The quaternary structure of streptavidin in urea

G. P. Kurzban, E. A. Bayer, M. Wilchek, P. M. Horowitz

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63 Scopus citations

Abstract

We report on the interactions of urea and guanidinium salts with streptavidin. Gel filtration chromatography in 0, 4, 6, and 7 M urea indicates that the streptavidin tetramer remains intact in urea. Biotin alters the electrophoretic mobility of streptavidin whether or not 6 M urea is present. The intrinsic fluorescence of streptavidin is increased and blue-shifted in 6 M urea. The fluorescence changes indicate the absence of unfolding. A conformational response to urea is possible, but much of the fluorescence change is due to urea binding as a weak biotin analog (K(a) ~ 1.3 M-1). The resistance to structural perturbation by urea reflects the structural stability of streptavidin's anti-parallel β-barrel motif. Unfolding is sluggish in 6 M guanidinium hydrochloride (half-time, ~50 days). After guanidinium thiocyanate unfolding, streptavidin can be refolded, but the unfolding and refolding transitions are centered at different concentrations of perturbant. Slow unfolding, with a 15th power dependence on guanidinium thiocyanate concentration, may be partially responsible for the noncoincidence of the unfolding and refolding processes. Nonequilibrium behavior is also seen in 6 M urea, as native streptavidin does not unfold and guanidinium thiocyanate unfolded streptavidin does not refold. Refolding does occur at lower concentrations of urea. Guanidinium thiocyanate only slowly unfolds the biotin-streptavidin complex. In the presence of biotin, unfolded streptavidin does not refold in 6 M guanidinium thiocyanate or in 6 M urea.

Original languageEnglish (US)
Pages (from-to)14470-14477
Number of pages8
JournalJournal of Biological Chemistry
Volume266
Issue number22
StatePublished - 1991
Externally publishedYes

ASJC Scopus subject areas

  • Biochemistry
  • Molecular Biology
  • Cell Biology

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