title: Carbon-13 nuclear magnetic resonance study of microtubule protein: evidence for a second colchicine site involved in the inhibition of microtubule assembly.
abstract: A 13C nuclear magnetic resonance study of bovine microtubule protein was carried out at 43 kG in the presence and absence of colchicine 13C labeled at the tropolone methoxy. Analysis indicated that tubulin has at least two colchicine binding sites: a quasi-irreversibly bound, high-affinity site (i.e., the KD less than 5 microM site generally accepted as the site of colchicine action) as well as a low-affinity site(s) (KD approximately 650 microM) with which free colchicine rapidly exchanges (greater than 100 s-1). The methoxy resonance is broadened to different apparent extents as a result of binding at these two sites (50- vs. 150-Hz broadening for the high- and low-affinity sites, respectively) but undergoes no change in chemical shift upon binding. The low-affinity sites are interpreted to be analogous to the sites deduced by Schmitt and Atlas [Schmitt, H., & Atlas, D. (1976) J. Mol. Biol. 102, 743-758] from labeling studies using bromocolchicine. These sites are likely to be the sites responsible for the abrupt halt in microtubule assembly ("capping") observed at high colchicine concentrations (greater than 20 microM)--a qualitatively different behavior from that observed at low colchicine concentrations [Sternlicht, H., Ringel, I., & Szasz, J. (1983) Biophys. J. 42, 255-267]. Carbon-13 spectra from the aliphatic carbons of microtubule protein consists of narrow resonances--many with line widths less than 30 Hz--superimposed on a broad background. The narrow resonances were assigned to flexible regions in nontubulin proteins [microtubule-associated proteins (MAPs)], in accord with an earlier 1H nuclear magnetic resonance study of microtubule protein [Woody, R. W., Clark, D. C., Roberts, G. C. K., Martin, S. R., & Bayley, P. M. (1983) Biochemistry 22, 2186-2192]. This assignment was supported by 13C NMR analysis of phosphocellulose-purified (MAP-depleted) tubulin as well as heat-stable MAPs. Aliphatic carbons in the MAP preparations were characterized by narrow resonances indicative of carbons with considerable motional freedom whereas the aliphatic regions of phosphocellulose-purified tubulin were, for the most part, characterized by broad resonances indicative of carbons with restricted mobility. However, a moderately narrow resonance (approximately less than 50-Hz line width) coincident with the C gamma resonance of glutamate was detected in 13C NMR spectra of tubulin which indicated that a fraction of the glutamic acid residues is relatively mobile.(ABSTRACT TRUNCATED AT 400 WORDS)
mesh_terms: D000818:Animals; D001665:Binding Sites; D001923:Brain Chemistry; D002417:Cattle; D003078:Colchicine / Q000378:metabolism*; D005971:Glutamates; D018698:Glutamic Acid; D009682:Magnetic Resonance Spectroscopy; D008869:Microtubule-Associated Proteins / Q000378:metabolism*; D014404:Tubulin / Q000378:metabolism*
chemical_list: D005971:Glutamates; D008869:Microtubule-Associated Proteins; D014404:Tubulin; D018698:Glutamic Acid; D003078:Colchicine