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Association of high-molecular-weight proteins with microtubules and their role in microtubule assembly in vitro.

High-molecular-weight components (HMW) specifically associated with microtubule protein purified from porcine brain tissue were separated from tubulin by DEAE-Sephadex ion exchange chromatography. Analysis by viscometry, sedimentation, and electron microscopy of the unfractionated microtubule protein, separated HMW and tubulin fractions, and reconstituted mixtures showed that HMW promoted formation of ring structures at 5 degrees and tubule polymerization at 37 degrees. The HMW reassociated with tubulin and was identified in thin sections as 18.9 x 5.6 nm projections attached to the microtubules with a longitudinal periodicity of 32.5 nm. These studies: (1) indicate that the HMW fraction stimulates microtubule assembly by facilitating the formation of ring structures which are apparently intermediates in polymerization, and (2) demonstrate that the HMW associates with microtubules as a structural component projecting from the surface of the microtubule wall.