Characterization of the cassava geminivirus transcription activation protein putative nuclear localization signal.

The geminivirus transcription activation protein (TrAP) localizes to the nucleus and contains a putative nuclear localization signal (NLS) ((28)PRRRR(32)) on the N-terminus. The role of individual residues of this putative NLS on nuclear localization and symptom induction was investigated using TrAP of East African cassava mosaic Cameroon virus (EACMCV). Subcellular localization was conducted using the green fluorescent protein (GFP). Results showed that the proline residue at position 28 (Pro-28) is essential for nuclear localization whereas individually, none of the four contiguous arginines is necessary for nuclear targeting. The role of each of the five NLS amino acid residues on TrAP-mediated disease phenotype and gene silencing suppression was investigated by expressing these mutants in Nicotiana benthamiana from the PVX vector and under the control of the Cauliflower mosaic virus 35S promoter. Results showed that all five residues of the NLS play a role on disease phenotype production in N. benthamiana plants. Furthermore, each of the NLS residues appeared to be required for suppression of VIGS but appeared not to be required for the ability of TrAP to transactivate transcription and interact with adenosine kinase (ADK).