GTP hydrolysis controls the dynamic instability of microtubules. (A) Tubulin dimers carrying GTP (red) bind more tightly to one another than do tubulin dimers carrying GDP (dark green). Therefore, rapidly growing plus ends of microtubules, which have freshly added tubulin dimers with GTP bound, tend to keep growing. (B) From time to time, however, especially if microtubule growth is slow, the dimers in this GTP cap will hydrolyze their GTP to GDP before fresh dimers loaded with GTP have time to bind. The GTP cap is thereby lost. Because the GDP-carrying dimers are less tightly bound in the polymer, the protofilaments peel away from the plus end, and the dimers are released, causing the microtubule to shrink (Movie 17.3).
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