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Folding of a dimeric β-barrel: Residual structure in the urea denatured state of the human papillomavirus E2 DNA binding domain

Published online by Cambridge University Press:  01 April 2000

YU-KEUNG MOK
Affiliation:
MRC Unit for Protein Function and Design, Cambridge University Chemical Laboratory, Lensfield Road, Cambridge CB2 1EW, United Kingdom Present address: Department of Biochemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Sai Hung, Hong Kong.
LEONARDO G. ALONSO
Affiliation:
Instituto de Investigaciones Bioquímicas, Fundación Campomar, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Patricias Argentinas 435, (1405) Buenos Aires, Argentina
L. MAURICIO T.R. LIMA
Affiliation:
Departamento de Bioquimica Medica, Universidade Federal do Rio de Janeiro, Cidade Universitaria, Rio de Janeiro 21914-590, Rio de Janeiro, Brazil
MARK BYCROFT
Affiliation:
MRC Unit for Protein Function and Design, Cambridge University Chemical Laboratory, Lensfield Road, Cambridge CB2 1EW, United Kingdom
GONZALO DE PRAT-GAY
Affiliation:
Instituto de Investigaciones Bioquímicas, Fundación Campomar, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Patricias Argentinas 435, (1405) Buenos Aires, Argentina
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Abstract

The dimeric β-barrel is a characteristic topology initially found in the transcriptional regulatory domain of the E2 DNA binding domain from papillomaviruses. We have previously described the kinetic folding mechanism of the human HPV-16 domain, and, as part of these studies, we present a structural characterization of the urea-denatured state of the protein. We have obtained a set of chemical shift assignments for the C-terminal domain in urea using heteronuclear NMR methods and found regions with persistent residual structure. Based on chemical shift deviations from random coil values, 3JNHNα coupling constants, heteronuclear single quantum coherence peak intensities, and nuclear Overhauser effect data, we have determined clusters of residual structure in regions corresponding to the DNA binding helix and the second β-strand in the folded conformation. Most of the structures found are of nonnative nature, including turn-like conformations. Urea denaturation at equilibrium displayed a loss in protein concentration dependence, in absolute parallel to a similar deviation observed in the folding rate constant from kinetic experiments. These results strongly suggest an alternative folding pathway in which a dimeric intermediate is formed and the rate-limiting step becomes first order at high protein concentrations. The structural elements found in the denatured state would collide to yield productive interactions, establishing an intermolecular folding nucleus at high protein concentrations. We discuss our results in terms of the folding mechanism of this particular topology in an attempt to contribute to a better understanding of the folding of dimers in general and intertwined dimeric proteins such as transcription factors in particular.

Type
Research Article
Copyright
© 2000 The Protein Society

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