Identification of the amino acid residues of proteins S5 and S8 adjacent to each other in the 30 S ribosomal subunit of Escherichia coli.

When Escherichia coli 30 S ribosomal subunits are reacted with protein-protein bifunctional reagents, a number of protein pairs as well as aggregates containing three or more ribosomal proteins are formed. In the present study we have purified one of the protein pairs obtained by reaction of 30 S ribosomal subunits with either radioactive or nonradioactive dimethylsuberimidate. Following molecular weight determination and ammonolysis, the pair was shown to consist of ribosomal proteins S5 and S8. The "native" structure of the complex was surmised from its capacity to be reconstituted into a biologically active 30 S ribosomal subunit. From peptide maps and primary structure determination of various peptides it was demonstrated that the cross-linking bond between ribosomal proteins S5 and S8 involves primarily the residues Lys-93 of protein S8 and the COOH-terminal lysine (Lys-166) of ribosomal protein S5. This result is substantiated by the finding that a mutant carrying an altered S5 lacking the COOH-terminal lysine yields a greatly reduced amount of S5-S8 cross-link. In addition to the points of cross-linking it was found that Lys-30, Lys-68, and Lys-86 of S8 and Lys-5 of S5 react with dimethylsuberimidate, indicating that these residues are available for reaction and suggesting their topographical localization on the ribosomal surface.