title: Multiple equilibria of the Escherichia coli chaperonin GroES revealed by mass spectrometry.
abstract: 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 <--> Dimer; 3 Dimers <--> Hexamer; Hexamer + Monomer <--> Heptamer. Equilibrium constants for each of these steps, and an overall constant for the Monomer <--> Heptamer equilibrium, have been estimated from the data. These constants imply a standard free-energy change, DeltaG(0), of about 9 kcal/mol for each contact surface formed between GroES subunits, except for the addition of the last subunit, where DeltaG(0) = 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.
authors.affiliation: Department of Chemistry, 507 Parker Building, University of Manitoba, Winnipeg, Manitoba, R3T 2N2, Canada. ldonald@cc.umanitoba.ca
authors.forename: Lynda J
authors.initials: LJ
authors.lastname: Donald
authors.forename: David J
authors.initials: DJ
authors.lastname: Stokell
authors.forename: Neil J
authors.initials: NJ
authors.lastname: Holliday
authors.forename: Werner
authors.initials: W
authors.lastname: Ens
authors.forename: Kenneth G
authors.initials: KG
authors.lastname: Standing
authors.forename: Harry W
authors.initials: HW
authors.lastname: Duckworth
mesh_terms: D018835:Chaperonin 10 / Q000737:chemistry*; D019281:Dimerization; D004926:Escherichia coli / Q000737:chemistry*; D013058:Mass Spectrometry / Q000379:methods*
chemical_list: D018835:Chaperonin 10