We then investigated using a combination of mathematical modelling, live-cell imaging of endogenous labelled HURP and in vitro reconstitution using purified tubulin, EBs and HURP and discovered that HURP binds preferentially to microtubules built from GDP-tubulin while avoiding GTP-tubulin. i.e. HURP can read the nucleotide state! HURP gaps also lack EBs – a known marker of GTP-tubulin which forms at cap at the end of growing microtubules. This work defines the existence of a mixed nucleotide zone within the mitotic spindle and raises the possibility for equivalent regions in other microtubule-based assemblies. How the formation of HURP-stipes and HURP-gaps in the spindle impacts chromosome segregation is an open question.
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