Protein synthesis in rabbit reticulocytes XXXI: Purification of Co-eIF-2C and studies of its roles in peptide chain initiation

Thermodynamic characterization of the cooperativity of 40S complex formation during the initiation of eukaryotic protein synthesis

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-tRNA(fMet), 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-tRNA(fMet). The association constants for the binary reactions were as follows: Ka(Q,M) < or = 0.14 x 10(6) M-1, Ka(R,M) = 1.78 x 10(6) M-1, and Ka(Q,R) = 0.94 x 10(6) 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.

Protein synthesis in rabbit reticulocytes: Requirements for Met-tRNAf·40S preinitiation complex formation with AUG-codon and physiological mRNAs

Under standard conditions, in the presence of GTP, highly purified eIF-2 and Co-eIF-2 factor preparations efficiently stimulated AUG-codon dependent but not physiological mRNA-dependent Met-tRNAf binding to 40S ribosomes. Replacement of GTP by a nonhydrolyzable GTP analog, GMP-PNP, in the above system, gave significant stimulation of Met-tRNAf binding to 40S ribosomes dependent on physiological mRNAs. Lower but significant stimulation of Met-tRNAf binding to 40S ribosomes was also observed when GTP was used in the presence of nucleoside 5'-diphosphate kinase (NDK) and ATP. ATP alone in the absence of NDK had no significant effect. This is the first report on the formation of a stable Met-tRNAf . 40S initiation complex dependent on physiological mRNAs and the factor requirements for such complex formation.

Protein synthesis in rabbit reticulocytes: characteristics of the protein factor RF that reverses inhibition of protein synthesis in heme-deficient reticulocyte lysates

During heme deficiency in reticulocyte lysates, the heme-regulated translational inhibitor of protein synthesis (HRI) is activated and shuts off protein synthesis. In partial reactions, HRI phosphorylates the Mr 38,000 subunit (alpha subunit) of eukaryotic initiation factor 2 (eIF-2), which forms a ternary complex, Met-tRNAf X eIF-2 X GTP. The eIF-2 alpha (P) thus formed is not recognized by two eIF-2 ancillary factors, Co-eIF-2B (which promotes the dissociation of the ternary complex at high Mg2+) and Co-eIF-2C (which reverses the inhibition of ternary complex formation), and thus, is presumably inactive in peptide chain initiation. A protein factor, designated RF, which reverses inhibition of protein synthesis in heme-deficient reticulocyte lysates, has been purified from reticulocyte cell supernatant. RF is a high molecular weight (Mr approximately equal to 450,000) protein complex composed of multiple polypeptides. An active RF preparation contains Co-eIF-2B and Co-eIF-2C activities, and these two activities in RF preparation are not inhibited by HRI and ATP--i.e., eIF-2 alpha (P) is recognized. During purification, RF remains associated with eIF-2 activity (eIF-2 X RF) and can be freed of this eIF-2 activity by CM-Sephadex chromatography. Both eIF-2 X RF and RF contain a Mr 38,000 polypeptide component that is indistinguishable from the Mr 38,000 subunit of eIF-2 by two-dimensional gel electrophoresis. It has been observed that a significant part of this Mr 38,000 polypeptide component in eIF-2 X RF and almost the entire Mr 38,000 polypeptide component in RF remain unphosphorylated after prolonged incubation with HRI and ATP. A possible role of this free Mr 38,000 polypeptide in RF action is discussed.

Protein synthesis in rabbit reticulocytes: characteristics of CO-eIF-2 protein complex

A high molecular weight reticulocyte protein factor, named Co-eIF-2, contains Co-eIF-2A, Co-eIF-2B, and Co-eIF-2C activities and stimulates Met-tRNAf binding to eIF-2 both in the presence and absence of Mg2+. Some characteristics of this stimulation in the absence of Mg2+ are: (1) Stimulation is most pronounced at low eIF-2 levels. (2) Stimulation is partially resistant to heat and NEM treatment, and thus appears to be due to the combined action of both heat and NEM-insensitive Co-eIF-2A, and heat and NEM-sensitive Co-eIF-2C activities. (3) [3H]GDP bound in eIF-2 . [3H]GDP complex is rapidly displaced by unlabelled GTP during ternary complex formation Co-eIF-2 stimulates Met-tRNAf binding to eIF-2 even when added after the [3H]GDP from eIF-2 . [3H]GDP has been completely displaced. This indicates that Co-eIF-2-stimulation is not due to GDP displacement from eIF-2 . GDP. We propose that eIF-2 molecules become inactive in the presence of Mg2+ and at high dilution, and Co-eIF-2 restores the inactive eIF-2 molecules into an active form.