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- Antoun, Ayman, et al.
(författare)
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How initiation factors maximize the accuracy of tRNA selection in initiation of bacterial protein synthesis
- 2006
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Ingår i: Molecular Cell. - : Elsevier BV. - 1097-2765 .- 1097-4164. ; 23:2, s. 183-193
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Tidskriftsartikel (refereegranskat)abstract
- During initiation of bacterial protein synthesis, messenger RNA and fMet-tRNA(fMet) bind to the 30S ribosomal subunit together with initiation factors IF1, IF2, and IF3. Docking of the 30S preinitiation complex to the 50S ribosomal subunit results in a peptidyl-transfer competent 70S ribosome. Initiation with an elongator tRNA may lead to frameshift and an aberrant N-terminal sequence in the nascent protein. We show how the occurrence of initiation errors is minimized by (1) recognition of the formyl group by the synergistic action of IF2 and IF1, (2) uniform destabilization of the binding of all tRNAs to the 30S subunit by IF3, and (3) an optimal distance between the Shine-Dalgarno sequence and the initiator codon. We suggest why IF1 is essential for E. coli, discuss the role of the G-C base pairs in the anticodon stem of some tRNAs, and clarify gene expression changes with varying IF3 concentration in the living cell.
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- Antoun, Ayman, et al.
(författare)
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How initiation factors tune the rate of initiation of protein synthesis in bacteria.
- 2006
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Ingår i: EMBO Journal. - : Wiley. - 0261-4189 .- 1460-2075. ; 25:11, s. 2539-50
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Tidskriftsartikel (refereegranskat)abstract
- The kinetics of initiator transfer RNA ( tRNA) interaction with the messenger RNA ( mRNA)-programmed 30S subunit and the rate of 50S subunit docking to the 30S preinitiation complex were measured for different combinations of initiation factors in a cell-free Escherichia coli system for protein synthesis with components of high purity. The major results are summarized by a Michaelis-Menten scheme for initiation. All three initiation factors are required for maximal efficiency ( k(cat)/K-M) of initiation and for maximal in vivo rate of initiation at normal concentration of initiator tRNA. Spontaneous release of IF3 from the 30S preinitiation complex is required for subunit docking. The presence of initiator tRNA on the 30S subunit greatly increases the rate of 70S ribosome formation by increasing the rate of IF3 dissociation from the 30S subunit and the rate of 50S subunit docking to the IF3-free 30S preinitiation complex. The reasons why IF1 and IF3 are essential in E. coli are discussed in the light of the present observations.
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- Antoun, Ayman, 1975-
(författare)
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Mechanism and Regulation of Initiation of Protein Synthesis in Eubacteria
- 2005
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Initiation of protein synthesis in E.coli involves several steps, which lead to the formation of the first peptide bond. This process requires three initiation factors: IF1, IF2 and IF3. Using a novel technique of combined light scattering and stopped-flow, we elucidated the importance of IF2•GTP conformation for the recruitment of 50S to 30S pre-initiation complex. Moreover, GTP hydrolysis is essential for IF2 release and later binding of ternary complex. Interestingly, a switch in IF2 affinity to G-nucleotides is induced during 30S pre-initiation complexes formation. We found that IF1, previously with unknown functions in vitro, increases the rate of naked 70S dissociation by a factor 80 and acts as a fidelity factor in preventing 70S formation containing elongator tRNA instead of fMet-tRNAfMet. We showed that RRF/EFG/IF3 split both naked and post-termination complexes while IF1/IF3 split only naked ribosomes. The mechanisms of action of RRF/ EFG, the order of their binding to 70S, as well as, the three different conformation of EF-G on the ribosomes are emphasized. Interestingly, 70S formation rate is dependent on the concentration of IF3 and not linear with 50S subunits concentration. We demonstrated that the rate-limiting step in 70S formation is IF3 dissociation from 30S complexes.The interplay between initiation factors in the rate and accuracy of protein synthesis was thoroughly studied. Using fMet-tRNAfMet (initiator tRNA), Met-tRNAfMet (non-formylated initiator tRNA) and Phe-tRNAPhe (elongator tRNA), we showed that the major player in the accuracy is IF2 through recognizing the formyl group on fMet-tRNAfMet, while IF3 acts by increasing both the on- and off-rate of tRNA from 30S pre-initiation complexes.Collectively, these novel results describe a comprehensive model of initiation of protein synthesis. In this model, initiation factors increase the rate of fMet-tRNAfMet binding to 30S subunits, subsequently; the stabilization of fMet-tRNAfMet by IF2 increases the rate of IF3 dissociation. Later, IF2 in GTP conformation allows 50S docking to 30S pre-initiation complex free of IF3 followed by GTP hydrolysis allowing IF2 release for ternary complex to bind and start elongation of protein synthesis.
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- Antoun, Ayman, et al.
(författare)
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Ribosome formation from subunits studied by stopped-flow and Rayleigh light scattering
- 2004
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Ingår i: Biol Procedure Online. ; 6, s. 35–54-
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Tidskriftsartikel (refereegranskat)abstract
- Light scattering and standard stopped-flow techniques were used to monitor rapid association of ribosomal subunits during initiation of eubacterial protein synthesis. The effects of the initiation factors IF1, IF2, IF3 and buffer conditions on subunit association were studied along with the role of GTP in this process. The part of light scattering theory that is essential for kinetic measurements is high-lighted in the main text and a more general treatment of Rayleigh scattering from macromolecules is given in an appendix.
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