Microtubules In Solution Research Articles

Flexural rigidity of singlet microtubules estimated from statistical analysis of their contour lengths and end-to-end distances

Microtubules in solutions, observed under a dark-field microscope, show incessant Brownian movement such as translational, rotational and flexing motion. A large number of microtubules, spontaneously stuck to the under surface of a coverslip, were photographed and the contour lengths and end-to-end distances of their images were measured. From the statistical analysis of the contour lengths and end-to-end distances, a value for the parameter γ representing the flexibility of singlet microtubules was estimated to γ = (6.8 ± 0.8) · 10 −3 μm −1. From the value of γ, the elastic modulus for bending, ε, and Young's modulus, Y, of singlet microtubules were computed to be ε = ∼ 10 −16 dyne·cm 2 and Y = ∼ 10 9 dyne·cm −2, respectively. The microscopic elastic constant, k, of bonding between two tubulin monomers neighboring along the singlet microtubule was computed to be k = ∼ 10 2 dyne·cm −1. A singlet microtubule is an order of magnitude as strong against bending and as weak against stretching as an F-actin filament.

Shape of microtubules in solutions

Although several authors have presented dark-field micrographs of axonemes or of outer doublet microtubules from sperm tails, singlet microtubules have not been observed individually by light microscopy. This study demonstrates the technical possibility of observing individual microtubules by dark-field microscopy. While polymerized brain microtubules were always observed to be straight, the outer doublet microtubules from sperm tails assumed a coiled form, as demonstrated by Summers & Gibbons [1] and Zobel [2]. The coiled conformation of the outer doublets was found to be a left-handed helix, with a nearly uniform diameter.