Kinetics of microtubule-AtMAP65-1 bond studied with dual-optical tweezers

The unbinding force of microtubule-AtMAP65-1 bond is measured with dual-optical tweezers. Force histograms reveal quantized force distributions under physiological condition and with 100 mM NaCl treatment. Based on Bell-Evans-model of multiple bonds, histograms are fitted and kinetic parameters are obtained. Under the force loading rate of 9pN/s, the most probable unbinding forces for the single bond are 16.8 +/- 0.8 and 14.7 +/- 0.8 pN under the physiological and treatment conditions, respectively. Position of transition state of the bond is given to be Delta x = 2.1 +/- 0.1 nm both with and without treatment of NaCl. Intrinsic dissociation rate constants k(0) are 0.0010 +/- 0.0002 and 0.0020 +/- 0.0005 s(-1) corresponding to without and with NaCl treatment, which means that 100 mM NaCl shortens the natural lifetime of the bond by 50%. In addition, the activation barrier also changes by 3.4% after treatment with NaCl, which indicates that NaCl makes the dissociation of the bond easier.