Measuring and modeling polymer gradients argues that spindle microtubules regulate their own nucleation

Spindle microtubules are nucleated by accessory proteins whose activity is
spatially regulated. It was shown that many spindle assembly factors bind
microtubules. Here, we investigate whether binding microtubules changes the
activity of nucleators. To study this issue, we use novel FLIM-FRET measurements
to map the concentration of microtubules and monomeric tubulin in and around the
spindle. We find that oligomeric tubulin is constrained to the spindle, with no
detectable gradient around it. This argues that microtubule nucleation is
restricted to the spindle. Using mathematical modelling we demonstrate that this
is indicative of a feedback from microtubule binding to nucleator activity. Our
results strongly suggest that nucleators binding to microtubules stimulates
their activity.