Nuclear import of the capsid protein of tomato yellow leaf curl virus (TYLCV) in plant and insect cells

The tomato yellow leaf curl virus (TYLCV) found in Israel is a whitefly-transmitted monopartite geminivirus. Although geminiviruses have been found in the nuclei of phloem-associated cells, the mechanism of viral invasion is poorly understood. The possible role of the TYLCV capsid protein (CP), the only known component of the viral coat, in virus transport into the host cell nucleus was investigated by monitoring its specific nuclear accumulation in plant and insect cells. CP was fused to the β-glucuronidase (GUS) reporter enzyme to assay nuclear import in petunia protoplasts, and micro-injection of purified fluorescently labeled CP was used to examine its nuclear uptake in Drosophila embryos. Both assays demonstrated that TYLCV CP is transported into plant-and insect-cell nuclei by an active process of nuclear import via a nuclear localization signal (NLS)-specific pathway. Using the GUS assay and deletion analysis, the TYLCV CP NLS sequence was identified in the amino-terminus of the protein.