The and inactivate,serine/threonine-related kinases such as sucrose

The TrAP is ~15 kDa protein, functions as a regulator of viral gene expression and isessential for efficient transcription of the late expressing viral genes (AV1 and BV1) locatedin positive sense orientation. TrAP localizes in the nucleus, containing a putative nuclearlocalization signal consisting of five residues (PRRRR), which are essential for suppressionof RNA silencing (Chowda-Reddy et al., 2009). TrAP of Tomato golden mosaic virus(TGMV) binds non-specifically to ssDNA and weakly interacts with dsDNA, which suggeststhat this protein might be interacting with host proteins for transcriptional activation. Itconsists of three functional domains namely nuclear localization signal domain at the Nterminus,DNA-binding Zinc-finger domain in the centre and an acidic activator domain atthe C-terminal (Hartitz et al., 1999). TrAP has an ability to interact with, and inactivate,serine/threonine-related kinases such as sucrose non-fermenting 1 protein kinase (SNF1) andadenosine kinases (ADK). SNF1 and ADK are involved in host defense response and mediatevarious plant anti-stress pathways (Hao et al., 2003; Wang et al., 2003). ADK also play a rolein maintenance of methyl cycle thus inhibition of ADK will also suppress transcriptionalgene silencing preventing methylation of geminiviral DNA. Arabidopsis PEAPOD2 (PPD2)transcription factor interacts with TrAP and its CP promoter, suggesting that TrAP is targetedto the CP promoter through interaction with PPD2, leading to activation of CP geneexpression (Lacatus and Sunter, 2009). It has been studied that TrAP of Mungbean yellowmosaic virus transactivates the viral promoter-driven transgene and causes toxicity intransgenic tobacco plants. All three domains of TrAP are essential for transactivation,suppression of gene silencing and the toxic effect.Phosphorylation appears to influence its subcellular localization. Followingexpression in insect cells, non-phosphorylated AL2 is present in both the nucleus and thecytoplasm whereas the phosphorylated form preferentially accumulates in the nucleus (Wanget al., 2003). In a yeast two-hybrid screen, TGMV AC2 and BCTV C2 were found tospecifically interact with adenosine kinase (ADK), a nucleoside kinase that catalyzes thesynthesis of 5VAMP from adenosine and ATP (Wang et al., 2003). ADK activity is reducedin transgenic plants expressing AC2 and C2, and is also significantly reduced in virus infectedtissue in an C2-dependent manner. Interestingly, plants infected with BCTV C2mutants and unrelated RNA viruses actually show enhanced ADK activity, suggesting thatincreased activity of this enzyme is part of the response to virus infection (Wang et al., 2003).


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