TITLE: Molecular Motors in vitro and in Live Cells: the effect of load on velocity

SPEAKER: Professor George Holzwarth,

TIME: Thursday Mar. 18, 2004 at 4 PM

PLACE: George P. Williams, Jr. Lecture Hall, (Olin 101)

Motor proteins such as kinesin have been carefully studied as single molecules in buffer but their mechanical behavior in cells is largely unknown.^* We have tracked single vesicles undergoing fast retrograde transport in PC12 neurites, with a spatial precision of +/- 30 nm and a time resolution of 120 ms. Individual vesicles move at a constant velocity for 1-2 s, then move at a different but again constant velocity, probably because the number of motors changes by +/- 1 or +/- 2. To estimate the forces generated by individual motors in cells, we determined the viscoelastic modulus of the cellular environment by analyzing the Brownian motion of the vesicles in the same cell, using Tom Mason's generalization of the Stokes-Einstein equation. We find that the drag force for one motor is 4.2 +/- 0.6 pN, about half the stall force for conventional kinesin in buffer.

* Why should it be different? One reason is that the load on the isolated motor in solution is an optical trap plus the viscous drag of water, whereas the load on the motor pulling a vesicle in a cell is viscoelastic drag on the vesicle. This is 1000X the viscous drag on the beads used in the solution experiments.