STRUCTURE OF GROWING MICROTUBULE ENDS - 2-DIMENSIONAL SHEETS CLOSE INTO TUBES AT VARIABLE RATES

Observation of microtubule growth at different rates by cryo-electron microscopy reveals that the ends range from blunt to long, gently curv ed sheets. The mean sheet length increases with the growth rate while the width of the distributions increases with the extent of assembly. The combination of a concentration dependent growth rate of the tubuli n sheet with a variable closure rate of the microtubule cylinder, resu lts in a model in which stochastic fluctuations in sheet length and tu bulin conformation confine GTP-tubulins to microtubule ends. We propos e that the variability of microtubule growth rate observed by video mi croscopy (Gildersleeve, R. F., A. R. Cross, K. E. Cullen, A. P. Fagen, and R. C. Williams. 1992. J. Biol. Chem. 267: 7995-8006, and this stu dy) is due to the variation in the rate of cylinder closure. The curva ture of the sheets at the end of growing microtubules and the small ol igomeric structures observed at the end of disassembling microtubules, indicate that tubulin molecules undergo conformational changes both d uring assembly and disassembly.