Protein-protein- and protein-RNA-binding properties of the movement protein and VP25 coat protein of Apple latent spherical virus

Studies on the Japanese soil-borne wheat mosaic virus movement protein highlight its ability to bind plant RNA

BACKGROUND

Plant viral movement protein (MP) function is decisive for virus cell-to-cell movement. Often, MPs also induce membrane alterations, which are believed to play a role for the establishment of viral replication compartments. Despite these central roles in virus infection, knowledge of the underlying molecular mechanisms by which MPs cause changes in plasmodesmata (PD) size exclusion limit and contribute to the formation of viral replication compartments remain far from being complete.

METHODS

To further identify host processes subverted by viral MPs, we here characterized the MP of Japanese soil-borne wheat mosaic virus (JSBWMV). We used confocal fluorescence microscopy to study the subcellular localization of MPJSBWMV and to address its functionality in promoting virus cell-to-cell movement. Using the biochemical and biophysical methods co-immunoprecipitation, fluorescence lifetime imaging, microscale thermophoresis and RNA immunoprecipitation we investigate the capacity of MPJSBWMV to multimerize and to bind viral and cellular RNAs.

RESULTS

MPJSBWMV localized to PD, promoted cell-to-cell movement by complementing a movement-deficient unrelated virus, formed multimers in-vivo and bound to viral RNA with high affinity. Using RNA immunoprecipitation, we identified host RNAs associated with the viral MP. Within the MP-RNA complexes we found RNAs encoding proteins with key functions in membrane modification, signaling, protein folding, and degradation. We propose that binding of MP to these RNAs during infection and regulation of their spatio-temporal translation may represent a mechanism for MPs to achieve PD and host control during replication and movement.

CONCLUSION

This study provides new insight into the complex interactions between viral MPs and host cellular processes.

Apple latent spherical virus structure with stable capsid frame supports quasi-stable protrusions expediting genome release

Picorna-like plant viruses are non-enveloped RNA spherical viruses of ~30 nm. Part of the survival of these viruses depends on their capsid being stable enough to harbour the viral genome and yet malleable enough to allow its release. However, molecular mechanisms remain obscure. Here, we report a structure of a picorna-like plant virus, apple latent spherical virus, at 2.87 Å resolution by single-particle cryo-electron microscopy (cryo-EM) with a cold-field emission beam. The cryo-EM map reveals a unique structure composed of three capsid proteins Vp25, Vp20, and Vp24. Strikingly Vp25 has a long N-terminal extension, which substantially stabilises the capsid frame of Vp25 and Vp20 subunits. Cryo-EM images also resolve RNA genome leaking from a pentameric protrusion of Vp24 subunits. The structures and observations suggest that genome release occurs through occasional opening of the Vp24 subunits, possibly suppressed to a low frequency by the rigid frame of the other subunits.

Tobacco mosaic virus (TMV) replicase and movement protein function synergistically in facilitating TMV spread by lateral diffusion in the plasmodesmal desmotubule of Nicotiana benthamiana

Virus spread through plasmodesmata (Pd) is mediated by virus-encoded movement proteins (MPs) that modify Pd structure and function. The MP of Tobacco mosaic virus ((TMV)MP) is an endoplasmic reticulum (ER) integral membrane protein that binds viral RNA (vRNA), forming a vRNA:MP:ER complex. It has been hypothesized that (TMV)MP causes Pd to dilate, thus potentiating a cytoskeletal mediated sliding of the vRNA:MP:ER complex through Pd; in the absence of MP, by contrast, the ER cannot move through Pd. An alternate model proposes that cell-to-cell spread takes place by diffusion of the MP:vRNA complex in the ER membranes which traverse Pd. To test these models, we measured the effect of (TMV)MP and replicase expression on cell-to-cell spread of several green fluorescent protein-fused probes: a soluble cytoplasmic protein, two ER lumen proteins, and two ER membrane-bound proteins. Our data support the diffusion model in which a complex that includes ER-embedded MP, vRNA, and other components diffuses in the ER membrane within the Pd driven by the concentration gradient between an infected cell and adjacent noninfected cells. The data also suggest that the virus replicase and MP function together in altering Pd conductivity.