A Micromechanical Model of Cargo Transport by Multiple Microtubule Motors

Abstract

Although the motions of many sub-cellular cargos are known to be driven by teams of microtubule motor proteins, understanding the role collective motor function plays in intracellular transport has been difficult to characterize. One confounding issue is that a team of molecular motors can adopt a spectrum of microtubule-associated configurations, depending on the number of bound motors and their organization on the microtubule. Since each microstate configuration can confer different mechanical and dynamic properties to a cargo, understanding how molecular motors function collectively ultimately requires knowledge of their relative contribution to cargo motility. To address this issue, we have developed methods to examine the load-dependent transport properties of structurally-defined multiple motor systems composed of two kinesin-1 molecules. Herein, we describe a discrete microstate transition-rate model of two-kinesin mechanics that can explain several unexpected behaviors observed in our assays. While this model accounts for a comprehensive spectrum of geometric arrangements of motors between a cargo and the microtubule, transition rates between microstate configurations are almost exclusively parameterized using data from single-kinesin optical trapping measurements. Overall, our model shows strong agreement with our data and recapitulates the central experimental finding that configurations where two kinesins both assume a portion of the applied load are rare and short-lived, causing two kinesins to exhibit negative cooperativity and properties that resemble the action of a single motor molecule. The bottom-up construction of our model also allows us to explore the effects of changes in single motor properties such as compliance, force-velocity relationship, and forward/backward stepping behavior on the dynamics of the system as a whole. This now provides a foundation for analyses of transport behaviors produced by more complex systems composed of different numbers, types and geometric arrangements of motors.