The Kinesin Speed Paradox Revisited: How Can Cargoes Move Faster Than Their Motors?

Intracellular transport is essential for cellular organization and function. Kinesin-1, the founding member of the kinesin superfamily, delivers a wide range of cargoes along microtubules. Since its discovery, extensive work has identified cargo-kinesin interactions and regulatory mechanisms. Yet a basic question remains: why does kinesin-driven transport in cells often appear faster than the single-molecule velocity of kinesin-1 measured in vitro? Cargoes moved by kinesin-1 can exhibit markedly higher velocities in cells than isolated kinesin-1 molecules under commonly used assay conditions. How can kinesin-1-driven transport be accelerated in the cytoplasm crowded with macromolecules and other structures? In this Perspective, we discuss potential mechanisms underlying this "kinesin speed paradox" and consider what they reveal about motor behavior in complex intracellular milieus.