Inside living cells organelles and other cell constituents are constantly moving to reach where they are needed and establish internal cellular organization. This transport is accomplished by molecular motor proteins that haul the cargoes through stepping along the network of intracellular filaments, microtubules and actin filaments. In vitro work with artificial cargoes had immensely contributed to understanding the function of individual isolated microtubule motors of opposite polarity (kinesin and dynein). However, in contrast to the unidirectional artificial cargos, those in living cells spend energy to constantly move back and forth while their net direction is precisely controlled by the cell. The particulars of how opposite polarity motors work together and how their function is regulated in vivo remain for the most part enigmatic at many levels of detail.
|
|
|
An optical trap (tweezers) is a focused laser beam that captures the lipid droplets. Motors pulling a droplet out of the trap center experience an ever-increasing backwards force that eventually stalls the motors. The maximum distance the motors were able to pull the droplet is proportional to the motors’ stall force. |
A histogram of stall forces for plus-end directed lipid droplets shows peaks at commensurate values of force. The three peaks are due to lipid droplets hauled by 1, 2 and 3 motors respectively. Measuring the force enables us to count the number of motors pulling a cargo as it moves along the microtubule. |