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Review
. 2018 Mar;19(3):744-763.
doi: 10.1111/mpp.12553. Epub 2017 May 26.

The HCPro from the Potyviridae family: an enviable multitasking Helper Component that every virus would like to have

Adrián A Valli  1 , Araiz Gallo  1 , Bernardo Rodamilans  1 , Juan José López-Moya  2 , Juan Antonio García  1
Affiliations
Review

The HCPro from the Potyviridae family: an enviable multitasking Helper Component that every virus would like to have

Adrián A Valli et al. Mol Plant Pathol. 2018 Mar.

Abstract

RNA viruses have very compact genomes and so provide a unique opportunity to study how evolution works to optimize the use of very limited genomic information. A widespread viral strategy to solve this issue concerning the coding space relies on the expression of proteins with multiple functions. Members of the family Potyviridae, the most abundant group of RNA viruses in plants, offer several attractive examples of viral factors which play roles in diverse infection-related pathways. The Helper Component Proteinase (HCPro) is an essential and well-characterized multitasking protein for which at least three independent functions have been described: (i) viral plant-to-plant transmission; (ii) polyprotein maturation; and (iii) RNA silencing suppression. Moreover, multitudes of host factors have been found to interact with HCPro. Intriguingly, most of these partners have not been ascribed to any of the HCPro roles during the infectious cycle, supporting the idea that this protein might play even more roles than those already established. In this comprehensive review, we attempt to summarize our current knowledge about HCPro and its already attributed and putative novel roles, and to discuss the similarities and differences regarding this factor in members of this important viral family.

Keywords: RNA silencing suppressor; multifunctional proteins; proteinase; transmission.

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Figures

Figure 1
Figure 1
‘Bridge hypothesis’ for aphid transmission of potyviruses. Left: an aphid is feeding from an infected plant. Centre: longitudinal section of the mandibular stylet (the external flanking maxillae have been omitted to simplify the figure), including the two parallel channels (the food canal that connects to the digestive system and the salivary canal that allows secretions during feeding) joining at the common duct. Right: a helper component proteinase (HCPro) complex (depicted in a dimeric form) is bound at one end of the viral particle and allows a reversible interaction with potential receptors located over the cuticle lining (internal side of the stylet tip). It should be noted that this figure is a predictive representation of the viral transmission process based on very limited available experimental data on interactions and the consequent role of HCPro during this process (see text for details). Hence, it cannot be ruled out, for instance, that the HCPro–CP interaction might occur all along the viral particle and non‐dimeric forms of HCPro may play a role in viral transmission.
Figure 2
Figure 2
Potential targets of helper component proteinase (HCPro) in the antiviral RNA silencing pathway. Simplified schematic representation of the RNA silencing‐mediated defences in plants that are deployed against RNA viruses. Steps of the cascade at which HCPro from different potyvirids may be acting in order to block this defensive response are indicated. AGO, Argonaute protein; DCL, Dicer‐like protein; DRB, double‐stranded RNA‐binding protein; HEN1, HUA enhancer 1; RDR, RNA‐dependent RNA polymerase; RISC, RNA‐induced silencing complex; SGS3, supressor of gene silencing 3.
Figure 3
Figure 3
Helper component proteinase (HCPro) structural and functional features. (A) Schematic representation of a representative potyviral HCPro (from Tobacco etch virus, TEV) divided into three main regions. The best‐characterized motifs are shown in squares. Amino acids relevant for a given function, which are conserved at least among the Potyvirus genus, are marked with triangles at their corresponding positions. Amino acids relevant for viral movement (marked in light blue) were described before the characterization of HCPro RNA silencing suppression activity; therefore, their real role might be misassigned. Pentapeptide insertions that render Plum pox virus (PPV) HCPro poorly functional or non‐functional as an RNA silencing suppressor (RSS) are depicted as grey circles at the equivalent TEV HCPro positions. A two‐dimensional representation of the Turnip mosaic virus HCPro structure solved by Guo et al. (2011) encompasses the equivalent C‐terminal region of TEV HCPro. Superscript numbers indicate the following references: 1Carrington et al. (1989a); 2Oh and Carrington (1989); 3Carrington and Herndon (1992); 4Atreya et al. (1992); 5Atreya and Pirone (1993); 6Huet et al. (1994); 7Dolja et al. (1993); 8Blanc et al. (1998); 9Kasschau and Carrington (2001); 10González‐Jara et al. (2005); 11Shiboleth et al. (2007); 12Torres‐Barceló et al. (2008); 13Cronin et al. (1995); 14Valli et al. (2014); 15Varrelmann et al. (2007). (B) Crystal structure of the cysteine protease domain of Turnip mosaic virus HCPro (Guo et al., 2011; PDB code 3RNV). The corresponding L and R domains of papain‐like proteases would be represented by the α‐helices shown in green and the β‐sheets shown in orange, respectively. Those amino acids highlighted in (A) are also indicated in (B).
Figure 4
Figure 4
Schematic representation of genomic organization in viruses from different genera of the family Potyviridae. The long open reading frame is shown as a box divided into mature viral products. PIPO open reading frame is indicated as the box below P3. The terminal protein VPg is depicted as a black ellipse. P1a and P1a‐like proteins are represented by grey boxes, whereas P1b and P1b‐like proteins are represented by black boxes. Features that are not shared by all potyvirids are highlighted in different colours. (A) Potyvirus and Rymovirus genera. The PISPO open reading frame in sweet potato‐infecting potyviruses is indicated as a pale green box below P1. The extra protein HAM between NIb and CP in Euphorbia ringspot virus (Knierim et al., 2017) is highlighted in pink. (B) Tritimovirus and Poacevirus. (C) Ipomovirus. The diversity among members of this genus has been reviewed (Dombrovsky et al., 2014). A HAM extra protein (in pink) was also present in a subset of ipomoviruses. (D) Brambyvirus. The AlkB domain in P1 from Blackberry virus Y is highlighted in pale orange. (E) Macluravirus. (F) Bymovirus. RNA2, unique in the Potyviridae family, is highlighted in yellow.
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