Analysis of reorganization of cortical microtubules in cell morphogenesis at M/G_1 interface.

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We have shown by immunofluorescence microscopy that, during the cell cycle transition from M to G_1 phase in synchronized tobacco BY-2 cells, MTs were organized on the perinuclear region. MT elongation up to the cell cortex was followed by further elongation parallel to the long axis of cells. As soon as the tips of the elongated MTs reached the distal end of the division plane, the transversely oriented MTs were formed proximal to the division plane. Although the nuclear surface is the site of MT organization, the organized MTs are not the direct precursors of the cortical MTs. To examine the sites of MT reorganization, the MTs of BY-2 cells were completely destroyed by a combination of cold and propyzamide treatment. After the cells were washed and cultured, the sites of MT reorganization at M/G_1 interface were identified solely on the perinuclear region or on the perinuclear region and on the cell cortex(Hasezawa et al.1997). These patterns of MT organization correspond to the five … More types observed during the cell cycle transition from M to G_1 phase ; reorganization of MTs solely on the perinuclear region reflecting earlier types of the patterns and MT reorganization on both the perinuclear region and the cell cortex reflecting later types of them.We have also shown that EF-1 alpha(a 49 kDa protein)is associated with organizing MTs in tobacco BY-2 cells, and this association occurs exclusively with organizing MTs on the perinuclear region during the cell cycle transition from M to G_1 phase(Kumagai et al.1995). We prepared partially purified protein fractions which had a microtubule organizing center(MTOC)activity and involved EF-1 alpha, from evacuolated protoplasts of BY-2 cells. Using these fractions, we developed a system, in which the addition of purified tubulin caused the formation of small aster-like structure. We called them"aster". Our efforts to find factors affecting the formation of the"aster"would be expected to provide further insight into MT organization in plant cells. Less