Molecular flexibility in microtubule proteins: proton nuclear magnetic resonance characterization.

Bovine microtubule protein preparations have been examined by proton nuclear magnetic resonance (1H NMR) spectroscopy at 270 MHz. Sharp resonances have been 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 (Mr = 270 000), releases the pendant portion of HMW2 (Mr = 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 whereas HMW2 is unaffected. The observations may have important implications for the interactions between microtubules and other cytoskeletal structures.