Structure of Microtubular Crystals Induced by Vinblastine in Vitro

The treatment of any of a wide variety of cells with vincristine or vinblastine results in the dissolution of cytoplasmic microtubules and the formation of both 90-100 A filaments and ordered arrays of hexagonally packed, circular structures (1-4) . These ordered structures have been named "microtubular crystals" (2, 3), and this nomenclature will be followed here with the awareness that conclusive evidence of true crystalline order is lacking. Ultracentrifugal studies by Weisenberg and Timasheff (5) revealed a vinblastine-induced aggregation of purified microtubule subunit protein. Vinblastine selectively and quantitatively precipitates microtubule protein from supernatants of cell and tissue homogenates, if adequate concentrations of vinblastine and magnesium are used (6, 7) . Electron microscopic studies of vinblastineprecipitated microtubule protein were previously reported from this laboratory (8) . These studies on embedded, sectioned material revealed ordered acriform arrays of ladder-like structures with "rungs" spaced approximately 300 A apart surrounding circular profiles with a diameter on the order of 360 A . On the basis of this sectioned material it was not possible to ascertain what relationship, if any, existed between the vinblastine-induced precipitated microtubule protein in vitro and the vinblastine-induced microtubular crystals seen in vivo . The negative contrast studies which we now report demonstrate the remarkable similarity between the structures induced by vinblastine in vitro and in vivo. In addition, the three-dimensional structure of the former is clarified, and the results are extrapolated to the latter .