CLASP1 stabilizes unstable microtubule ends in a nucleotide-dependent manner. (A) Representative kymographs of mixed-nucleotide microtubule extensions (green) undergoing depolymerization after tubulin dilution. Microtubules were grown with 7 µM A488 tubulin, 0.8 mM GMPCPP, and 0.2 mM GTP, and then the buffer was exchanged while imaging (dotted white line) for nucleotides alone or in the presence of 20 nM CLASP1. (B) Representative kymographs of GDP microtubule extensions (green) undergoing depolymerization after tubulin dilution. Microtubules were first grown with 12 µM A488 tubulin and 100 µM GTP, and then the buffer was exchanged while imaging (dotted white line) for nucleotides alone or in the presence of 20 nM CLASP1. (C) Quantification of the mixed-lattice and GDP-lattice microtubule depolymerization rates following dilution with and without CLASP1 in the presence of different nucleotides. N = 25–64 microtubules for each condition obtained across three experimental days. Individual data points from different experiments are plotted in different shades. The means for each experimental repeat are plotted as larger points in the same color. The squares indicate the average of the experimental means and the vertical bars are the standard errors of the means. The mean rates and statistics can be found in Table S1. (C–E) The data in C were replotted to show dilution-induced microtubule depolymerization rates in the presence of nucleotides alone (D) and in the presence of CLASP (E) for the different microtubule templates. Welch’s two-tailed unequal variances t tests were performed, and the corresponding P values are indicated on the graphs.