Identification of a RNA silencing suppressor in the genome of Grapevine virus A.

Higher plants use post-transcriptional gene silencing (PTGS), an RNA-degradation system, as a
defence mechanism against viral infections. To counteract this, plant viruses encode and express
PTGS suppressor proteins. Four of the five proteins encoded by the Grapevine virus A (GVA)
genome were screened using a green fluorescent protein (GFP)-based transient expression assay,
and the expression product of ORF5 (protein p10) was identified as a suppressor of silencing.
ORF5 p10 suppressed local and systemic silencing induced by a transiently expressed
single-stranded sense RNA. This protein was active towards both a transgene and exogenous
GFP mRNAs. Ectopic expression of GVA-ORF5 by a Potato virus X vector enhanced symptom
severity. The findings that p10 markedly reduces the levels of small interfering RNAs (siRNAs)
and that the recombinant protein is able to bind single-stranded and double-stranded forms of
siRNAs and microRNAs, suggest the existence of a potential mechanism of suppression based
on RNA sequestering.