FIGURE 5.

Active Gα_s promotes microtubule depolymerization and increases the catastrophe frequency. A, tubulin (23 μm) was incubated with Gα_s^QL for 1 h at 30 °C in the presence of 1 mm GTP. Microtubules were pelleted by centrifugation, and the amount of tubulin in supernatant and pellet fractions was quantified (see “Experimental Procedures”). Gα_s^QL inhibited microtubule assembly with an IC₅₀ of 3.5 ± 0.4 μm (n = 3). B–D, life history plot of microtubules in the absence (B) or presence of inactive (C) or active (D) Gα_s. Gα_s was added to microtubules polymerized on sea urchin axoneme seeds, and the length of microtubules was determined over time (see “Experimental Procedures”). Three representative microtubules are shown in each panel. E, effect of inactive (Gα_s^WT) and active (Gα_s^QL) G proteins on the catastrophe frequency (events/min). ***, p < 0.001. Dashed lines, 95% confidence interval for best fit curves; error bars, S.E.