The Potyviridae P1a leader protease contributes to host range specificity

- Cucumber vein yellowing virus (CVYV) P1a is a leader self-cleaving protease.
- CVYV P1a shows incomplete processing in the experimental host Nicotiana benthamiana.
- Incomplete P1a cleavage impairs a downstream silencing suppressor and viral viability.
- Supply of an extra cleavage site restores P1a processing and P1a-derived defects.
- P1a shows no negative effects in cucumber, the CVYV natural host.

The P1a protein of the ipomovirus Cucumber vein yellowing virus is one of the self-cleavage serine proteases present in Potyviridae family members. P1a is located at the N-terminal end of the viral polyprotein, and is closely related to potyviral P1 protease. For its proteolytic activity, P1a requires a still unknown host factor; this might be linked to involvement in host specificity. Here we built a series of constructs and chimeric viruses to help elucidate the role of P1a cleavage in host range definition. We demonstrate that host-dependent separation of P1a from the remainder of the polyprotein is essential for suppressing RNA silencing defenses and for efficient viral infection. These findings support the role of viral proteases as important determinants in host adaptation.