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Aging of tubulin at neutral pH
Authors:V. Prakash  Serge N. Timasheff
Abstract:The aging of calf brain tubulin in neutral solution has been investigated using the techniques of sedimentation velocity and equilibrium, microtubule assembly, fluorescence spectroscopy, circular dichroism and sodium dodecyl sulfate/polyacrylamide gel electrophoresis. The results indicate that tubulin incubated at 4 °C undergoes a slow association process leading to the generation of a 9 S component. The fraction of 9 S component increases progressively with incubation time and appears to follow mono-molecular kinetics. The generation of the 9 S species is paralleled closely by inhibition of microtubule assembly and loss of colchicine-binding ability. Fluorescence spectroscopy and circular dichroism spectra indicate that tryptophan moieties are perturbed during the aggregation process and that the tubulin dimer undergoes a conformational change. There is no protein degradation up to an incubation period of 50 hours. Sedimentation equilibrium experiments in 6 m-guanidine hydrochloride, both in the presence and absence of 2-mercaptoethanol, indicate that the aggregates are stabilized by disulfide bonds and hydrophobic interactions. At periods of incubation >50 hours, the protein starts to be degraded.
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