Dynamical and mechanical study of immobilized microtubules with atomic force microscopy

The dynamics of the assembly/disassembly reaction play an important role in the study of microtubule (MT) polymers. The polymerization rate constant (2 +/- 1 x 10(-3) s(-1)) of this process was determined on the basis of atomic force microscopy images of air-dried samples, taken at discrete time intervals. The decay of tubulin oligomers (rings) is ten times faster, and consequently is not a rate-limiting step. The time dependence of turbidity measurements is compared with atomic force microscopy results expressed as the average MT length and mass per unit surface. Further, the elastic behavior of microtubules under liquid is analyzed as a function of a cross-linking agent's concentration (glutaraldehyde). Recording force-distance curves allows the indentation to be determined, from which Young's modulus E is derived. Extrapolating these data provides the intrinsic stiffness of the MTs (E = 3.1 +/- 0.9 MPa). MTs prepared in the presence of the drugs taxol and taxotere have values of, respectively, 1.3 +/- 0.6 and 2.7 +/- 1.2 MPa as Young's modulus. Experimental MT heights are quantitatively explained by the indentation values found in this study, and a similar approach to determine true heights for any kind of soft sample is suggested. (C) 1996 American Vacuum Society.