Structural basis for RNA-silencing suppression by Tomato aspermy virus protein 2b

The 2b proteins encoded by cucumovirus act as post‐transcriptional gene silencing suppressors to counter host defence during infection. Here we report the crystal structure of Tomato aspermy virus 2b (TAV2b) protein bound to a 19 bp small interfering RNA (siRNA) duplex. TAV2b adopts an all α‐helix structure and forms a homodimer to measure siRNA duplex in a length‐preference mode. TAV2b has a pair of hook‐like structures to recognize simultaneously two α‐helical turns of A‐form RNA duplex by fitting its α‐helix backbone into two adjacent major grooves of siRNA duplex. The conserved π‐stackings between tryptophan and the 5′‐terminal base of siRNA duplex from both ends enhance the recognition. TAV2b further oligomerizes to form a dimer of dimers through the conserved leucine‐zipper‐like motif at its amino‐terminal α‐helix. Biochemical experiments suggest that TAV2b might interfere with the post‐transcriptional gene silencing pathway by directly binding to siRNA duplex.