DYNAMICAL AND MECHANICAL STUDY OF IMMOBILIZED MICROTUBULES WITH ATOMIC-FORCE MICROSCOPY

The dynamics of the assembly/disassembly reaction play an important ro le in the study of microtubule (MT) polymers. The polymerization rate constant (2 +/- 1 x 10(-3) s(-1)) of this process was determined on th e 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 ti me dependence of turbidity measurements is compared with atomic force microscopy results expressed as the average MT length and mass per uni t surface. Further, the elastic behavior of microtubules under liquid is analyzed as a function of a cross-linking agent's concentration (gl utaraldehyde). Recording force-distance curves allows the indentation to be determined, from which Young's modulus E is derived. Extrapolati ng 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 taxoter e have values of, respectively, 1.3 +/- 0.6 and 2.7 +/- 1.2 MPa as You ng'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) 1 996 American Vacuum Society.