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Originally published as Biophys J. BioFAST on January 21, 2005.
doi:10.1529/biophysj.104.055871
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Biophysical Journal 88:2855-2866 (2005)
© 2005 The Biophysical Society

Protein Self-Association Induced by Macromolecular Crowding: A Quantitative Analysis by Magnetic Relaxation Dispersion

Karim Snoussi and Bertil Halle

Department of Biophysical Chemistry, Lund University, SE-22100 Lund, Sweden

Correspondence: Address reprint requests to Dr. Karim Snoussi, E-mail: karim.snoussi{at}port.ac.uk; or Dr. Bertil Halle, E-mail: bertil.halle{at}bpc.lu.se.

In the presence of high concentrations of inert macromolecules, the self-association of proteins is strongly enhanced through an entropic, excluded-volume effect variously called macromolecular crowding or depletion attraction. Despite the predicted large magnitude of this universal effect and its far-reaching biological implications, few experimental studies of macromolecular crowding have been reported. Here, we introduce a powerful new technique, fast field-cycling magnetic relaxation dispersion, for investigating crowding effects on protein self-association equilibria. By recording the solvent proton spin relaxation rate over a wide range of magnetic field strengths, we determine the populations of coexisting monomers and decamers of bovine pancreatic trypsin inhibitor in the presence of dextran up to a macromolecular volume fraction of 27%. Already at a dextran volume fraction of 14%, we find a 30-fold increase of the decamer population and 5105-fold increase of the association constant. The analysis of these results, in terms of a statistical-mechanical model that incorporates polymer flexibility as well as the excluded volume of the protein, shows that the dramatic enhancement of bovine pancreatic trypsin inhibitor self-association can be quantitatively rationalized in terms of hard repulsive interactions.




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