Single-molecule behavior of monomeric and heteromeric kinesins

Conventional kinesin is capable of long-range, processive movement along mi crotubules, a property that has been assumed to be important for its role i n membrane transport. Here we have investigated whether the Caenorhabditis elegans monomeric kinesin unc104 and the sea urchin heteromeric kinesin KRP 85/95, two other members of the kinesin superfamily that function in membra ne transport, are also processive. Both motors were fused to green fluoresc ent protein, and the fusion proteins were tested for processive ability usi ng a single-molecule fluorescence imaging microscope. Neither unc104-GFP no r KRP85/95-GFP exhibited processive movement (detection limit similar to 40 nm), although both motors were functional in multiple motor microtubule gl iding assays (upsilon = 1760 +/- 540 and 202 +/- 37 nm/s, respectively). Mo reover, the ATP turnover rates (5.5 and 3.1 ATPs per motor domain per secon d, respectively) are too low to give rise to the observed microtubule glidi ng velocities, if only a single motor were driving transport with an 8 nm s tep per ATPase cycle. Instead, the results suggest that these motors have l ow duty cycles and that high processivity may not be required for efficient vesicle transport. Conventional kinesin's unusual processivity may be requ ired for efficient transport of protein complexes that cannot carry multipl e motors.