Kinesin's backsteps under mechanical load

TL;DR

This paper reviews kinesin's backstepping under load, challenging the ATP synthesis hypothesis and proposing an expanded mechanistic model that includes backward and nucleotide-free steps for better explanation.

Contribution

It introduces an extended mechanistic framework for kinesin backsteps, incorporating backward and nucleotide-free pathways, improving understanding of load-dependent stepping behavior.

Findings

Backstepping is not due to ATP synthesis, as evidenced by ATP-independent stall force.

Backsteps have a slower cycle time compared to forward steps.

Supplementing the model with backward and nucleotide-free routes better explains experimental data.

Abstract

Kinesins move processively toward the plus end of microtubules by hydrolyzing ATP for each step. From an enzymatic perspective, the mechanism of mechanical motion coupled to the nucleotide chemistry is often well explained using a single-loop cyclic reaction. However, several difficulties arise in interpreting kinesin's backstepping within this framework, especially when external forces oppose the motion of kinesin. We review evidence, such as an ATP-independent stall force and a slower cycle time for backsteps, that has emerged to challenge the idea that kinesin backstepping is due to ATP synthesis, i.e., the reverse cycle of kinesin's forward-stepping chemomechanics. Supplementing the conventional single-loop chemomechanics with routes for ATP-hydrolyzing backward steps and nucleotide-free steps, especially under load, gives a better physical interpretation of the experimental data on backsteps.