| Summary: | The number of motor proteins attached to cellular cargos is widely believed to influence intracellular transport processes and may play a role in transport regulation. However, to date, investigating the biophysics of multiple-motor dynamics has been challenging since the number of motors responsible for cargo motion is not easily characterized. This work examines the transport properties of structurally-defined motor complexes containing two kinesin-1 motors, from both an experimental and theoretical perspective. Motor complexes were synthesized using DNA as a molecular scaffold and engineered DNA-conjugated protein polymers as linkers to couple motors to scaffolds. After anchoring the motor complexes to a bead their dynamic properties were measured using an automated optical trapping instrument that could be used to perform both static (increasing load) and force-feedback (constant load) optical trapping experiments. Data from these experiments is compared to predictions from a microscopic transition rate model of multiple kinesin dynamics. Together, these studies uncovered that multiple kinesins typically cannot cooperate since the microtubule-bound configuration of a motor complex often prevents both kinesins from sharing cargo loads. Furthermore, multiple-motor behaviors are influenced by the fact that motor complexes display hysteretic force-velocity behaviors when applied loads change rapidly in time. Overall, such behaviors suggest the number of kinesins on a cargo will not be a key determinant of intracellular transport processes, and in turn, will not contribute appreciably to mechanisms that regulate cargo motion. However, this work also provides evidence that processive microtubule motors that are less efficient than kinesin (e.g., dynein) will cooperate productively, produce greater responses to motor number, and may therefore act as a regulator of cargo transport.
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