Filament-filament switching can be regulated by separation between filaments together with cargo motor number

Abstract

How intracellular transport controls the probability that cargos switch at intersections between filaments is not well understood. In one hypothesis some motors on the cargo attach to one filament while others attach to the intersecting filament, and the ensuing tug-of-war determines which filament is chosen. We investigate this hypothesis using 3D computer simulations, and discover that switching at intersections increases with the number of motors on the cargo, but is not strongly dependent on motor number when the filaments touch. Thus, simply controlling the number of active motors on the cargo cannot account for in vivo observations that found reduced switching with increasing motor number, suggesting additional mechanisms of regulation. We use simulations to show that one possible way to regulate switching is by simultaneously adjusting the separation between planes containing the crossing filaments and the total number of active motors on the cargo. Heretofore, the effect of filament-filament separation on switching has been unexplored. We find that the switching probability decreases with increasing filament separation. This effect is particularly strong for cargos with only a modest number of motors. As the filament separation increases past the maximum head-to-head distance of the motor, individual motors walking along a filament will be unable to reach the intersecting filament. Thus, any switching requires that other motors on the cargo attach to the intersecting filament and haul the cargo along it, while motor(s) engaged on the original filament detach. Further, if the filament separation is large enough, the cargo can have difficulty proceeding along the initial filament because the engaged motors can walk underneath the intersecting filament, but the cargo itself cannot fit between the filaments. Thus, the cargo either detaches entirely from the original filament, or must dip to the side of the initial filament and then pass below the crossing filament.

Figure 1. Cargo filament geometry used in simulations.

(A) Geometry of two actin filaments crossing. (B) Cargo initially starts above filament 1. (C) Cargo initially starts below filament 1.

Figure 2. Probability of different outcomes for a cargo approaching an intersection versus the total number of motors on the cargo with no vertical separation between filaments.

“Above” means the cargo started on top of the initial filament. “Below” means the cargo started on the bottom of the initial filament. The intersecting filament lay on top of the initial filament at an angle of 70 degrees. (A) Probability that a cargo got stopped at an intersection, i.e., probability that a cargo detached at an intersection or before reaching the intersection. (B) Probability that a cargo went through an intersection without switching filaments or getting stuck. (C) Probability that a cargo switched actin filaments. The error bars illustrate the standard error of the outcomes, which is no greater than approximately 0.3%. The lines connecting points are merely guides for the eye; they do not imply a specific functional relationship.

Figure 3. Average number of engaged motors that actively hauled a cargo versus the total number of motors that were on the cargo for different vertical separations between the two filaments.

The lines represent linear regression fits to the results. Standard error of the average engaged motors is less than 0.1 and is not shown since it is smaller than the plot markers.

Figure 4. Probability of different outcomes for a cargo approaching an intersection versus the total number of motors on the cargo for vertical separations between filaments varying from 0 to 80 nm.

The cargo started on top of the initial filament. The intersecting filament lay on top of the initial filament at an angle of 70 degrees. (A) Percentage of cargos that stopped at an intersection, i.e., percentage of cargos that detached at an intersection or before reaching the intersection. (B) Percentage of cargos that went through an intersection without switching filaments or getting stuck. (C) Percentage of cargos that switched actin filaments. The standard error of the outcomes is less than 0.3% and is not shown because the errors are smaller than the size of the plot markers. The lines connecting points are merely guides for the eye; they do not imply a specific functional relationship.

Figure 5. Percentage of different outcomes for a cargo that approached an intersection versus vertical separation between the two filaments and versus the total number of motors on the cargo.

The cargo started on top of the initial filament. The intersecting filament lay on top of the initial filament at an angle of 70 degrees. (A) Percentage of cargos that stopped at an intersection, i.e., percentage of cargos that detached at an intersection or before reaching the intersection. (B) Percentage of cargos that went through an intersection without switching filaments or getting stuck. (C) Percentage of cargo that switched actin filaments. The standard error of the outcomes is no greater than 0.3% and is not shown.