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translation elongation factor activity Source: ..
Addition of each incoming amino acid requires the cooperation of three elongation factors. Elongation factor Tu (EF-Tu) is the most abundant protein in E. coli, with about 100,000 copies per cell, or 5% of the cell’s protein. This protein is a GTPase, and the EF-Tu:GTP complex specifically binds aminoacyl-tRNAs (AA-tRNAs). Formation of the ternary complex (EF-Tu:GTP:AA-tRNA) protects the ester bond (linking the amino acid to its cognate tRNA) from hydrolysis, and transports the AA-tRNA to the ribosomal A-site. Once the correct codon-anticodon interaction is confirmed, ribosome-triggered hydrolysis of EF-Tu-bound GTP occurs. EF-Tu:GDP is then released from the ribosome, and the AA-tRNA occupies the A-site. While EF-Tu transports all elongator tRNAs amino-acylated with natural amino acids to the ribosome, this factor has negligible affinity for formylated or nonformy-lated tRNAfMet. The unpaired first position in the tRNAfMet acceptor stem helix apparently is a negative recognition element for EF-Tu:GTP, because this element prevents the initiator tRNA from pairing with internal AUG or GUG codons.
FIGURE 8 Steps in elongation. With the peptidyl-tRNA bound in the P-site and the incoming aminoacyl-tRNA in the A-site, the peptidyl transferase activity of the large ribosomal subunit catalyzes peptide bond formation. The growing polypeptide chain is then attached to the A-site-tRNA, and the deacylated tRNA is in the P-site. Elongation factor G facilitates translocation of the peptidyl-tRNA to the P-site and the empty tRNA to the E-site prior to its release from the ribosome.
Roles of Protein Synthesis Elongation Factor EF-Tu in …
The elongation factor EF-Ts is a nucleotide exchange factor that regenerates active EF-Tu:GTP (from EF-Tu:GDP) for binding subsequent AA-tRNAs following GTP hydrolysis. Before their functions were known, elongation factors Tu and Ts were named for their observed thermal stabilities in vitro—Tu indicates that this protein is Temperature unstable, while Ts stands for Temperature stable. In eukaryotes, the two subunits of elongation factor EF-1 perform the functions of EF-Tu and EF-Ts.
Following formation of the peptide bond, a major rearrangement of components in the functional center of the ribosome must take place. Because the most recently entered tRNA has become the peptidyl-tRNA, it must be moved from the A-site to the P-site. The former peptidyl-tRNA has been deacylated and needs to vacate the P-site. Finally the mRNA must move three nucleotides further in the 3′-direction so that the next codon can be read. The concerted movement of tRNAs and mRNA at the end of each elongation round is called translocation, and is catalyzed by elongation factor G (EF-G), another of the GTPase proteins in the translational machinery.
Protein synthesis elongation factor Tu ..
The best known and studied bacterial toxin is the diphtheria toxin,produced by . Diphtheria toxin is abacterialexotoxin of the A/B prototype. It is produced as single polypeptidechainwith a molecular weight of 60,000 daltons. The function of the proteinis distinguishable into two parts: subunit A, with a m.w. of 21,000daltons,contains the enzymatic activity for inhibition of elongation factor-2involvedin host protein synthesis; subunit B, with a m.w. of 39,000 daltons, isresponsible for binding to the membrane of a susceptible host cell. TheB subunit possesses a region T (translocation) domain which insertsinto the endosome membrane thus securing the release of the enzymaticcomponent into the cytoplasm.
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