Diffusion of kinesin motors on cargo can enhance binding and run lengths during intracellular transport

Abstract

Cellular cargos, including lipid droplets and mitochondria, are transported along microtubules using molecular motors such as kinesins. In the cell, it is unclear how motors are coordinated to achieve transport outcomes that are cargo-specific. One possibility is that transport is modulated by differences in organization and mobility of motors on the cargo's surface. We use mechanochemical 3D computational modeling to compare different motor anchoring modes, and find that organizational changes can optimize for different objectives. Cargos with clustered motors are transported efficiently, but are slow to bind to microtubules. Cargos with motors dispersed rigidly on their surface bind microtubules quickly, but are transported inefficiently. Cargos with freely-diffusing motors have both fast binding and efficient transport, although less efficient than clustered motors. These results point to a functional role for observed changes in motor organization on cargos, and suggest motor diffusivity as a control point for transport, either by modulation of adaptor proteins or changes in lipid composition. Author summaryThe molecular motors of the kinesin family are responsible for moving the parts of cells 1 to their subcellular destinations, organizing the cell interior. Computational modeling 2 has historically played an important role in understanding how these motors work. But, 3 strikingly, most models assume the subcellular cargos have rigid surfaces. Simulating 4 fluid surfaces is more challenging but more applicable to biological cargo. In this work, 5 we build a computational model of molecular motor transport of cargo with fluid 6 surfaces. Surface fluidity gives cargo transport new properties, like enhanced transport 7 efficiency, and the ability to push against opposing forces. Because the effects of surface 8 fluidity vary strongly with some of the properties of the cargo, like its size, our results 9 hint at one of the ways that the same transport machinery can distinguish different 10 cargo despite using the same motors. 13 kinesin and dynein superfamilies to transport organelles and other cargo along 14 microtubules. Despite having only a limited set of cargo transport motors (kinesin-1, 15 kinesin-2 and kinesin-3 families [1], along with cytoplasmic dynein), different cargos are 16 transported to different locations, even though they are transported along the same set 17 of microtubule "roads". For example, under normal conditions COS-7 cells direct lipid 18 droplets toward microtubule plus end, localizing them near the plasma membrane, and 19 mitochondria toward the minus end, localizing them near the nucleus. Under glucose 20 starvation, localization of both organelles changes to spread them out around the cell, 21 allowing them to come into contact with each other [2]. How do cells achieve these 22 cargo-specific routing outcomes? In some cases, cells use molecular specificity to achieve 23 cargo specificity, such as using specific linkers or cargo-bound regulators [3,4]. 24 However, recent experiment...

Processivity of molecular motors under vectorial loads

“…It explained that the stall force of multiple kinesin can be similar to that of a single kinesin when the vertical load component is high, as observed in [31,32], due to slip-bond behaviour. However, multiple kinesins can produce forces much larger than the single-motor force when the vertical load component is small, e.g., in gliding assays [33,34] and bead assays [35][36][37], due to catch-bond behaviour. This later prediction has been recently confirmed in a three-bead assay where lower detachment rates were observed for a single kinesin with low kinesin-MT angle [38].…”

Roles of motor on-rate and cargo mobility in intracellular transport

“…This would allow motors freedom to diffuse on the two-dimensional cargo surface, unlike in the bead experiments here, and more like [25]. In [22], we use computational simulation to demonstrate that this motor freedom allows motors to find the microtubule quickly, overcoming the slow cargo rotation times observed here. Alternatively or in coordination, MAPs may tether the cargo in place or otherwise spatially constrain the motors.…”

“…Heuristically, this conclusion follows from the inevitability that if the motor is away from the microtubule, it takes for the cargo to rotate so that the motor comes near it. Even for an infinitely high motor on rate, the mean time for the cargo to bind is 0.1 s to 1 s if the motor starts far away [22]. This effect is exacerbated the fact that the motor is small in comparison to the cargo.…”

“…Our results suggest that a severely limiting timescale for intracellular transport is the timescale for reorganization of the cargo necessary to allow the motor access to the microtubule. Even with an infinitely high motor on rate, the time for the cargo to bind is long, especially at high viscosities [22]. To overcome this, we hypothesize that many biological cargos have surfaces with liquid properties, like lipid droplets or vesicles.…”

“…Simulations were done using models and code constructed previously [16,22]. We note that unlike in [16], we include only one microtubule and one motor on the bead.…”