Tomato protein of the resistance gene Ve2 to verticillium wilt [Verticillium spp.] is located in the endoplasmic reticulum

Ve1 and Ve2 are two closely linked inverted genes from tomato independently conferring resistance against verticillium wilt [Verticillium spp], a common fungal disease. The amino acid sequences of the proteins encoded by the two genes display structural domains reminiscent of cell-surface receptors. The cytosolic domain of Ve2 displays a YXX phi motif commonly involved in receptor-mediated endocytosis. In addition, Ve2 concludes with the residues KKF, similar to the KKX motif that signals endoplasmic reticulum (ER) retention in mammalian and plant cells. To define the subcellular localization of the Ve2 protein and to study the possible endocytic dynamics of the protein, we generated fusions of the green fluorescent protein (GFP) with proteins of Ve2 and a Ve2 mutant with an altered YXX phi signal sequence, for expression in tobacco (Nicotiana tabacum) 'Bright Yellow 2' (BY-2) suspension cells and tobacco plants. Analysis under fluorescence microscopy revealed that both fusion proteins are localized exclusively within the ER. Additional constructs with removed terminal KKF residues in the GFP fusion also appeared in the Golgi apparatus after expression in tobacco cells. These results indicate that the Ve2 protein is predominantly in the ER.