Identification of a novel plant-specific kinesin-like protein that is highly expressed in interphase tobacco BY-2 cells

Through reverse transcription-polymerase chain reaction and Northern blot analysis, we identified TBK5, a novel plant-specific kinesin-like protein (KLP) that is highly expressed in interphase tobacco BY-2 cells. TBK5 mRNA was present at a high level throughout the growth cycle, even in cells that had entered the stationary phase, where cell proliferation had ceased. However, transcripts for five other tobacco KLPs that we have identified were preferentially expressed in mitotic cells, and either not or only slightly accumulated in cells that had entered the stationary phase. Thus, TBK5 appears to be a KLP whose cellular function most closely relates to the cortical array of microtubules that plays a key role in plant cell morphogenesis. The predicted structure of TBK5 is characterized by a central motor domain that is phylogenetically distant from those of other reported KLPs, coiled-coil domains located on both sides of the motor domain, and a basic C-terminal domain. In addition, TBK5 has a putative neck domain which is closely related to the neck domain of KLPs with C-terminal motor domains, previously shown to control the direction of KLP movement towards the minus ends. Antibodies against truncated TBK5 recognized a polypeptide with a molecular mass of 74 kDa in cytoplasmic extracts of interphase cells, and this polypeptide cosedimented with microtubules assembled in the cytoplasmic extracts. The 74 kDa polypeptide corresponding to TBK5 dissociated from microtubules with high concentrations of NaCl but was not dissociated by MgATP. We hypothesize that TBK5 functions in the regulation of the arrangement of cortical microtubules.