Island dynamics are distinct from GDP-tubulin extensions and GMPCPP seeds.

<p>(A) Shortening rates of GDP-tubulin extensions (white solid line) and GXP islands (white dashed line) and the time to microtubule regrowth (yellow double arrow measured from extrapolated yellow dotted line) were measured from kymographs of depolymerizing microtubules. (B) GXP islands shorten more slowly than GDP-tubulin extensions. Distribution of shortening rates measured for extensions and islands (<i>N</i> = 30). The inset shows a box-plot of the data, which has a median shortening rate of 0.49 µm/s for extensions and 0.16 µm/s for islands (<i>p</i><0.00001, Wilcoxon rank-sum test). Only cases where extension and island shortening could both be measured in the same depolymerization event were analyzed. (C) Microtubules grow more readily from islands than from GMPCPP seeds. Time to regrowth was measured from kymographs of dynamic microtubules and was on average shorter from islands than from GMPCPP seeds. Median time to regrowth was 6 seconds for GXP islands (<i>N</i> = 60) and 27 seconds for GMPCPP seeds (<i>N</i> = 60) (inset, <i>p</i><0.00001, Wilcoxon rank-sum test; outliers >100 are not shown).