The family-specific K-loop functions to increase motor-microtubule affinity and does not contribute to processivity. Although the kinesin motor domain is highly conserved, different kinesin families contain unique surface features whose functional outputs are largely unexamined. We show that the K-loop, a positively charged segment on the kinesin-3 motor surface, does not contribute to the processivity of dimeric kinesin-3 motors but rather provides the motor with a high affinity for the microtubule in its weakly bound state.  These results clarify past results on the K-loop and provide one of the first systematic explorations of kinesin surface features. 

A new mechanism of regulation for kinesin motors.  Kinesin motors activity must be tightly regulated in cells to prevent futile ATP hydrolysis and clogging of microtubule tracks.  Most kinesin motors studied to date are kept inactive by autoinhibition in the dimeric state.  We show that kinesin-3 motors are regulated in a unique way that prevents dimerization (and thus processive motion) and we define two different mechanisms for preventing dimerization.  First, we show that KIF1 and KIF16 motors utilize an intra-molecular interaction between the neck coil (NC) and coiled-coil 1 (CC1) segments to prevent motor dimerization.  Second, we show that KIF13 motors contain a Proline-induced helical shift between the NC and CC1 segments that prevents motor dimerization.