Suppression of RNA silencing by TYLCV F-box protein

Summary The Israeli isolate of Tomato yellow leaf curl geminivirus (TYLCV-Is) is a major pathogen of tomato. Surprisingly, little is known about the molecular mechanisms underlying this virus's interactions with tomato cells. Earlier, we have identified the TYLCV-Is V2 protein as a suppressor of RNA silencing interacting with SGS3, a component of the plant silencing machinery. In this proposal, our major goal was to study the mechanism by which V2 suppresses the host plant defense. During this study we cloned the tomato ortholog of Skp1 from tomato and showed that V2 interacts with this protein, a pre-requisite for V2 to be involved in targeting SGS3 for degradation by the 26S proteasome. Also, we discovered that V2 interacts with yet another member of the plant defense machinery, CYP1 – a papain-like cysteine protease involved in numerous cellular activities including programmed cell death in response to pathogen attack. In this study we showed that V2 inhibits CYP1 enzymatic activity. Based on these data, we propose a model for dual action of V2 in suppressing the host antiviral defense: V2 targets SGS3 for ubiquitination and degradation, blocking the RNA silencing response, and inhibits CYP1, blocking the programmed cell death hypersensitive response.

In a parallel research supporting the main goal described, we also investigated Agrobacterium tumefaciens-encoded F-box protein VirF. It has been proposed that VirF targets a host protein for the UPS-mediated degradation, very much the way TYLCV V2 does.