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Translation Initiation

Translation Initiation. Emma Phifer , Lindsay Vendetta. Ribosome dissociates, eIF-3 prevents re-association eIF-2 (a GTP binding protein) recognizes initiator tRNA Ternary complex + ribosomal subunit eIF-4F recognizes the 5’ cap

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Translation Initiation

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  1. Translation Initiation Emma Phifer, Lindsay Vendetta

  2. Ribosome dissociates, eIF-3 prevents re-association • eIF-2 (a GTP binding protein) recognizes initiator tRNA • Ternary complex + ribosomal subunit • eIF-4F recognizes the 5’ cap • mRNA-bound subunit travels until it finds AUG (this requires ATP) • Once the small subunit reaches AUG, the large subunit binds via eIF-5 (which releases the other initiation factors) • Methionine is now located in the P site Source: nobelprize.org

  3. eIF1 and eIF1A • elF1 plays a role in the formation of the 40 S preinitiation complex . eIF1 binds to eIF3. • although both eIF1A and eIF3 are essential to generate a stable 40 S preinitiation complex, quantitative binding of the ternary complex to 40 S subunits also required eIF1. • The stable association of eIF1 with 40 S subunits requires the presence of eIF3. • In contrast, the binding of eIF1A to free 40 S ribosomes as well as to the 40 S preinitiation complex was stabilized by the presence of both eIF1 and eIF3. • studies suggest that it is possible for eIF1 and eIF1A to bind the 40 S preinitiation complex prior to mRNA binding • Source: http://www.jbc.org/content/278/8/6580.abstract

  4. Initiation of translation in prokaryotes and eukaryotes(1999) • Eukaryotic Initiation: scanning mechanism -> predicts that translation should start at the AUG codon nearest the 5* end of the mRNA • exceptions: use another AUG that isn't first, but is close to 5* end of mRNA • 3 escape mechanisms • context-dependent leaky scanning • reinitiation • direct internal initiation • hence, transcription promotors, and maybe evolution of splicing?

  5. (3.) small ribosomal (40S) subunit binds to and carries Met-tRNAi*eIF2*GTP*etc • binds the 5' end of the mRNA (never 3') • eIF-4E mediates the cap dependent step • methylation of 5' cap is important as well • scans the UTR to find start codon one base at a time--rec. by base pairing with Met-tRNAi • length of UTR doesn't seem to matter • when 60S subunit binds, start codon is fixed • eukaryotes usu. ONLY use AUG, but rarely ACG/CUG, but always starts with Met • context affects initiation AUG recognition--ideal sequence appears to be GCCRCCaugG, esp the -3R(usu. A) and the +4G--

  6. maybe b/c it slows scanning? • -eIF2 recognizes only initiation specific form of Met-tRNA, requires GTP to bind • all 3 called a ternary complex • eIF2 hydrolyzes GTP after binding the 40S subunit with the help of eIF5 (enters after scanning and finding AUG)--the new GDP*eIF2 is thus released from the 40S subunit, which allows 60S to join the 40S with the Met-tRNA in the P site • eIF2B catalyzes the replacement of GDP with GTP • eIF3 stimulates the ternary complex to the ribosome

  7. The Eukaryotic Translation Initiation Factors eIF1 and eIF1A Induce an Open Conformation of the 40S Ribosome • (2007) • Initiation of translation is the process by which initiator tRNA and the start codon of mRNA are positioned in the ribosomal P site. In eukaryotes, one of the first steps involves the binding of two small factors, eIF1 and eIF1A, to the small (40S) ribosomal subunit. This facilitates tRNA binding, allows scanning of mRNA, and maintains fidelity of start codon recognition. Using cryo-EM, we have obtained 3D reconstructions of 40S bound to both eIF1 and eIF1A, and with each factor alone. These structures reveal that together, eIF1 and eIF1A stabilize a conformational change that opens the mRNA binding channel. Biochemical data reveal that both factors accelerate the rate of ternary complex (eIF2*GTP*Met-tRNAiMet) binding to 40S but only eIF1A stabilizes this interaction. Our results suggest that eIF1 and eIF1A promote an open, scanning-competent preinitiation complex that closes upon start codon recognition and eIF1 release to stabilize ternary complex binding and clamp down on mRNA. • http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=frnrec&part=ch6 • (2009)

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