Geminiviruses are single-stranded DNA plant viruses with circular genomes packaged
within geminate particles. Among the Geminiviridae family, Begomovirus and Curtovirus
comprise the two best characterized genera. Curtovirus and Old World begomovirus
possess similar genome structures with six to seven open-reading frames (ORF). Among
them, begomovirus and curtovirus V2 ORFs share the same location in the viral genome,
encode proteins of similar size, but show extremely poor sequence homology between
the genera. V2 from Beet curly top virus (BCTV), the model species for the Curtovirus
genus, as it begomoviral counterpart, suppresses post-transcriptional gene silencing
(PTGS) by impairing the RDR6/SGS3 pathway and localizes in the nucleus spanning
from the perinuclear region to the cell periphery. By aminoacid sequence comparison
we have identified that curtoviral and begomoviral V2 proteins shared two hydrophobic
domains and a putative phosphorylation motif. These three domains are essential for
BCTV V2 silencing suppression activity, for the proper nuclear localization of the protein
and for systemic infection. The lack of suppression activity in the mutated versions of
V2 is complemented by the impaired function of RDR6 in Nicotiana benthamiana but
the ability of the viral mutants to produce a systemic infection is not recovered in gene
silencing mutant backgrounds. We have also demonstrated that, as its begomoviral
homolog, V2 from BCTV is able to induce systemic symptoms and necrosis associated
with a hypersensitive response-like (HR-like) when expressed from Potato virus X vector
in N. benthamiana, and that this pathogenicity activity does not dependent of its ability
to supress PTGS
