Changes in the number of molecular motors driving vesicle transport in PC12 /

by 1976- Hill, David Brooks

Motor proteins such as kinesin and dynein drive vesicle transport within cells by converting the chemical energy of ATP into mechanical work. Though isolated kinesin has been well studied in in-vitro assays, little is known of the performance of the motor protein inside a living cell. The mechanical properties of motor proteins in their native environment were quantified by examining the velocity patterns of vesicles undergoing active transport in PC12 neurites. As they are transported, vesicles travel at constant velocity for long intervals before changing to a different constant velocity. These changes in velocity are caused by a change in the number of motor proteins actively transporting the vesicle from 1, 2, 3 etc. To determine the number of motors pulling the load, the minimum sustained velocity for a vesicle, v0, was used to scale the individual vesicle velocities. Scaling the velocities in this way revealed quantized changes in vesicle velocity of ±1 v0 or occasionally ± 2, ± 3, and ± 4. To estimate the viscoelastic modulus and viscosity appropriate for large vesicles (0.25 – 0.5 µm radius) in PC12, the Brownian motion of vesicles within the distal expansions of neurites was measured. The corresponding Stokes’ drag on the transported vesicles is 4.2 ± 0.6 pN per motor. Assuming that kinesin hydrolyzes 1 ATP per 8 nm step in the cells as in solution, the motor efficiency is 33%. Magnetic bead assays were performed in hopes of emulating vesicle motion. Though the cells took up the beads, little bead motion was detectable in the presence of a strong magnetic gradient. ix