Molecular interactions between ribosomal proteins -- an analysis of S7-S9, S7-S19, S9-S19 and S7-S9-S19 interactions.

Ribosomal proteins S7, S9 and S 19 from Escherichia coli have been studied by
the sedimentation equilibrium technique for possible intermolecular interaction between
pairs of proteins as well as in a mixture of 3 proteins. The proteins were isolated to a
purity greater than 95% and were characterized in the reconstitution buffer. It was
observed that none of the proteins has a tendency to self-associate in the concentration
range studied in the temperature range 3–6°C. Protein S9 behaves differently in the
presence of other proteins. Analysis of the sedimentation equilibrium data for S7–S9, S9–
S19 and S7–S9–S19 complexes revealed the need for considering the presence of a
component of higher molecular weight in the system along with the monomers and their
complexes to provide a meaningful curve-fitting of the data. Proteins S7 and S19 were
found to interact with an equilibrium constant of association of 3 ± 2 × 104 M–1 at 3°C
with a Gibbs free energy of interaction ΔG° of -5·7 kcal/mol. These data are useful for the
consideration of the stabilization of the 30S subunit through protein-protein interactions
and also help in building a topographical model of the proteins of the small subunit from
an energetics point of view.