| AL2 and L2 are related proteins encoded by geminiviruses of the Begomovirus and Curtovirus genera, respectively. Both are pathogenicity determinants that cause enhanced susceptibility when expressed in transgenic plants. To understand how geminiviruses defeat host mechanisms that limit infectivity, we searched for cellular proteins that interact with AL2 and L2. Here, evidence is presented which indicates that the viral proteins interact with and inactivate adenosine kinase (ADK), a nucleoside kinase that catalyzes the salvage synthesis of 5′ -AMP from adenosine and ATP. We show that the AL2 and L2 proteins inactivate ADK in vitro and following coexpression in E. coli and yeast. We also demonstrate that ADK activity is reduced in transgenic plants expressing the viral proteins and in geminivirus infected plant tissue. In contrast, ADK activity is increased following inoculation of plants with diverse RNA viruses or a geminivirus lacking a functional L2 gene. Consistent with its ability to interact with multiple cellular kinases, we also demonstrate that AL2 is present in both the nucleus and the cytoplasm of infected plant cells. To our knowledge this is the first evidence that ADK is targeted by viral pathogens, and the first evidence that this “housekeeping” enzyme might be a part of host defense responses.; In previous work, we showed that AL2 and L2 also interact with and inactivate SNF1 kinase, a global regulator of metabolism that is activated by 5 ′-AMP. Together these observations suggest that metabolic alterations mediated by SNF1 are an important component of innate antiviral defenses and that inactivation of ADK and SNF1 by the geminivirus proteins represents a dual strategy for countering this defense.; Another connection between ADK and viral pathogenesis relates to RNA silencing suppression, which is associated with methylation. By recycling adenosine, ADK plays a critical role in sustaining the methyl cycle and SAM-dependent methyltransferase activity. AL2 proteins have also been shown to act as suppressors of RNA silencing, an adaptive host defense response. In this thesis, we confirm that the geminiviruses TGMV and BCTV induce and are targeted by RNA silencing in the course of a normal infection, and that the AL2 and L2 proteins they encode are capable of suppressing RNA silencing in a transient three component system. Remarkably, we found that inhibiting ADK activity at the RNA level by expression of a dsRNA construct directed against ADK, or at the protein level by the use of an ADK inhibitor (the adenosine analogue RBI), also results in silencing suppression. (Abstract shortened by UMI.)... |
