Characterization of tubulin modified with the sulfhydryl-reactive fluorochrome monobromo(trimethylammonio)bimane

Aging of tubulin at neutral pH: the destabilizing effect of vinca alkaloids

The effect of the vinca alkaloid drugs, vincristine, vinblastine, catharanthine, and vindoline, on the aging process of tubulin has been examined. It was found that addition of vincristine or vinblastine accelerated by a factor of 3-3.5 the transformation of tubulin from the 5.8 S alpha-beta-tubulin dimer to paucidisperse polymers, with an average sedimentation coefficient of 9 S, previously observed in the absence of drugs (V. Prakash and S. N. Timasheff, 1982, J. Mol. Biol. 160, 499-515). This transformation of tubulin from 5.8 S to "9 S" followed pseudo-first-order kinetics whether the starting protein was predominantly dimeric (i.e., at low drug concentration) or self-associated into the reversible linear polymers induced by the vinca alkaloid drugs at high drug concentration (G. C. Na and S. N. Timasheff, 1980, Biochemistry 19, 1355-1365; V. Prakash and S. N. Timasheff, 1985, Biochemistry 24, 5004-5010). Identical kinetics were found in a fluorescence examination of the loss by tubulin of its ability to bind colchicine specifically, indicating that the rate determining step is a protein conformational change that induces a major change in the far uv circular dichroism spectrum of tubulin. The found lack of an effect of dithiothreitol on the aging and aggregation processes is consistent with the irreversible aggregation being due to the intermolecular coalescence of nonpolar patches on the protein. The observations that vincristine binds to aged tubulin and that the aging of tubulin is accompanied by quenching of the tryptophan fluorescence similar to that which occurs on the binding of the vinca drugs has led to the proposal that the vinca alkaloids stabilize the aged conformation of the protein by interacting with nonpolar regions that may be related to the aggregation sites.

Assembly properties of fluorescein-labeled tubulin in vitro before and after fluorescence bleaching

Brain tubulin has been conjugated with dichlorotriazinyl-aminofluorescein (DTAF) to form a visualizable complex for the study of tubulin dynamics in living cells. By using several assays we confirm the finding of Keith et al. (Keith, C. H., J. R. Feramisco, and M. Shelanski, 1981, J. Cell Biol., 88:234-240) that DTAF-tubulin polymerizes like control tubulin in vitro. The fluorescein moiety of the complex is readily bleached by the 488-nm line from an argon ion laser. When irradiations are performed over short times (less than 1 s) and in the presence of 2 mM glutathione, a mixture of DTAF-tubulin and control protein (as occurs after microinjection of the fluorescent conjugate into living cells) will retain full polymerization activity. Slow bleaching (approximately 5 min) or bleaching without glutathione promotes formation of covalent cross-links between neighboring polypeptides and kills the polymerization activity of DTAF-tubulin, including some molecules that are neither cross-linked nor bleached. Even under conditions that damage DTAF-tubulin, however, DTAF-microtubules are not destroyed by bleaching. They will continue to elongate by addition of DTAF-tubulin subunits to their free ends, and they neither bind nor exchange subunits along their lateral surfaces. These results suggest that DTAF-tubulin is a suitable analog for tubulin, both in studies of protein incorporation and for investigations of fluorescence redistribution after photobleaching.

Interaction of bimane-labeled fluorescent tubulin with the isolated mitotic apparatus

Fluorescent derivatives of cellular proteins that retain their native characteristics have become useful probes to investigate the dynamics of specific cytoskeletal proteins. In the experiments reported here, a previously characterized fluorescent derivative of tubulin, bimane-tubulin [Wadsworth and Sloboda, 1982a], was used to investigate microtubule assembly in vitro. The results demonstrate that bimane-tubulin was competent to assemble onto a variety of organizing centers in vitro, including microtubule organizing centers (MTOCs) present in homogenates of sea urchin eggs, isolated mitotic apparatuses (MAs), and lysed mitotic cells. When homogenates of fertilized sea urchin eggs containing MTOCs were incubated with bimane-tubulin at 37 degrees C, discrete areas of linear fluorescence were observed. Only diffuse fluorescence was observed when calcium or colchicine was added to the homogenate or if the temperature was maintained at 0 degrees C. Negative-stain electron microscopy of the fluorescent arrays revealed morphologically normal microtubules radiating from electron dense regions. When mitotic spindles, isolated in glycerol containing buffers and therefore cold stable, were incubated with bimane-tubulin, linear fluorescence was observed emanating from the spindle poles but not from the region occupied by the kinetochores. MAs incubated with bimane-labeled bovine serum albumin or bimane-labeled microtubule-associated proteins showed only diffuse fluorescence. However, when mitotic cells which were hypotonically lysed in the absence of detergents or microtubule stabilizing solvents, were perfused with bimane-tubulin intense fluorescence was observed in the asters and throughout the spindle. Two experiments suggested that the fluorescence observed in the results outlined above was due to the assembly of normal microtubules from the fluorescent subunits. First, the observed fluorescence was sensitive to cold temperature, which is known to disassemble microtubules. Second, when the isolated, fluorescent MAs were examined by thin section electron microscopy, microtubules of normal diameter were seen. No aggregated material appeared associated with the walls of the microtubules, which might have been expected if the fluorescent protein was nonspecifically adsorbed to the microtubules. The results of these experiments demonstrate that isolated, stabilized MAs support the growth of new microtubules from the spindle poles while labile spindles, present in lysed cells, incorporate fluorescent tubulin throughout the spindle and asters.(ABSTRACT TRUNCATED AT 400 WORDS)

Microinjection of fluorescent tubulin into dividing sea urchin cells

To follow the dynamics of microtubule (MT) assembly and disassembly during mitosis in living cells, tubulin has been covalently modified with the fluorochrome 5-(4,6-dichlorotriazin-2-yl)aminofluorescein and microinjected into fertilized eggs of the sea urchin Lytechinus variegatus. The changing distribution of the fluorescent protein probe is visualized in a fluorescence microscope coupled to an image intensification video system. Cells that have been injected with fluorescent tubulin show fluorescent linear polymers that assemble very rapidly and radiate from the spindle poles, coincident with the position of the astral fibers. No fluorescent polymer is apparent in other areas of the cytoplasm. When fluorescent tubulin is injected near the completion of anaphase, little incorporation of fluorescent tubulin into polymer is apparent, suggesting that new polymerization does not occur past a critical point in anaphase. These results demonstrate that MT polymerization is very rapid in vivo and that the assembly is both temporally and spatially regulated within the injected cells. Furthermore, the microinjected tubulin is stable within the sea urchin cytoplasm for at least 1 h since it can be reutilized in successive daughter cell spindles. Control experiments indicate that the observed fluorescence is dependent on MT assembly. The fluorescence is greatly diminished upon treatment of the cells with cold or colchicine agents known to cause the depolymerization of assembled MT. In addition, cells injected with fluorescent bovine serum albumin or assembly-incompetent fluorescent tubulin do not exhibit fluorescence localized in the spindle but rather appear diffusely fluorescent throughout the cytoplasm.