulation of PVX are mediated by the expression of potyviral 5' proximal sequences encoding P1, the helper component proteinase (HC-Pro), and a fraction of P3 [3].
• Here, we report that the same potyviral sequence (termed P1/HC-Pro) enhances the pathogenicity and accumulation of two other heterologous viruses: cucumber mosaic virus and tobacco mosaic virus [3].

High impact information on HC-Pro

• Genetic crosses and segregation analyses revealed that a P1/ HC-Pro transgene suppressed PTGS of the GUS sequence [4].
• By expressing PVY and CMV-encoded proteins in a PVX vector we have shown that the viral suppressors of gene silencing are the HCPro of PVY and the 2b protein of CMV [5].
• Expression in transgenic plants of a construct with a large deletion in the 35 kd protein coding region resulted in partial inhibition of HC-Pro N-terminal cleavage, suggesting that the 35 kd protein may affect the proteolytic event but not in a catalytic role [6].
• A point mutation affecting an amino acid residue essential for the proteolytic activity of HC-Pro had no effect on N-terminal processing [6].
• Moreover, high-level expression occurs without viral symptoms and, when HC-Pro is expressed from a mutant transgene, without detrimental developmental phenotypes [7].

Chemical compound and disease context of HC-Pro

• The first approximately 60 amino acids of the N-terminal part of the potyvirus helper component-proteinase (HC-Pro) include highly conserved residues comprising a Cys-rich region [8].
• However, when mutations were introduced into the highly conserved "PTK" and "KITC" motifs of HC-Pro, known for their involvement in protein/protein interaction during aphid-mediated transmission of potyvirus, the resulting change of amino acid residue from lysine (K) to glutamic acid (E) in the "KITC" motif had no obvious influence [9].

Biological context of HC-Pro

• However, it was recently reported that a sequence coding for HCPro is lacking in the genome of Cucumber vein yellowing virus (CVYV), a member of the genus Ipomovirus, the fourth monopartite genus of the family [2].
• One of the proposed activities of P1 is to assist HCPro, the product that viruses of the genus Potyvirus use to counteract antiviral defense mediated by RNA silencing [2].
• In the case of PVX-potyviral synergism, we show that the expression of the HC-Pro gene product, but not the RNA sequence itself, is sufficient to induce the increase in PVX pathogenicity and that both P1 and P3 coding sequences are dispensable for this aspect of the synergistic interaction [3].
• These results indicate that the potyviral P1/HC-Pro region affects a step in disease development that is common to a broad range of virus infections and suggest a mechanism involving transactivation of viral replication [3].
• In protoplasts, expression of the potyviral P1/HC-Pro region prolongs the accumulation of PVX (-) strand RNA and transactivates expression of a reporter gene from a PVX subgenomic promoter [3].

Anatomical context of HC-Pro

• The proteolytic activity of HC-Pro was analyzed by expression of the enzyme in bacterial and cell-free systems [10].
• Antisera to the bacterially expressed nonstructural proteins (NSP) HC-Pro, CI, NIa, and NIb and the coat protein (CP) of plum pox potyvirus (PPV) were used for analysing the composition of virus-induced cytoplasmic and nuclear inclusions by electron microscopy [11].

Associations of HC-Pro with chemical compounds

• The finding that p29-mediated symptom expression is influenced by two cysteine residues that are conserved in the potyvirus-encoded HC-Pro raises the possibility that these related viral-papain-like proteases function in their respective fungal and plant hosts by impacting ancestrally related regulatory pathways [12].
• Highly purified HC-Pro protein was obtained from plants infected with TEV by using a modified version of the virus that incorporates a histidine tag at the HC-Pro N terminus (hisHC-Pro) [13].
• To investigate the interrelationships between three of these functions, 25 alanine-scanning mutations affecting clusters of charged residues were introduced into the HC-Pro coding sequence [14].
• Mutations in the conserved His and two Cys residues within the Cys-rich region have a strong debilitating effect on self-interaction when introduced in the full-length HC-Pro, but not when introduced in the N-terminal fragment [8].
• To identify the class of proteinase to which HC-Pro belongs, tobacco etch virus (TEV) HC-Pro mutants containing single amino acid substitutions at serine, cysteine, aspartic acid, and histidine positions were synthesized by in vitro transcription and translation and were tested for autoproteolytic activity [15].

Regulatory relationships of HC-Pro

• In this study, we provide further evidence that P1 enhances the activity of HCPro in members of the genus Potyvirus and show that it is duplicated in the ipomovirus CVYV [2].

Other interactions of HC-Pro

• The greatest variability occurred in the 5' untranslated region, P1 protein, and NIa-VPg protein, the N-terminal two thirds of HC-Pro, and the C-terminal one third of P3 [16].
• The potato A potyvirus (PVA)-encoded proteins P1, HC-Pro, P3, CI, VPg, NIaPro, NIb and coat protein (CP) were expressed as 6 x His-tagged recombinant proteins in Escherichia coli and purified to homogeneity [17].
• The complete genome sequence of one of the recombinant isolates, BOR-3, was determined, as well as some partial sequences in the HC-Pro and P3 genes for additional natural recombinant isolates [18].

Analytical, diagnostic and therapeutic context of HC-Pro

• Our data from electron microscopy, biochemical analysis, and secondary structure predictions lead us to suggest a model for structure/function relationships in the HC-Pro protein [19].
• Comparison of projection maps of negatively stained crystals revealed that HC-Pro is composed of two domains separated by a flexible constriction [19].
• Characterization of these and additional variants derived by site-directed mutagenesis demonstrated that deletion of sequences coding for the HC-Pro N-terminal domain had a negative effect on accumulation of viral RNA and coat protein [20].
• Electrophoretic mobility-shift assays demonstrated that HC-Pro acts as a sequence non-specific RNA-binding protein and suggest that more than one molecule of protein was bound per molecule of RNA [21].
• Western blot comparison did not permit attribution of HC-Pro functionality in transmission to its level in the host [22].

References