The number of microtubule motors attached to vesicles, organelles, and other subcellular commodities is widely believed to influence their motile properties. There is also evidence that cells regulate intracellular transport by tuning the number and/or ratio of motor types on cargos. Yet, the number of motors responsible for cargo motion is not easily characterized, and the extent to which motor copy number affects intracellular transport remains controversial. Here, we examined the load-dependent properties of structurally defined motor assemblies composed of two kinesin-1 molecules. We found that a group of kinesins can produce forces and move with velocities beyond the abilities of single kinesin molecules. However, such capabilities are not typically harnessed by the system. Instead, two-kinesin assemblies adopt a range of microtubule-bound configurations while transporting cargos against an applied load. The binding arrangement of motors on their filament dictates how loads are distributed within the two-motor system, which in turn influences motor-microtubule affinities. Most configurations promote microtubule detachment and prevent both kinesins from contributing to force production. These results imply that cargos will tend to be carried by only a fraction of the total number of kinesins that are available for transport at any given time, and provide an alternative explanation for observations that intracellular transport depends weakly on kinesin number in vivo.

The collective function of motor proteins is known to be important for the directed transport of many intracellular cargos. However, understanding how multiple motors function as a group remains challenging and requires new methods that enable determination of both the exact number of motors participating in motility and their organization on subcellular cargos. Here we present a biosynthetic method that enables exactly two kinesin-1 molecules to be organized on linear scaffolds that separate the motors by a distance of 50 nm. Tracking the motions of these complexes revealed that while two motors produce longer average run lengths than single kinesins, the system effectively behaves as though a single-motor attachment state dominates motility. It is proposed that negative motor interference derived from asynchronous motor stepping and the communication of forces between motors leads to this behavior by promoting the rapid exchange between different microtubule-bound configurations of the assemblies.

Subcellular cargos are often transported by teams of processive molecular motors, which raises questions regarding the role of motor cooperation in intracellular transport. Although our ability to characterize the transport behaviors of multiple-motor systems has improved substantially, many aspects of multiple-motor dynamics are poorly understood. This work describes a transition rate model that predicts the load-dependent transport behaviors of multiple-motor complexes from detailed measurements of a single motor's elastic and mechanochemical properties. Transition rates are parameterized via analyses of single-motor stepping behaviors, load-rate-dependent motor-filament detachment kinetics, and strain-induced stiffening of motor-cargo linkages. The model reproduces key signatures found in optical trapping studies of structurally defined complexes composed of two kinesin motors, and predicts that multiple kinesins generally have difficulties in cooperating together. Although such behavior is influenced by the spatiotemporal dependence of the applied load, it appears to be directly linked to the efficiency of kinesin's stepping mechanism, and other types of less efficient and weaker processive motors are predicted to cooperate more productively. Thus, the mechanochemical efficiencies of different motor types may determine how effectively they cooperate together, and hence how motor copy number contributes to the regulation of cargo motion.

Transport of intracellular cargos by multiple microtubule motor proteins is believed to be a common and significant phenomenon in vivo, yet signatures of the microscopic dynamics of multiple motor systems are only now beginning to be resolved. Understanding these mechanisms largely depends on determining how grouping motors affect their association with microtubules and stepping rates, and hence, cargo run lengths and velocities. We examined this problem using a discrete state transition rate model of collective transport. This model accounts for the structural and mechanical properties in binding/unbinding and stepping transitions between distinct microtubule-bound configurations of a multiple motor system. In agreement with previous experiments that examine the dynamics of two coupled kinesin-1 motors, the energetic costs associated with deformations of mechanical linkages within a multiple motor assembly are found to reduce the system's overall microtubule affinity, producing attenuated mean cargo run lengths compared to cases where motors are assumed to function independently. With our present treatment, this attenuation largely stems from reductions in the microtubule binding rate and occurs even when mechanical coupling between motors is weak. Thus, our model suggests that, at least for a variety of kinesin-dependent transport processes, the net 'gains' obtained by grouping motors together may be smaller than previously expected.

Background: Multiple kinesin function is central to intracellular transport. Results: Unlike single-motor molecules, two kinesin velocities can depend on whether loads vary spatially or temporally. Conclusion: Kinesin cooperation is influenced appreciably by spatially dependent changes in load. Significance: Factors governing the force-time history and spatial dependence of loads must be examined to understand mechanisms regulating intracellular transport.