The first step in mammalian protein synthesis is the formation of the 40S initiation complex, composed of the 40S ribosomal subunit (R), mRNA (M, here, a 10-mer oligoribonucleotide analogue containing the initiation codon),and the quaternary complex (Q,composed of eIF-2, GTP, Met-tRNAfMet, and the ancillary protein factor Co-eIF-2C). The interdependence of the binding of R, M, and Q in forming the 40S complex is currently unclear. We have determined the thermodynamic parameters that characterize these interactions. The binary constants for R + M and Q + M were determined spectroscopically, measuring changes in the anisotropy of the fluorescence emission of 3′-fluorescein labeled M. The other binary constant, for Q + R, and the ternary constant were determined from Millipore filtration assays using radiolabeled Met-tRNAfMet. The association constants for the binary reactions were as follows: Ka(Q,M) ≤ 0.14 × 6 M-1, AT,(R,M) = 1.78 × 106 M-1, and Ka(Q,R) = 0.94 × 106 M-1. The binding of Q to R·M was markedly greater than that of Q to R (Ka(Q,R·M)/Ka(Q,R) > 62]. High cooperativity for this interaction occurs in either a single-site model or in lattice models for the binding of M to R. Data obtained using five other RNA 10-mers, each with the sequence altered at the AUG codon, suggest that this cooperativity is AUG dependent. The data are consistent with a scheme in which mRNA and Q bind independently to the 40S ribosome, but when the AUG codon is properly aligned with Q, a conformational change results in a 2.4 kcal/mol stabilization of the complex.
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