RNA silencing suppressors from different plant viruses are structurally diverse. In addition to inhibiting the antiviral silencing response to condition susceptibility, many suppressors are pathogenicity factors that cause disease or developmental abnormalities. Here, unrelated suppressors from multiple viruses were shown to inhibit microRNA (miRNA) activities and trigger an overlapping series of severe developmental defects in transgenic Arabidopsis thaliana. This suggests that interference with miRNA-directed processes may be a general feature contributing to pathogenicity of many viruses. A normally labile intermediate in the miRNA biogenesis/RNA-induced silencing complex (RISC) assembly pathway, miRNA*, accumulated specifically in the presence of suppressors (P1/HC-Pro, p21, or p19) that inhibited miRNA-guided cleavage of target mRNAs. Both p21 and p19, but not P1/HC-Pro, interacted with miRNA/miRNA* complexes and hairpin RNA-derived short interfering RNAs (siRNAs) in vivo. In addition, p21 bound to synthetic miRNA/miRNA* and siRNA duplexes in vitro. We propose that several different suppressors act by distinct mechanisms to inhibit the incorporation of small RNAs into active RISCs.[Keywords: MicroRNA; RNA silencing; RNAi; virus-encoded suppressors; RISC] Supplemental material is available at http://www.genesdev.org.

This study concerns the self-assembly of virus-like particles (VLPs) composed of an icosahedral virus protein coat encapsulating a functionalized spherical nanoparticle core. The recent development of efficient methods for VLP self-assembly has opened the way to structural studies. Using electron microscopy with image reconstruction, the structures of several VLPs obtained from brome mosaic virus capsid proteins and gold nanoparticles were elucidated. Varying the gold core diameter provides control over the capsid structure. The number of subunits required for a complete capsid increases with the core diameter. The packaging efficiency is a function of the number of capsid protein subunits per gold nanoparticle. VLPs of varying diameters were found to resemble to three classes of viral particles found in cells (T ‫؍‬ 1, 2, and 3). As a consequence of their regularity, VLPs form three-dimensional crystals under the same conditions as the wild-type virus. The crystals represent a form of metallodielectric material that exhibits optical properties influenced by multipolar plasmonic coupling. metamaterials ͉ protein cage ͉ self-assembly ͉ surface plasmon ͉ virus assembly E ngineered virus capsids and protein cage structures have shown increasing promise as therapeutic and diagnostic vectors (1-6), imaging agents (7-10), and as templates and microreactors for advanced nanomaterials synthesis (11)(12)(13)(14)(15)(16)(17). Here, we address the rules for the formation of symmetric protein cages consisting of viral capsid subunits formed over a functionalized inorganic nanoparticle core, called virus-like particles (VLPs).VLPs provide an example of how biomimetic self-organization can combine the natural characteristics of virus capsids with the exquisite physical properties of nanoparticles (18)(19)(20). Interactions between the artificial cargo and the protein carrier affects both the self-assembly and the stability of the resulting structure, yet very little is known about them. Progress toward the basic development and the practical use of VLPs requires an understanding of how relevant parameters contribute to complex formation.Symmetric VLPs may provide a technology for therapeutic or diagnostic agent delivery that is improved over amorphous shell nanoparticles that are already known to be efficient in similar applications (21). The advantage of a regular surface protein motif is that the binding domains are functionally identical by virtue of their equivalent environment. It has been shown in several situations that receptor-mediated targeting can be achieved even when using amorphous coatings (22). However, the principle challenges for nanoparticle delivery currently include: limited life-time in body fluids, nanoparticle transduction across the cellular membrane, avoidance of the exocytotic pathways, and target specificity. To optimize their infectivity, viruses have evolved to overcome these challenges. We still must learn how to apply virus strategies to targeted delivery. A simple question is central to this o...

Arrays of >5,000 Saccharomyces cerevisiae proteins were screened to identify proteins that can preferentially bind a small RNA hairpin that contains a clamped adenine motif (CAM). A CAM is required for the replication of Brome Mosaic Virus (BMV), a plant-infecting RNA virus that can replicate in S. cerevisiae. Several hits were selected for further characterization in Nicotiana benthamiana. Pseudouridine Synthase 4 (Pus4) and the Actin Patch Protein 1 (App1) modestly reduced BMV genomic plus-strand RNA accumulation, but dramatically inhibited BMV systemic spread in plants. Pus4 also prevented the encapsidation of a BMV RNA in plants and the reassembly of BMV virions in vitro. These results demonstrate the feasibility of using proteome arrays to identify specific RNAbinding proteins for antiviral activities. Furthermore, the effects of Pus4 suggest that the CAM-containing RNA motif provides a regulatory link between RNA replication and encapsidation.Brome mosaic virus ͉ protein-RNA interaction ͉ pseudouridine synthase 4 ͉ yeast proteome chip ͉ viral RNA replication

Using an Agrobacterium-mediated transient assay, we screened the 15.5-kb genome of the Beet yellows virus for proteins with RNA silencing suppressor activity. Among eight proteins tested, only a 21-kDa protein (p21) was able to suppress double-stranded (ds) RNA-induced silencing of the green fluorescent protein (GFP) mRNA. Restoration of GFP expression by p21 under these conditions had no apparent effect on accumulation of the small interfering RNAs. In addition, p21 elevated the transient expression level of the GFP mRNA in the absence of dsRNA inducer. Similar activities were detected using homologs of p21 encoded by other members of the genus Closterovirus. Computer analysis indicated that p21-like proteins constitute a novel protein family that is unrelated to other recognized suppressors of RNA silencing. Examination of the subcellular distribution in BYV-infected plants revealed that p21 is partitioned between soluble cytoplasmic form and proteinaceous inclusion bodies at the cell periphery.

A series of new cowpea mosaic virus (CPMV) RNA-2-based expression vectors were designed. The jellyfish green fluorescent protein (GFP) was introduced between the movement protein (MP) and the large (L) coat protein or downstream of the small (S) coat protein. Release of the GFP inserted between the MP and L proteins was achieved by creating artificial processing sites each side of the insert, either by duplicating the MP-L cleavage site or by introducing a sequence encoding the foot-and-mouth disease virus (FMDV) 2A catalytic peptide. Eight amino acids derived from the C-terminus of the MP and 14-19 amino acids from the N-terminus of the L coat protein were necessary for efficient processing of the artificial Gln/Met sites. Insertion of the FMDV 2A sequence at the C-terminus of the GFP resulted in a genetically stable construct, which produced particles containing about 10 GFP-2A-L fusion proteins. Immunocapture experiments indicated that some of the GFP is present on the virion surface. Direct fusion of GFP to the C-terminus of the S coat protein resulted in a virus which was barely viable. However, when the sequence of GFP was linked to the C-terminus by an active FMDV 2A sequence, a highly infectious construct was obtained.