Cucurbit chlorotic yellows virus p22 suppressor of RNA silencing binds single-, double-stranded long and short interfering RNA molecules in vitro.

Cucurbit chlorotic yellows virus (CCYV) is a new member of the genus Crinivirus (family Closteroviridae) with a bi-partite genome. CCYV RNA 1-encoded p22 has recently been reported to be a weak local suppressor of RNA silencing for which an interaction with cucumber SKP1LB1 through an F-box-like motif was demonstrated to be essential. Using a bacterially expressed maltose-binding protein (MBP) fusion of CCYV p22 in electrophoretic mobility shift assays (EMSA), we have examined in vitro its ability to bind different RNA templates. Our experiments showed that CCYV p22 is able to bind to ss and ds long RNAs, in addition to ss and ds small interfering (si) RNA molecules. CCYV p22 deletion mutants (MBP_CCYV DEL1-4) were produced that covered the entire protein, with MBP_CCYV DEL2 corresponding to the F-box motif and its flanking sequences. None of these deletions abolished the capacity of CCYV p22 to bind ss- and dsRNA molecules. However, deletions affecting the C-terminal half of the protein resulted in decreased binding efficiency for either ss- or dsRNA molecules indicating that essential elements for these interactions are located in this region. Taken together, our data add to current knowledge of the mode of action of suppressors of RNA silencing encoded by genes sited at the 3'-terminus of crinivirus genomic RNA 1, and shed light on the involvement of CCYV p22 in the suppression of RNA silencing and/or in another role in the virus life cycle via RNA binding.