Single-molecule behavior of monomeric and heteromeric kinesins

Conventional kinesin is capable of long-range, processive movement along microtubules, a property that has been assumed to be important for its role in membrane transport. Here we have investigated whether the Caenorhabditis elegans monomeric kinesin unc104 and the sea urchin heteromeric kinesin KRP85/95, two other members of the kinesin superfamily that function in membrane transport, are also processive. Both motors were fused to green fluorescent protein, and the fusion proteins were tested for processive ability using a single-molecule fluorescence imaging microscope. Neither unc104-GFP nor KRP85/95-GFP exhibited processive movement (detection limit similar to 40 nm), although both motors were functional in multiple motor microtubule gliding assays (upsilon = 1760 +/- 540 and 202 +/- 37 nm/s, respectively). Moreover, the ATP turnover rates (5.5 and 3.1 ATPs per motor domain per second, respectively) are too low to give rise to the observed microtubule gliding velocities, if only a single motor were driving transport with an 8 nm step per ATPase cycle. Instead, the results suggest that these motors have low duty cycles and that high processivity may not be required for efficient vesicle transport. Conventional kinesin's unusual processivity may be required for efficient transport of protein complexes that cannot carry multiple motors.