Testing the Tug-of-War Model of Bidirectional Transport In Vivo

Abstract

Molecular motor proteins are responsible for force generation in myriad cellular processes. Much of our understanding of how motors function has benefited from force measurements and manipulation at the single molecule level in vitro. We report on novel optical trapping methodology capable of precise in vivo stall-force measurements of individual endogenous cargoes hauled by molecular motors in their native environment. We present unprecedented stall force histograms of motor-driven lipid droplets in Drosophila embryos. Force measurements show that equal numbers of kinesin-1 and cytoplasmic dynein haul each cargo in opposite directions. Critically, by measuring cargo dynamics in the optical trap, we find that there is memory: it is more likely for a cargo to resume motion in the same direction -rather than reverse direction- after the motors transporting it detach from the microtubule due to the trap force. This suggests that only motors of one polarity are active on the cargo at any instant in time and is not consistent with the tug-of-war models of bidirectional transport where both polarity motors can bind the microtubules at all times. We further use the optical trap to measure, in vivo, the detachment rates from microtubules of kinesin-1 and dynein-driven lipid droplets. Unlike what is commonly assumed, we find that dynein's but not kinesin's detachment time in vivo increases with opposing load. This suggests that dynein's interaction with microtubules behaves like a catch bond and further argues in favor of a regulatory model for bidirectional transport.