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The effects of purified recombinant microtubule-associated protein 2c (rMAP2c) on the dynamic instability of microtubules were examined by direct observation of individual microtubules in vitro by video-enhanced differential interference contrast light microscopy. Microtubules were grown in the absence or presence of varying concentrations of rMAP2c and were analyzed to determine growth rates, shortening rates, and the frequencies of conversion between growing and shortening phases. We found rMAP2c to stabilize microtubules dramatically. The most notable effect is a reduction in both the frequency of catastrophes (transitions from growth to shortening) and the mean length of shortening events:  no microtubule catastrophes were observed at concentrations of rMAP2c as low as 1.06 μM in a solution of 10 μM tubulin. Even at lower rMAP2c concentrations, there is a marked stabilizing effect. As the …