Structural and Functional Adaptations in Kinesin Motors

Abstract

Kinesin-13s are members of the kinesin super-family of motor proteins that modulate microtubule (MT) dynamics in a variety of important cell processes such as mitosis, cytokinesis, axonal branching, and ciliogenesis. Like all kinesins, they have the highly conserved globular catalytic motor domain that defines the superfamily. However, distinct from most other members of the super-family, kinesin-13s are MT depolymerases with no motile activity. While motile kinesins alternate between strong and weakly bound states to the MT, in coordination with ATP hydrolysis and stepping, kinesin-13s interact weakly with the MT and undergo direction unbiased one-dimensional diffusion until reaching the MT ends where they induce depolymerization. In comparison with the more studied conventional kinesin, the kinesin-13 action mechanism and the structural basis of their functional differences with motile kinesins are less clear. To address this issue we used fluorescence polarization microscopy and electron microscopy to investigate the dynamic conformation of MT-bound kinesin constructs based on the D. melanogaster kinesin-13 KLP10A. We found that KLP10A interacts with the MT in two coexisting modes, one in which the motor domain binds with a specific orientation to the MT lattice, and another where the motor domain is very mobile and able to undergo one dimensional diffusion. Surprisingly, the KLP10A motor domain alone can undergo one dimensional diffusion and kinesin-13 class specific motifs are necessary to steer the motor domain into the typical microtubule bound configuration of motile kinesins. These results indicate that motifs located outside the motor, such as the kinesin-13 specific neck domain, are not required to set the distinct interaction modes of kinesin-13 with the MT. They also indicate that the kinesin-13 motor domain is uniquely adapted to elude forming the tight MT bound states that motile kinesins form at different points of their ATPase cycle.