Elongation Factors


Elongation Factors in more detail


The three elongation factors each have their own specialised role to play in protein translation. 


EF1A (EF-Tu)

            EF1A deposits the aa-tRNA on the A-site of the ribosome by forming a ternary complex with GTP and the aa-tRNA.  The aa-tRNA stimulates EF1A to convert GTP to GDP, causing a conformational change in EF1A that detaches GDP-bound EF1A from the ribosome, leaving the aa-tRNA attached at the A-site.  Only the correct (cognate) aa-tRNA anticodon that matches the mRNA codon exposed at the A-site will cause the tight codon-anticodon interaction necessary to induce a conformational change in EF1A.  Therefore, EF1A helps achieve accurate translation. 

            EF1A is a G-protein with a low GTP-hydrolysis rate when compared to other G-proteins, such as small GTPases and heterotrimeric G-proteins.  This low basal rate of GTP hydrolysis plays an important part in maintaining translation accuracy, since it prevents non-cognate aa-tRNAs from binding to the ribosome through their inability to stimulate rapid GTP hydrolysis.


EF1B (EF-Ts)

EF1B is a nucleotide exchange factor that is required to regenerate active EF1A.  Once EF1A‑GDP has detached from the ribosome, EF1B acts as a catalyst to convert it into active EF1A‑GTP.  The EF1A is then ready to interact with a new aa-tRNA, to begin the cycle again.  In eukaryotes, EF1B is more complex, consisting of three subunits, EF-1beta, EF-1gamma and EF-1delta, in the order EF-1bgd.


EF2 (EF-G)

EF2 is another G-protein.  EF2 assists in the translocation of tRNA and mRNA (base-paired together) through the ribosome, acting to move the mRNA-bound tRNA from the A-site to the P‑site, thereby freeing the A-site ready for the next aa-tRNA to bind.  At the same time, it moves the deacylated-tRNA from the P-site to the E-site, where it exits the ribosome.

EF2 brings about the translocation of tRNA and mRNA through a ratchet-like mechanism that makes use of the two ribosomal subunits acting independently.  The initial binding of GTP-bound EF2 to the ribosome causes a counter-clockwise rotation of about 6o in the small ribosomal subunit.  The movement of the small subunit is in the same direction as the movement of the two tRNAs from the A and P-sites to the P and E-sites, respectively.  The subsequent hydrolysis of GTP to GDP by EF2 causes the small ribosome to rotate back (clockwise) by about 3 o, and all the way back to the starting position (another 3 o) with the release of EF2. 

The twisting action of the small ribosomal subunit appears to destabilise tRNA-ribosome interactions, freeing it to translocate along the ribosome upon GTP-hydrolysis by EF2.  In addition, the twisting of the small subunit changes the entry and exit channel openings for the mRNA.  When EF2 is bound, the opening is widened to enable movement of the mRNA along the ribosome, but when EF2 dissociates, it narrows in order to hold the mRNA rigidly in place for the next aa-tRNA anticodon to bind.


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