Kinesin Is an Evolutionarily Fine-Tuned Molecular Ratchet-and-Pawl Device of Decisively Locked Direction

TL;DR

This study uses physical modeling to reveal how kinesin's molecular structure acts as a ratchet-and-pawl device, ensuring unidirectional movement along microtubules, with adaptations for back-stepping under extreme loads.

Contribution

It provides a detailed physical mechanism explaining kinesin's directionality and introduces a quantitative model showing evolutionary fine-tuning of its molecular properties.

Findings

Kinesin's energy landscape enforces unidirectional stepping.

Asymmetric ground state locks kinesin's movement toward the microtubule plus-end.

Extreme loads can induce back-steps, but the ratchet mechanism remains partially functional.

Abstract

Conventional kinesin is a dimeric motor protein that transports membranous organelles toward the plus-end of microtubules (MTs). Individual kinesin dimers show steadfast directionality and hundreds of consecutive steps, yetthe detailed physical mechanism remains unclear. Here we compute free energies for the entire dimer-MT system for all possible interacting configurations by taking full account of molecular details. Employing merely first principles and several measured binding and barrier energies, the system-level analysis reveals insurmountable energy gaps between configurations, asymmetric ground state caused by mechanically lifted configurational degeneracy, and forbidden transitions ensuring coordination between both motor domains for alternating catalysis. This wealth of physical effects converts a kinesin dimer into a molecular ratchet-and-pawl device, which determinedly locks the dimer's movement into the MT plus-end and ensures consecutive steps in hand-over-hand gait.Under a certain range of extreme loads, however, the ratchet-and-pawl device becomes defective but not entirely abolished to allow consecutive back-steps. This study yielded quantitative evidence that kinesin's multiple molecular properties have been evolutionarily adapted to fine-tune the ratchet-and-pawl device so as to ensure the motor's distinguished performance.