Activation of cytoplasmic dynein through microtubule crossbridging

Cytoplasmic dynein is the main microtubule-minus-end-directed transporter of cellular cargo in animal cells [1, 2]. Cytoplasmic dynein also functions in the organisation and positioning of mitotic spindles [3, 4] and the formation of ordered microtubule arrays in neurons and muscle [5, 6]. Activation of the motor for cargo transport is thought to require formation of a complex with dynactin and a cargo adapter [7-10]. Here we show that recombinant human dynein can crossbridge neighbouring microtubules and can be activated by this crossbridging to slide and polarity-sort microtubule bundles. While single molecules of human dynein are predominantly static or diffusive on single microtubules, they walk processively for 1.5 μm on average along the microtubule bundles they form. Speed and force output of dynein are doubled on bundles compared to single microtubules, indicating that the crossbridging dynein steps equivalently on two microtubules. Our data are consistent with a model of autoactivation through the physical separation of dynein motor domains when crossbridging two microtubules. This enables cytoplasmic dynein to function effectively as a microtubule organiser and transporter without needing to first form a complex with dynactin and a cargo adapter.