Microtubule-associated protein 2 alters the dynamic properties of microtubule assembly and disassembly.

The influence of microtubule-associated protein 2 (MAP2) on the dynamics of microtubule assembly and disassembly from axonemal fragments was characterized in vitro in solutions of pure tubulin and varying concentrations of MAP2. A mechanistic description of interactions between MAP2 and individual microtubules was developed from analysis of recorded images obtained by video-enhanced differential-interference-contrast light microscopy. MAP2 decreased the rates and lengths of shortening events and decreased the frequency of transitions between growth and shortening over a wide range of concentrations, thereby producing the increases in net microtubule growth previously described by light-scattering techniques. Increases in rates and lengths of elongation phases, as well as rescue frequencies (i.e. transition from shortening to growth), were observed under conditions in which microtubules are expected to be saturated with MAP2. During early stages of nucleated assembly, MAP2 greatly increased the number of microtubules growing from a given axoneme and caused elongation of "curved" structures which may be sheet-like microtubules.