The Putative RNA-Processing Protein, THO2, is a Microtubule-Associated Protein in Tobacco

THO2 is a component of the THOTREX (transcription and export factor) complex that participates in mRNA metabolism and export from the nucleus in yeast and animal cells. Here we report that tobacco putative THO2-related protein (NtTHO2) is a microtubule-associated protein, which directly binds to microtubules in vitro and co-localizes with cortical microtubules in vivo. We purified endogenous NtTHO2 by cycles of microtubule polymerizationdepolymerization from crude extracts of tobacco BY-2 miniprotoplasts. Purified NtTHO2 sedimented with microtubules in vitro. Immunofluore-scence revealed that NtTHO2 was present in both the nucleus and cytoplasm. In interphase, cytoplasmic NtTHO2 was localized along cortical microtubules. In the mitotic phase, NtTHO2 was localized to the mitotic spindle but not to either the preprophase band or the phragmoplast. In mature cells of seedling roots, and in BY-2 cells in which proliferation was stopped by removing 2,4-D, NtTHO2 staining was confined mainly to the nucleolus. These results suggest that NtTHO2 is a multifunctional protein that participates in mRNA metabolism, and also functions within the cortical microtubules and mitotic spindle.