Motor proteins of the conserved kinesin-14 family have important roles in mitotic spindle organization and chromosome segregation. Previous studies have indicated that kinesin-14 motors are non-processive enzymes, working in the context of multi-motor ensembles that collectively organize microtubule networks. In this study, we show that the yeast kinesin-14 Kar3 generates processive movement as a heterodimer with the non-motor proteins Cik1 or Vik1. By analyzing the single-molecule properties of engineered motors, we demonstrate that the non-catalytic domain has a key role in the motility mechanism by acting as a ‘foothold’ that allows Kar3 to bias translocation towards the minus end. This mechanism rivals the speed and run length of conventional motors, can support transport of the Ndc80 complex in vitro and is critical for Kar3 function in vivo. Our findings provide an example for a non-conventional translocation mechanism and can explain how Kar3 substitutes for key functions of Dynein in the yeast nucleus.Molecules can be transported around a cell by so-called motor proteins that move along a network of filaments called microtubules. Many motor proteins—including the kinesin family of these proteins—can only move in one direction along a microtubule. In most cells, kinesins tend to transport other molecules away from the center and towards the cell edge.Kinesins can have different structures, but most are made up of two subunits that are joined and work together to create a walking-like movement. Each subunit has a region called a motor domain (also known as its ‘head’) that can bind to the microtubule and to a molecule called ATP, which provides the energy required for the motor to step forward.Kinesins can be classed either as processive or non-processive motors. Processive motors can walk continuously along a microtubule for several hundred steps, whereas non-processive motors fall off after just a few steps. A motor protein called Kar3 belongs to a group of non-processive kinesins. Kar3 is unusual; unlike most of the motors in this group (which work together in pairs), Kar3 motor protein subunits each bind to and work with non-motor protein subunits, including one called Cik1. The head of the non-motor protein cannot bind to ATP, although it can bind to microtubules. This means that the non-motor protein subunits are not provided with the energy to make a stepping motion; this raises questions about how the Kar3 motor protein moves along the microtubule, and whether this affects the roles the motor performs.Mieck et al. studied how a molecular motor made up of Kar3 and Cik1 moves along microtubules using sensitive microscopy that allows single molecules to be observed. This revealed that, contrary to what is expected from a non-processive motor, Kar3–Cik1 moves long distances on microtubules without detaching from them. Further investigation showed that Cik1 provides a ‘foothold’ for the motor, binding it to the microtubule in such a way that allows it to move along the microtubule in the opposite direction to most kinesins. In addition, Mieck et al. found that the Kar3–Cik1 motor binds to and transports a protein complex that is crucial for separating chromosomes during cell division.A challenge for the future is to understand in even greater detail how the movement of such a motor occurs. If it doesn't ‘walk’ like other motors, then how can its motion be explained? The benefits for the cell that underlie why this mechanism evolved also remain to be discovered.
Mieck, C., Molodtsov, M. I., Drzewicka, K., Van Der Vaart, B., Litos, G., Schmauss, G., … Westermann, S. (2015). Non-catalytic motor domains enable processive movement and functional diversification of the kinesin-14 kar3. ELife, 2015(4). https://doi.org/10.7554/eLife.04489