# Molecular interplay between HURP and Kif18A in mitotic spindle regulation

## TL;DR

This study explores how HURP and Kif18A interact to regulate microtubule dynamics during cell division, revealing a mechanism for spindle length control.

## Contribution

The novel contribution is the discovery of how HURP regulates Kif18A motility through steric exclusion and microtubule binding.

## Key findings

- HURP activates Kif18A motility at low concentrations but inhibits it at higher concentrations.
- Cryo-EM shows HURP binds microtubules via two motifs, overlapping with Kif18A's binding site.
- HURP and Kif18A together suppress microtubule plus-end dynamics, aiding spindle length control.

## Abstract

During mitosis, microtubule dynamics are regulated to ensure proper alignment and segregation of chromosomes. The dynamics of kinetochore-attached microtubules are regulated by hepatoma-upregulated protein (HURP) and the mitotic kinesin-8 Kif18A, but the underlying mechanism remains elusive. Using single-molecule imaging in vitro, we demonstrate that Kif18A motility is regulated by HURP. While sparse decoration of HURP activates the motor, higher concentrations hinder processive motility. To shed light on this behavior, we determined the binding mode of HURP to microtubules using Cryo-EM. The structure reveals that one HURP motif spans laterally across β-tubulin, while a second motif binds between adjacent protofilaments. HURP partially overlaps with the microtubule-binding site of the Kif18A motor domain, indicating that excess HURP inhibits Kif18A motility by steric exclusion. We also observed that HURP and Kif18A function together to suppress dynamics of the microtubule plus-end, providing a mechanistic basis for how they collectively serve in spindle length control.