Role of Plant Virus Movement Proteins in Suppression of Host RNAi Defense

Abstract

One of the systems of plant defense against viral infection is RNA silencing, or RNA interference (RNAi), in which small RNAs derived from viral genomic RNAs and/or mRNAs serve as guides to target an Argonaute nuclease (AGO) to virus-specific RNAs. Complementary base pairing between the small interfering RNA incorporated into the AGO-based protein complex and viral RNA results in the target cleavage or translational repression. As a counter-defensive strategy, viruses have evolved to acquire viral silencing suppressors (VSRs) to inhibit the host plant RNAi pathway. Plant virus VSR proteins use multiple mechanisms to inhibit silencing. VSRs are often multifunctional proteins that perform additional functions in the virus infection cycle, particularly, cell-to-cell movement, genome encapsidation, or replication. This paper summarizes the available data on the proteins with dual VSR/movement protein activity used by plant viruses of nine orders to override the protective silencing response and reviews the different molecular mechanisms employed by these proteins to suppress RNAi.

Begomovirus–Host Interactions: Viral Proteins Orchestrating Intra and Intercellular Transport of Viral DNA While Suppressing Host Defense Mechanisms

“…RNA silencing is a broad defense mechanism that plants employ against viral infections. To counteract this defense, the begomovirus genome encodes viral suppressors of RNA silencing (VSRs) [ 86 , 87 ]. Regarding begomoviral proteins associated with virus movement, V2 and C4 inhibit both transcriptional gene silencing (TGS) and posttranscriptional gene silencing (PTGS) [ 86 , 87 , 88 ].…”

“…To counteract this defense, the begomovirus genome encodes viral suppressors of RNA silencing (VSRs) [ 86 , 87 ]. Regarding begomoviral proteins associated with virus movement, V2 and C4 inhibit both transcriptional gene silencing (TGS) and posttranscriptional gene silencing (PTGS) [ 86 , 87 , 88 ]. V2 from cotton leaf curl Multan virus (CLCuMuV) effectively counteracts TGS by directly interacting with Argonaute 4 (AGO4) and suppressing RNA-dependent DNA Methylation (RdDM)-mediated TGS [ 89 ].…”

Complete sequence and genome characterization of Miscanthus virus M, a new betaflexivirus from Miscanthus sp.

“…ORF4 encodes an 74 aa (M r : 8.5 kDa) TGB3 and has a transmembrane domain (aa 3-23) in the N-terminus. All three TGB proteins are putatively involved in cell-to-cell movement (12) and TGB1 is likely associated with the suppression of RNA silencing (13). ORF5 encodes a 244 aa (M r : 27.1 kDa) coat protein (CP).…”

A novel ilarvirus protein CP-RT is expressed via stop codon readthrough and suppresses RDR6-dependent RNA silencing

“…The mode of action of VSRs is also very diverse and they target various steps of the RNAi pathway (Kasschau et al, 1998; Burgyan and Havelda, 2011; Jin et al, 2021). The ability to suppress RNAi is often found as an additional activity of viral proteins with another established function and in most cases this function requires a close association with nucleic acids such as found in movement proteins (reviewed in Atabekova et al, 2023), coat proteins (Qu et al, 2003; Martínez-Turiño and Hernández, 2009) or viral replicase proteins (Kubota et al, 2003; Csorba et al, 2007). VSRs have been identified in almost all groups of plant viruses and they provide one of the most widespread viral counter-defence strategies (Baulcombe, 2022).…”