Microtubule-based mitotic spindles contain a micron-sized mixed-nucleotide zone

Current models infer that the microtubule-based mitotic spindle is built from GDP-tubulin with small GTP caps at microtubule plus-ends, including those that attach to kinetochores (K-fibres). Here we reveal that K-fibres additionally contain a dynamic mixed-nucleotide zone that reaches several microns in length. This zone becomes visible in cells expressing fluorescently labelled EBs, a known marker for GTP-tubulin, and endogenously-labelled HURP - a protein which we show to preferentially bind the GDP microtubule lattice in vitro . In living cells HURP accumulates on the ends of depolymerising K-fibres, whilst avoiding recruitment to nascent polymerising K-fibres. This gives rise to a growing “HURP-gap” which we can recapitulate in a minimal computational simulation. We therefore postulate that the K-fibre lattice contains a dynamic, micron-sized mixed-nucleotide zone. One Sentence Summary We reveal that the microtubules of the mitotic spindle contain a third, uncharacterized domain, a mixed nucleotide zone that resides between the GTP-cap and the GDP-tubulin lattice.