Geminiviruses are small, single-stranded DNA viruses that replicate through double-stranded DNA intermediates in nuclei of their plant hosts. Bipartite geminivíruses (Begomovirus), such as Cabbage leaf curl virus (CaLCuV), possess two genomic components of ± 2600kb, DNA-A and DNA-B. The trans-acting factors involved in viral replication and transcription are encoded by the A-component whereas the B component encodes two movement proteins: movement protein (MP) and nuclear shuttle protein (NSP), both required for systemic infection. We have previously demonstrated that the NSP from CaLCuV interacts with a leucine rich repeat (LRR)-receptor like kinases (RLK) designated NIK (NSP-Interacting Kinase) through an 80 amino acid region of the kinase domain (aa 422-502) that encompasses the putative active site for ser/thr kinases (sub-domain VIb-HrDvKssNxLLD) and the activation loop (sub-domain VII-DFGAk/rx, plus subdomain VIII - GtxGyiaPEY) and inhibits their kinase activity. Their kinase domain was expressed as GST fusion in E. coli and the affinity-purified recombinant protein behaved as authentic kinase proteins as they undergo autophosphorylation in vitro. To map the phosphorylation sites involved in activation of NIK1 we have replaced the Thr-468, Thr-469 and Thr-474 in the activation loop for Ala or Glu residues and expressed the mutated protein as GST-fusions in E. coli. Either the Thr-474-Glu or Thr-474-Ala replacement on NIK1 reduces both autoplosphorylarion and its phosphorylation activity of exogenous substrates, although to different extent. Likewise, deletion of the C-terminal Ser-rich region of NIK1 reduces the phosphorylation signal of the truncated protein, but did not alter its activity to phosphorylate exogenous substrates. We are currently expressing the mutated and truncated versions of the protein in Arabidopsis thaliana to examine the effects of these mutations on viral infection.