Molecular flexibility in microtubule proteins: proton nuclear magnetic resonance characterization

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Abstract
Bovine microtubule protein preparations were examined by 1H NMR spectroscopy at 270 MHz. Sharp resonances were identified as deriving from microtubule-associated proteins. These resonances persist after self-assembly of microtubule protein. Brief tryptic treatment of assembled microtubules, specifically cleaving the microtubule-associated protein HMW2 (MW = 270,000), releases the pendant portion of HMW2 (MW = 240,000), three-quarters of which is in a flexible conformation. Isolated .tau. protein and HMW2 protein both show substantial flexibility; on recombination with tubulin dimer, .tau. shows considerable decrease in flexibility wheras HMW2 is unaffected. The observations may have important implications for the interactions between microtubules and other cytoskeletal structures.