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Published online before print March 31, 2005, 10.1110/ps.041164305
Protein Science (2005), 14:1375-1379. Published by Cold Spring Harbor Laboratory Press. Copyright © 2005 The Protein Society
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FOR THE RECORD

Multiple equilibria of the Escherichia coli chaperonin GroES revealed by mass spectrometry

Lynda J. Donald1, David J. Stokell1, Neil J. Holliday2, Werner Ens3, Kenneth G. Standing3 and Harry W. Duckworth1

1 Department of Chemistry, 2 Department of Entomology, and 3 Department of Physics and Astronomy, University of Manitoba, Winnipeg, Manitoba, R3T 2N2, Canada

(RECEIVED October 21, 2004; FINAL REVISION December 21, 2004; ACCEPTED January 7, 2005)

Nanospray time-of-flight mass spectrometry has been used to study the assembly of the heptamer of the Escherichia coli cochaperonin protein GroES, a system previously described as a monomer–heptamer equilibrium. In addition to the monomers and heptamers, we have found measurable amounts of dimers and hexamers, the presence of which suggests the following mechanism for heptamer assembly: 2 Monomers {leftrightarrow} Dimer; 3 Dimers {leftrightarrow} Hexamer; Hexamer + Monomer {leftrightarrow} Heptamer. Equilibrium constants for each of these steps, and an overall constant for the Monomer {leftrightarrow} Heptamer equilibrium, have been estimated from the data. These constants imply a standard free-energy change, {Delta}G0, of about 9 kcal/mol for each contact surface formed between GroES subunits, except for the addition of the last subunit, where {Delta}G0 = 6 kcal/mol. This lower value probably reflects the loss of entropy when the heptamer ring is formed. These experiments illustrate the advantages of electrospray mass spectrometry as a method of measuring all components of a multiple equilibrium system.

Keywords: electrospray ionization; time-of-flight mass spectrometry; E. coli chaperone protein; GroES equilibrium

Article published online ahead of print. Article and publication date are at http://www.proteinscience.org/cgi/doi/10.1110/ps.041164305.

Reprint requests to: Lynda J. Donald, Department of Chemistry, 507 Parker Building, University of Manitoba, Winnipeg, Manitoba, R3T 2N2, Canada; e-mail: ldonald{at}cc.umanitoba.ca; fax: (204) 474-7608.


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