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The polymerization of ␣-and ␤-tubulin into microtubules results in a complex network of microfibrils that have important structural and functional roles in all eukaryotic cells. In addition, microtubules can interact with a diverse family of polypeptides which are believed to directly promote the assembly of microtubules and to modulate their functional activity. We have demonstrated that the c-Myc oncoprotein interacts in vivo and in vitro with ␣-tubulin and with polymerized microtubules and have defined the binding site to the N-terminal region within the transactivation domain of c-Myc. In addition, we have shown that c-Myc colocalizes with microtubules and remains tightly bound to the microtubule network after detergent extraction of intact cells. These findings suggest a potential role for Myc-tubulin interaction in vivo.The product of the c-myc gene is a nuclear phosphoprotein that has been implicated in the regulation of cell differentiation, apoptosis, and the development of human tumors (5,19,28). The c-Myc protein includes multifunctional domains that include an N-terminal transactivation domain within exon II and a C-terminal domain, within exon III, which is necessary for heterodimerization (5, 19). These observations have supported the model that c-Myc functions as a transcription factor whose activity can be regulated by its protein binding partners. In addition, the N-terminal transactivation region of c-Myc binds to the retinoblastoma protein-related tumor suppressor protein p107, and it has been recently shown that this interaction can suppress the activity of the c-Myc transactivation domain (12). Further, the observation that Burkitt's lymphoma frequently contains naturally occurring somatic mutations within the transactivation domain (3, 34) which result in loss of p107 suppression confirms that this is an important functional domain of c-Myc.Although c-Myc is characterized as a nuclear protein (10, 15), it has recently been proposed that the subcellular localization of c-Myc can vary according to the proliferation state of the cell. For example, in actively growing NIH 3T3 cells c-Myc is predominantly found in the nucleus, while a shift to the cytoplasm occurs in contact-inhibited cells (31). An increased cytoplasmic/nuclear ratio for c-Myc has also been noted following monocytic differentiation of human myeloid leukemia cells (8) and in nondividing prefertilized Xenopus oocytes (14). In addition, comparison of c-Myc localization patterns of normal tissue, adenomas, and colorectal tumors showed that tumor progression was associated with an accumulation of cytoplasmic c-Myc protein (27). These experiments suggested that nucleus-cytoplasm exchanges of c-Myc protein play an important functional role in cell proliferation. Since it had been proposed that tubulin mediates the cytoplasm-to-nucleus translocation of the glucocorticoid and vitamin D receptors (2, 25), we examined whether c-Myc could also interact with tubulin and microtubules. This hypothesis was also suggested by the observation tha...

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