Identification of the amino acid residues and domains in the cysteine-rich protein of Chinese wheat mosaic virus that are important for RNA silencing suppression and subcellular localization

Cysteine-rich proteins (CRPs) encoded by some plant viruses in diverse genera function as RNA silencing suppressors. Within the N-terminal portion of CRPs encoded by furoviruses, there are six conserved cysteine residues and a Cys-Gly-X-X-His motif (Cys, cysteine; Gly, glycine; His, histidine; X, any amino acid residue) with unknown function. The central domains contain coiled-coil heptad amino acid repeats that usually mediate protein dimerization. Here, we present evidence that the conserved cysteine residues and Cys-Gly-X-X-His motif in the CRP of Chinese wheat mosaic virus (CWMV) are critical for protein stability and silencing suppression activity. Mutation of a leucine residue in the third coiled-coil heptad impaired CWMV CRP activity for suppression of local silencing, but not for the promotion of cell-to-cell movement of Potato virus X (PVX). In planta and in vitro analysis of wild-type and mutant proteins indicated that the ability of the CRP to self-interact was correlated with its suppression activity. Deletion of up to 40 amino acids at the C-terminus did not abolish suppression activity, but disrupted the association of CRP with endoplasmic reticulum (ER), and reduced its activity in the enhancement of PVX symptom severity. Interestingly, a short region in the C-terminal domain, predicted to form an amphipathic α-helical structure, was responsible for the association of CWMV CRP with ER. Overall, our results demonstrate that the N-terminal and central regions are the functional domains for suppression activity, whereas the C-terminal region primarily functions to target CWMV CRP to the ER.