Assessment by sedimentation equilibrium analysis of a heterologous macromolecular interaction in the presence of self-association: interaction of S5 with S8.

The proteins S5 and S8 from the Escherichia coli 30S ribosomal subunit have been examined by sedimentation equilibrium methods for behavior in solution as isolated components and in mixtures. The means of resolving two simultaneous associations in this system is discussed, and the energy of association of S5 and S8 is reported. It was found that protein S5 from the MRE 600 strain tends to self-associate weakly at 4 degree C in a manner that can be described as an isodesmic self-association with an association constant and corresponding standard Gibbs free energy equal to (7.7 +/- 0.7) X 10(3) M-1 and -4.9 +/- 0.1 kcal/mol, respectively. Protein S8 was found to have a molecular weight of 15800 and was monomeric in a pure state. Mixtures of S5 and S8 clearly demonstrated the presence of an S5-S8 complex in addition to the self-association of S5. The equilibrium constant of association for the formation of a simple S5-S8 complex at 4 degree C and the corresponding standard Gibbs free energy were found to be (5.5 +/- 1.0) X 10(4) M-1 and -6.0 +/- 0.1 kcal/mol, respectively.