FIGURE 1:
Schematics of the theoretical idea of FT. (A) The schematics of the force–velocity relations for motor proteins (FPUs). The curves represent the case of one FPU (red), two FPUs (yellow), and three FPUs (green). The lines represent the relation Γv = F, where F is a force exerted by motors and v and Γ are the velocity and friction coefficient of a cargo vesicle. The values of drag force for one FPU, two FPUs, and three FPUs are considered to be discrete in the high viscous case (purple line), similarly to the values of stall force. (B) A cargo (yellow) is transported by motors (red) along a microtubule (pink) at a constant velocity (v) (top). Here the dashed circles represent fluctuating movement of the cargo, and the blue and green circles represent water molecules, vesicles, or organelles that collide with the cargo. The cargo fluctuates due to the collision with other vesicles and organelles in addition to thermal noise, while exhibiting directional motion. In the macroscopic description of the cargo transport (bottom), force (F) exerted by motors is balanced by the drag force (Γv). In the macroscopic description, the fluctuation of the cargo is described by the effective temperature (Teff) representing the additional noise. In this paper, F is estimated from this fluctuation via the FT.