Detection and characterization of partially unfolded oligomers of the SH3 domain of alpha-spectrin.

TitleDetection and characterization of partially unfolded oligomers of the SH3 domain of alpha-spectrin.
Publication TypeJournal Article
Year of Publication2004
AuthorsCasares, S., M. Sadqi, O. Lopez-Mayorga, F. Conejero-Lara, and N. A. J. van Nuland
JournalBiophys J
Date Published2004 Apr
KeywordsAnimals, Calorimetry, Differential Scanning, Chickens, Circular Dichroism, Cross-Linking Reagents, Models, Molecular, Nuclear Magnetic Resonance, Biomolecular, Protein Denaturation, Protein Folding, Spectrin, src Homology Domains, Thermodynamics

For the purpose of equilibrium and kinetic folding-unfolding studies, the SH3 domain of alpha-spectrin (spc-SH3) has long been considered a classic two-state folding protein. In this work we have indeed observed that the thermal unfolding curves of spc-SH3 measured at pH 3.0 by differential scanning calorimetry, circular dichroism, and NMR follow apparently the two-state model when each unfolding profile is considered individually. Nevertheless, we have found that protein concentration has a marked effect upon the thermal unfolding profiles. This effect cannot be properly explained in terms of the two-state unfolding model and can only be interpreted in terms of the accumulation of intermediate associated states in equilibrium with the monomeric native and unfolded states. By chemical cross-linking and pulsed-field gradient NMR diffusion experiments we have been able to confirm the existence of associated states formed during spc-SH3 unfolding. A three-state model, in which a dimeric intermediate state is assumed to be significantly populated, provides the simplest interpretation of the whole set of thermal unfolding data and affords a satisfactory explanation for the concentration effects observed. Whereas at low concentrations the population of the associated intermediate state is negligible and the unfolding process consequently takes place in a two-state fashion, at concentrations above approximately 0.5 mM the population of the intermediate state becomes significant at temperatures between 45 degrees C and 80 degrees C and reaches up to 50% at the largest concentration investigated. The thermodynamic properties of the intermediate state implied by this analysis fall in between those of the unfolded state and the native ones, indicating a considerably disordered conformation, which appears to be stabilized by oligomerization.

Alternate JournalBiophys. J.
PubMed ID15041678
PubMed Central IDPMC1304089