Molecular interactions between ribosomal proteins. Evidence for specificity of interaction between isolated proteins

The proteins S2, S3, S5, and S10 from the 30S ribosomal subunit of Escherichia coli was studied by analytical ultracentrifugation to characterize them in solution and to determine whether isolated protein-protein interactions exist. Such interactions, if specific, may therefore bear some relationship to the spatial organization of the subunit structure. It was found that protein S2 self-associates to a slight extent and that solution mixtures of S2 and S3 contain only enough dimeric species to account for the S2 dimer. Hence, no observable interaction was detected between S2 and S3. Solution mixtures of proteins S5 and S10 revealed a species of molecular weight greater than either protein. The proposal is that S5 and S10 interact with an association equilibrium constant of 7.6 X 10(-5) M-1 at 3 degrees in a Tris buffer at pH 7.4. It was also shown that solution with a 1:1:1 mixture by mass, of S2, S5, and S10 contained a species possessing a molecular weight consistent with a simple ternary complex of the three proteins.