Comparison of the effects of microtubule-associated protein 2 and tau on the packing density of in vitro assembled microtubules.

I have compared the effects of microtubule-associated protein 2 (MAP-2) and tau on the packing density of sedimented microtubules. Microtubules assembled in vitro in taxol were pelleted by centrifugation. The volumes of the resulting pellets were calculated from their weights assuming a specific gravity of 1 and then were normalized to the amount of protein in the pellet, yielding a value for pellet specific volume in microliter/mg of protein. The specific volume of the pellets reflects the intermicrotubule spacing within the pellet. Microtubules were assembled from tubulin alone or tubulin plus various amounts of MAP-2 or tau and collected by centrifugation, and the pellet specific volume was measured. The specific volume of microtubules composed of pure tubulin ranged from 6.4 to 7.7 microliter/mg of protein. Tau had no detectable effect on this value even at saturating levels on the microtubules. In contrast, MAP-2 increased pellet specific volume as the MAP-2/tubulin weight ratio increased; at the highest ratio examined, 0.43, the pellet specific volume was approximately 33. Even at the relatively low MAP-2/tubulin ratio of 0.09, pellet specific volume was approximately 2-fold greater than that of microtubules containing tubulin alone or tubulin plus tau. Electron microscopy confirmed that the observations on pellet specific volume reflected differences in the effects of MAP-2 and tau on the packing density of sedimented microtubules. These results are discussed in the context of observations showing that neighboring microtubules are more widely spaced in dendrites than in axons and that MAP-2 is enriched on microtubules in dendrites compared to microtubules in axons, whereas the converse is true for tau.