DIFFERENCES IN SURFACE-MORPHOLOGY OF MICROTUBULES RECONSTITUTED FROM PURE BRAIN TUBULIN USING 2 DIFFERENT MICROTUBULE-ASSOCIATED PROTEINS - HIGH MOLECULAR-WEIGHT MAP-2 PROTEINS AND TAU PROTEINS

Abstract

Microtubules were reconstituted from homogeneous [porcine] brain tubulin and homogeneous preparations of 2 different microtubule-associated proteins [MAP], the high MW MAP 2 proteins or the tau proteins. The resulting microtubules were characterized by 3 EM procedures: thin sectional analysis of embedded material, negative staining analysis using a STEM microscope and high resolution metal-shadowing analysis. By all 3 procedures MAP 2 microtubules have a much rougher surface morphology than tau microtubules, in agreement with the much higher MW of the MAP 2 proteins. Tau microtubules do not show the very smooth surface of microtubules assembled from pure tubulin in the absence of any microtubule-associated proteins. In the case of MAP 2 microtubules thin sectional analysis and metal shadowing reveals that the globular protrusions seen in negative staining analysis appear as linear side arms which may extend by much as 30 nm on both sides from the microtubular wall proper, giving rise to an overall structure with a diameter close to 100 nm. The possible implication of such structures for in vivo situations is briefly discussed, as is the possibility that the halo-effect around microtubules seen in vivo may be due to a structural organization similar to that of MAP 2 tubules in vitro.