Identification of the Ribosomal Proteins Present in the Vicinity of Globin mRNA in the 40S Initiation Complex

The interaction of ribosomal proteins with mRNA in the 40S initiation complex was examined by chemical cross-linking. 40S initiation complexes were formed by incubating rat liver [3H]Met-tRNAi, rat liver 40S ribosomal subunits, rabbit globin mRNA, and partially purified initiation factors of rabbit reticulocytes in the presence of guanylyl(β, γ-methylene)-diphosphonate. The initiation complexes were then treated with 1, 3-butadiene diepoxide to introduce crosslinks between the mRNA and proteins. The covalent mRNA-protein conjugates were isolated by chromatography on an oligo(dT) cellulose column in the presence of sodium dodecyl sulfate, followed by sucrose density gradient centrifugation. Proteins cross-linked to the mRNA were labeled with Na125I, extracted by extensive ribonuclease digestion, and analyzed by two-dimensional and diagonal poly-acrylamide gel electrophoresis. Three ribosomal proteins, S6, S8, and S23/S24, together with small amounts of S3/S3a, S27, and S30, were identified as the protein components cross-linked to the globin mRNA protein complex, and were shown to attach directly to the mRNA. It is suggested that these proteins constitute the ribosomal binding site for mRNA in the 40S initiation complex.