Contribution of plus and minus end pathways to microtubule turnover

Life cycle of MTs: persistent growth in the cell interior, asymmetric transition frequencies and effects of the cell boundary

Microtubule dynamics were investigated in CHO and NRK cells by novel experimental approaches designed to evaluate the microtubule behavior in the cell interior. These approaches were: (1) laser photobleaching of a path through the centrosome; (2) direct observation of microtubules in centrosome-containing cytoplasts; (3) GFP-CLIP-170 expression as a marker for microtubule plus end growth; and (iv) sequential subtraction analysis. The combination of these approaches allowed us to obtain data where the density of microtubules had previously prevented conventional methods to be applicable.

In the steady state, nascent microtubules grew persistently from the centrosome towards the cell margin. Frequently, they arrived at the cell margin without undergoing any transition to the shortening phase. In contrast to the growth of microtubules, shortening of the plus ends from the periphery was non-persistent; that is, rescue was frequent and the extent of shortening showed a distribution of lengths reflecting a stochastic process. The combination of persistent growth and a cell boundary led to a difference in apparent microtubule behavior in the cell interior compared with that near the cell margin. Whereas microtubules in the cell interior showed asymmetric transition frequencies, their behavior near the cell margin showed frequent fluctuations between phases of shortening and growth. Complete microtubule turnover was accomplished by the relatively rare episodes of shortening back to the centrosome. Release from the centrosome with subsequent minus end shortening also occurred but was a minor mechanism for microtubule turnover compared with the plus end pathway.

We propose a life cycle for a microtubule which consists of rapid growth from the centrosome to the cell margin followed by an indefinite period of fluctuations of phases of shortening and growth. We suggest that persistent growth and asymmetric transition frequencies serve the biological function of providing a mechanism by which microtubules may rapidly accommodate to the changing shape and advancing edge of motile cells.

The effect of stathmin phosphorylation on microtubule assembly depends on tubulin critical concentration

Stathmin is a phosphorylation-regulated tubulin-binding protein. In vitro and in vivo studies using nonphosphorylatable and pseudophosphorylated mutants of stathmin have questioned the view that stathmin might act only as a tubulin-sequestering factor. Stathmin was proposed to effectively regulate microtubule dynamic instability by increasing the frequency of catastrophe (the transition from steady growth to rapid depolymerization), without interacting with tubulin. We have used a noninvasive method to measure the equilibrium dissociation constants of the T(2)S complexes of tubulin with stathmin, pseudophosphorylated (4E)-stathmin, and diphosphostathmin. At both pH 6.8 and pH 7.4, the relative sequestering efficiency of the different stathmin variants depends on the concentration of free tubulin, i.e. on the dynamic state of microtubules. This control is exerted in a narrow range of tubulin concentration due to the highly cooperative binding of tubulin to stathmin. Changes in pH affect the stability of tubulin-stathmin complexes but do not change stathmin function. The 4E-stathmin mutant mimics inactive phosphorylated stathmin at low tubulin concentration and sequesters tubulin almost as efficiently as stathmin at higher tubulin concentration. We propose that stathmin acts solely by sequestering tubulin, without affecting microtubule dynamics, and that the effect of stathmin phosphorylation on microtubule assembly depends on tubulin critical concentration.

Cell cycle-dependent changes in microtubule dynamics in living cells expressing green fluorescent protein-alpha tubulin

LLCPK-1 cells were transfected with a green fluorescent protein (GFP)-alpha tubulin construct and a cell line permanently expressing GFP-alpha tubulin was established (LLCPK-1alpha). The mitotic index and doubling time for LLCPK-1alpha were not significantly different from parental cells. Quantitative immunoblotting showed that 17% of the tubulin in LLCPK-1alpha cells was GFP-tubulin; the level of unlabeled tubulin was reduced to 82% of that in parental cells. The parameters of microtubule dynamic instability were compared for interphase LLCPK-1alpha and parental cells injected with rhodamine-labeled tubulin. Dynamic instability was very similar in the two cases, demonstrating that LLCPK-1alpha cells are a useful tool for analysis of microtubule dynamics throughout the cell cycle. Comparison of astral microtubule behavior in mitosis with microtubule behavior in interphase demonstrated that the frequency of catastrophe increased twofold and that the frequency of rescue decreased nearly fourfold in mitotic compared with interphase cells. The percentage of time that microtubules spent in an attenuated state, or pause, was also dramatically reduced, from 73.5% in interphase to 11.4% in mitosis. The rates of microtubule elongation and rapid shortening were not changed; overall dynamicity increased 3.6-fold in mitosis. Microtubule release from the centrosome and a subset of differentially stable astral microtubules were also observed. The results provide the first quantitative measurements of mitotic microtubule dynamics in mammalian cells.