Single-Molecule Nanomechanics of Kinesin and Kinesin-Family Proteins

Abstract

There are roughly 45 kinesin superfamily members coded by the human genome, and 25 by the fruit fly genome: these motor proteins can be classified into at least 14 different subtypes. An individual cell typically expresses up to a dozen-or-more different versions at any given time. Although the nanomechanical properties of the founding member of the superfamily, kinesin-1, are well established, it's been an ongoing challenge to understand the physiology of the other kinesin members, and in particular, how that physiology ties into their varied cellular functions. It's become clear that different kinesin motors exhibit radically different nanomechanical properties, including wide variations in translocation speed, processivity, randomness, and response to load. These properties dictate not only the diverse range of behaviors exhibited by individual motors, but also the behavior of motors working in ensembles, for example, when multiple motors are attached to a common cargo. For a number of years, my lab has been using single-molecule optical trapping to characterize the nanomechanical properties of kinesin family motors in detail, including Eg-5 (kinesin-5), KIF17 and KIF3AB (kinesin-2), and KIF15 (kinesin-12), as well as variants of kinesin-1. A common feature that has emerged is that different kinesin motors can all be modeled by a similar four-state, biochemical cycle involving a partial docking of the neck linker. This minimal model accounts quantitatively for forward stepping, backward stepping, and eventual microtubule release, adjusting only the transition rates among states for different motors. In particular, the substantial nanomechanical differences exhibited by two kinesin-2 motors, KIF17 and KIF3AB, which act jointly to drive intraflagellar transport, can explain in vivo data showing that these motors “trade off” while transporting vesicular cargo along the length of cilium (Prevo at el., 2015, Nat. Cell. Biol. 17:1546-45; Milic et al., 2017 Proc. Nat. Acad. Sci. USA 114:E6830-E6838).