Science 325, 870–873 (2009)

Motor proteins that carry cargo around the cell by 'walking' along microtubule filaments are slowed by the friction caused by the rupturing of chemical bonds as they go.

Jonathon Howard of the Max Planck Institute of Molecular Cell Biology and Genetics in Dresden, Germany, Erik Schäffer of the Technical University of Dresden and their colleagues measured this friction in vitro. Using optical tweezers, they dragged a microsphere coated in kinesin-8 — a motor protein from yeast — over an immobilized microtubule. They calculate that, at top speed, friction between a kinesin molecule and its filament dissipates about half of the chemical energy that the cell usually supplies to drive the motor along its track.