release factor

During translation of mRNA, most codons are recognized by "charged" tRNA molecules, called aminoacyl-tRNAs because they are adhered to specific amino acids corresponding to each tRNA's anticodon. In the standard genetic code, there are three mRNA stop codons: UAG ("amber"), UAA ("ochre"), and UGA ("opal" or "umber"). Although these stop codons are triplets just like ordinary codons, they are not decoded by tRNAs. It was discovered by Mario Capecchi in 1967 that, instead, tRNAs do not ordinarily recognize stop codons at all, and that what he named "release factor" was not a tRNA molecule but a protein.{{cite journal | vauthors = Capecchi MR | title = Polypeptide chain termination in vitro: isolation of a release factor | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 58 | issue = 3 | pages = 1144–1151 | date = September 1967 | pmid = 5233840 | pmc = 335760 | doi = 10.1073/pnas.58.3.1144 | doi-access = free | bibcode = 1967PNAS...58.1144C }} Later, it was demonstrated that different release factors recognize different stop codons.{{cite journal | vauthors = Scolnick E, Tompkins R, Caskey T, Nirenberg M | title = Release factors differing in specificity for terminator codons | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 61 | issue = 2 | pages = 768–774 | date = October 1968 | pmid = 4879404 | pmc = 225226 | doi = 10.1073/pnas.61.2.768 | doi-access = free | bibcode = 1968PNAS...61..768S }}

There are two classes of release factors. Class 1 release factors recognize stop codons; they bind to the A site of the ribosome in a way mimicking that of tRNA, releasing the new polypeptide as it disassembles the ribosome.{{cite journal | vauthors = Brown CM, Tate WP | title = Direct recognition of mRNA stop signals by Escherichia coli polypeptide chain release factor two | journal = The Journal of Biological Chemistry | volume = 269 | issue = 52 | pages = 33164–33170 | date = December 1994 | pmid = 7806547 | doi = 10.1016/S0021-9258(20)30112-5 | doi-access = free }}{{cite journal | vauthors = Scarlett DJ, McCaughan KK, Wilson DN, Tate WP | title = Mapping functionally important motifs SPF and GGQ of the decoding release factor RF2 to the Escherichia coli ribosome by hydroxyl radical footprinting. Implications for macromolecular mimicry and structural changes in RF2 | journal = The Journal of Biological Chemistry | volume = 278 | issue = 17 | pages = 15095–15104 | date = April 2003 | pmid = 12458201 | doi = 10.1074/jbc.M211024200 | doi-access = free }} Class 2 release factors are GTPases that enhance the activity of class 1 release factors. It helps the class 1 RF dissociate from the ribosome.{{cite journal | vauthors = Jakobsen CG, Segaard TM, Jean-Jean O, Frolova L, Justesen J | title = [Identification of a novel termination release factor eRF3b expressing the eRF3 activity in vitro and in vivo] | journal = Molekuliarnaia Biologiia | volume = 35 | issue = 4 | pages = 672–681 | date = 2001 | pmid = 11524954 }}

Bacterial release factors include RF1, RF2, and RF3 (or PrfA, PrfB, PrfC in the "peptide release factor" gene nomenclature). RF1 and RF2 are class 1 RFs: RF1 recognizes UAA and UAG while RF2 recognizes UAA and UGA. RF3 is the class 2 release factor.{{cite book | vauthors = Weaver RF | date = 2005 | title = Molecular Biology | pages = [https://archive.org/details/molecularbiology00weav/page/616 616–621] | publisher = McGraw-Hill | location = New York, NY | isbn = 978-0-07-284611-9 | url-access = registration | url = https://archive.org/details/molecularbiology00weav/page/616 }} Eukaryotic and archaeal release factors are named analogously, with the naming changed to "eRF" for "eukaryotic release factor" and vice versa. a/eRF1 can recognize all three stop codons, while eRF3 (archaea use aEF-1α instead) works just like RF3.{{cite journal | vauthors = Saito K, Kobayashi K, Wada M, Kikuno I, Takusagawa A, Mochizuki M, Uchiumi T, Ishitani R, Nureki O, Ito K | display-authors = 6 | title = Omnipotent role of archaeal elongation factor 1 alpha (EF1α in translational elongation and termination, and quality control of protein synthesis | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 107 | issue = 45 | pages = 19242–19247 | date = November 2010 | pmid = 20974926 | pmc = 2984191 | doi = 10.1073/pnas.1009599107 | doi-access = free | bibcode = 2010PNAS..10719242S }}

The bacterial and archaeo-eukaryotic release factors are believed to have evolved separately. The two groups class 1 factors do not show sequence or structural homology with each other.{{cite journal | vauthors = Buckingham RH, Grentzmann G, Kisselev L | title = Polypeptide chain release factors | journal = Molecular Microbiology | volume = 24 | issue = 3 | pages = 449–456 | date = May 1997 | pmid = 9179839 | doi = 10.1046/j.1365-2958.1997.3711734.x | quote = Standard methods of sequence comparison do not show significant similarity between the prokaryotic factors RF1/2 and RF1 | doi-access = free }}{{cite journal | vauthors = Kisselev L |title=Polypeptide Release Factors in Prokaryotes and Eukaryotes |journal=Structure |date=January 2002 |volume=10 |issue=1 |pages=8–9 |doi=10.1016/S0969-2126(01)00703-1 |pmid=11796105 |doi-access=free}} The homology in class 2 is restricted to the fact that both are GTPases. It is believed that (b)RF3 evolved from EF-G while eRF3 evolved from eEF1α.{{cite journal | vauthors = Inagaki Y, Ford Doolittle W | title = Evolution of the eukaryotic translation termination system: origins of release factors | journal = Molecular Biology and Evolution | volume = 17 | issue = 6 | pages = 882–889 | date = June 2000 | pmid = 10833194 | doi = 10.1093/oxfordjournals.molbev.a026368 | doi-access = free }}

In line with their symbiotic origin, eukaryotic mitochondria and plastids use bacterial-type class I release factors.{{cite journal | vauthors = Duarte I, Nabuurs SB, Magno R, Huynen M | title = Evolution and diversification of the organellar release factor family | journal = Molecular Biology and Evolution | volume = 29 | issue = 11 | pages = 3497–3512 | date = November 2012 | pmid = 22688947 | pmc = 3472500 | doi = 10.1093/molbev/mss157 }} {{As of|2019|04}}, no definite reports of an organellar class II release factor can be found.

• RF1 (mitochondrial): MTRF1, MTRF1L, MRPL58 (ICT1), MTRFR (C12orf65)
• eRF1: ETF1
• eRF3: GSPT1, GSPT2

Crystal structures have been solved for bacterial 70S ribosome bound to each of the three release factors, revealing details in codon recognition by RF1/2 and the EF-G-like rotation of RF3.{{cite journal | vauthors = Zhou J, Korostelev A, Lancaster L, Noller HF | title = Crystal structures of 70S ribosomes bound to release factors RF1, RF2 and RF3 | journal = Current Opinion in Structural Biology | volume = 22 | issue = 6 | pages = 733–742 | date = December 2012 | pmid = 22999888 | pmc = 3982307 | doi = 10.1016/j.sbi.2012.08.004 }} Cryo-EM structures have been obtained for eukaryotic mamallian 80S ribosome bound to eRF1 and/or eRF3, providing a view of structural rearrangements caused by the factors. Fitting the EM images to previously known crystal structures of individual parts provides identification and a more detailed view of the process.{{cite journal | vauthors = Taylor D, Unbehaun A, Li W, Das S, Lei J, Liao HY, Grassucci RA, Pestova TV, Frank J | display-authors = 6 | title = Cryo-EM structure of the mammalian eukaryotic release factor eRF1-eRF3-associated termination complex | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 109 | issue = 45 | pages = 18413–18418 | date = November 2012 | pmid = 23091004 | pmc = 3494903 | doi = 10.1073/pnas.1216730109 | doi-access = free | bibcode = 2012PNAS..10918413T }}{{cite journal | vauthors = des Georges A, Hashem Y, Unbehaun A, Grassucci RA, Taylor D, Hellen CU, Pestova TV, Frank J | display-authors = 6 | title = Structure of the mammalian ribosomal pre-termination complex associated with eRF1.eRF3.GDPNP | journal = Nucleic Acids Research | volume = 42 | issue = 5 | pages = 3409–3418 | date = March 2014 | pmid = 24335085 | pmc = 3950680 | doi = 10.1093/nar/gkt1279 }}

In both systems, the class II (e)RF3 binds to the universal GTPase site on the ribosome, while the class I RFs occupy the A site.

The bacterial class 1 release factors can be divided into four domains. The catalytically-important domains are:

• The "tripeptide anticodon" motif in domain 2, {{code|P[AV]T}} in RF1 and {{code|SPF}} in RF2. Only one residue actually participates in stop codon recognition via hydrogen bonding.
• The GGQ motif in domain 3, critical for peptidyl-tRNA hydrolase (PTH) activity.

As RF1/2 sits in the A site of the ribosome, domains 2, 3, and 4 occupy the space that tRNAs load into during elongation. Stop codon recognition activates the RF, promoting a compact to open conformation change,{{cite journal | vauthors = Fu Z, Indrisiunaite G, Kaledhonkar S, Shah B, Sun M, Chen B, Grassucci RA, Ehrenberg M, Frank J | display-authors = 6 | title = The structural basis for release-factor activation during translation termination revealed by time-resolved cryogenic electron microscopy | journal = Nature Communications | volume = 10 | issue = 1 | pages = 2579 | date = June 2019 | pmid = 31189921 | pmc = 6561943 | doi = 10.1038/s41467-019-10608-z | bibcode = 2019NatCo..10.2579F }} sending the GGQ motif to the peptidyl transferase center (PTC) next to the 3′ end of the P-site tRNA. By hydrolysis of the peptidyl-tRNA ester bond, which displayed pH-dependence in vitro,{{cite journal | vauthors = Indrisiunaite G, Pavlov MY, Heurgué-Hamard V, Ehrenberg M | title = On the pH dependence of class-1 RF-dependent termination of mRNA translation | journal = Journal of Molecular Biology | volume = 427 | issue = 9 | pages = 1848–1860 | date = May 2015 | pmid = 25619162 | doi = 10.1016/j.jmb.2015.01.007 | series = Translation: Regulation and Dynamics | doi-access = free | url = http://uu.diva-portal.org/smash/get/diva2:821760/FULLTEXT01 }} the peptide is cut loose and released. RF3 is still needed to release RF1/2 from this translation termination complex.

After releasing the peptide, ribosomal recycling is still required to empty the P-site tRNA and mRNA out to make the ribosome usable again. This is done by splitting the ribosome with factors like IF1–IF3 or RRF–EF-G.{{cite journal | vauthors = Pavlov MY, Antoun A, Lovmar M, Ehrenberg M | title = Complementary roles of initiation factor 1 and ribosome recycling factor in 70S ribosome splitting | journal = The EMBO Journal | volume = 27 | issue = 12 | pages = 1706–1717 | date = June 2008 | pmid = 18497739 | pmc = 2435134 | doi = 10.1038/emboj.2008.99 }}

eRF1 can be broken down into four domains: N-terminal (N), Middle (M), C-terminal (C), plus a minidomain:

• The N domain is responsible for stop codon recognition. Motifs include {{code|TASNIKS}} and {{code|YxCxxxF}}.
• A GGQ motif in the M domain is critical for peptidyl-tRNA hydrolase (PTH) activity.

Unlike in the bacterial version, eRF1–eRF3–GTP binds together into a sub-complex, via a {{code|GRFTLRD}} motif on RF3. Stop codon recognition makes eRF3 hydrolyze the GTP, and the resulting movement puts the GGQ into the PTC to allow for hydrolysis. The movement also causes a +2-nt movement of the toeprint of the pre-termination complex. The archaeal aRF1–EF1α–GTP complex is similar.{{cite journal | vauthors = Kobayashi K, Saito K, Ishitani R, Ito K, Nureki O | title = Structural basis for translation termination by archaeal RF1 and GTP-bound EF1α complex | journal = Nucleic Acids Research | volume = 40 | issue = 18 | pages = 9319–9328 | date = October 2012 | pmid = 22772989 | pmc = 3467058 | doi = 10.1093/nar/gks660 }} The triggering mechanism is similar to that of aa-tRNA–EF-Tu–GTP.

A homologous system is Dom34/Pelota–Hbs1, a eukaryotic system that breaks up stalled ribosomes. It does not have GGQ. The recycling and breakup is mediated by ABCE1.{{cite journal | vauthors = Becker T, Franckenberg S, Wickles S, Shoemaker CJ, Anger AM, Armache JP, Sieber H, Ungewickell C, Berninghausen O, Daberkow I, Karcher A, Thomm M, Hopfner KP, Green R, Beckmann R | display-authors = 6 | title = Structural basis of highly conserved ribosome recycling in eukaryotes and archaea | journal = Nature | volume = 482 | issue = 7386 | pages = 501–506 | date = February 2012 | pmid = 22358840 | pmc = 6878762 | doi = 10.1038/nature10829 | bibcode = 2012Natur.482..501B }}{{cite journal | vauthors = Hellen CU | title = Translation Termination and Ribosome Recycling in Eukaryotes | journal = Cold Spring Harbor Perspectives in Biology | volume = 10 | issue = 10 | pages = a032656 | date = October 2018 | pmid = 29735640 | pmc = 6169810 | doi = 10.1101/cshperspect.a032656 }}

{{Reflist}}

• {{MeshName|Termination+Release+Factor}}

{{GeneticTranslation}}

Category:Proteins

Category:Protein biosynthesis