Editing Five-prime cap (section)

== Structure ==
[[File:5' cap structure.png|thumb|5′ cap structure (cap-2).]]
[[File:Ribose Structure 2.svg|thumb|Ribose structure showing the positions of the 2′, 3′ and 5′ carbons.]]

In [[eukaryotes]], the 5′ cap (cap-0), found on the 5′ end of an mRNA molecule, consists of a [[guanine]] nucleotide connected to mRNA via an unusual 5′ to 5′ [[triphosphate]] linkage.  This [[guanosine]] is [[methylation|methylated]] on the 7&nbsp;position directly after capping ''in vivo'' by a [[methyltransferase]].<ref name="shatkin">{{cite journal|last1=Shatkin|first1=A|title=Capping of eucaryotic mRNAs|journal=Cell|date=December 1976|volume=9|issue=4|pages=645–653|doi=10.1016/0092-8674(76)90128-8|pmid=1017010|s2cid=26743858}}</ref><ref name="banerjee">{{cite journal | vauthors = Banerjee AK | title = 5′-terminal cap structure in eucaryotic messenger ribonucleic acids | journal = Microbiological Reviews | volume = 44 | issue = 2 | pages = 175–205 | date = June 1980 | pmid = 6247631 | pmc = 373176 | doi = 10.1128/mmbr.44.2.175-205.1980 }}</ref><ref name="sonenberg">{{cite journal | vauthors = Sonenberg N, Gingras AC | title = The mRNA 5′ cap-binding protein eIF4E and control of cell growth | journal = Current Opinion in Cell Biology | volume = 10 | issue = 2 | pages = 268–275 | date = April 1998 | pmid = 9561852 | doi = 10.1016/S0955-0674(98)80150-6 }}</ref><ref name="marcotrigiano">{{cite journal | vauthors = Marcotrigiano J, Gingras AC, Sonenberg N, Burley SK | title = Cocrystal structure of the messenger RNA 5′ cap-binding protein (eIF4E) bound to 7-methyl-GDP | journal = Cell | volume = 89 | issue = 6 | pages = 951–961 | date = June 1997 | pmid = 9200613 | doi = 10.1016/S0092-8674(00)80280-9 | s2cid = 15200116 | doi-access = free }}</ref> It is referred to as a [[7-Methylguanosine|7-methylguanylate]] cap, abbreviated m<sup>7</sup>G. The Cap-0 is the base cap structure, however, the first and second transcribed nucleotides can also be 2' O-methylated, leading to the Cap-1 and Cap-2 structures, respectively. This is more common in higher eukaryotes and thought to be part of the innate immune system to recognize mRNAs from other organisms.<ref>{{Cite journal |last=Despic |first=Vladimir |last2=Jaffrey |first2=Samie R. |date=February 2023 |title=mRNA ageing shapes the Cap2 methylome in mammalian mRNA |url=https://www.nature.com/articles/s41586-022-05668-z |journal=Nature |language=en |volume=614 |issue=7947 |pages=358–366 |doi=10.1038/s41586-022-05668-z |issn=1476-4687|pmc=9891201 }}</ref>

In multicellular eukaryotes and some viruses,<ref name="fechter2005">{{cite journal | vauthors = Fechter P, Brownlee GG | title = Recognition of mRNA cap structures by viral and cellular proteins | journal = The Journal of General Virology | volume = 86 | issue = Pt 5 | pages = 1239–1249 | date = May 2005 | pmid = 15831934 | doi = 10.1099/vir.0.80755-0 | doi-access = free }}</ref> further modifications can be made, including the methylation of the 2′ [[hydroxyl|hydroxy-groups]] of the first 2 [[ribose]] sugars of the 5′ end of the mRNA. cap-1 has a methylated 2′-hydroxy group on the first ribose sugar, while cap-2 has methylated 2′-hydroxy groups on the first two ribose sugars, shown on the right. The 5′ cap is chemically similar to the [[3′ end]] of an RNA molecule (the 5′ carbon of the cap ribose is bonded, and the 3′ unbonded). This provides significant resistance to 5′ [[exonuclease]]s.<ref name="Furuichi2015">{{cite journal
 | last1 = Furuichi
 | first1 = Yasuhiro
 | title = Discovery of m7G-cap in eukaryotic mRNAs
 | journal = Proceedings of the Japan Academy, Series B
 | volume = 91
 | number = 8
 | pages = 394-409
 | doi = 10.2183/pjab.91.394
 | year = 2015
 | pmid = 26460318
| pmc = 4729855
 }}</ref>

[[Small nuclear RNA]]s contain unique 5′-caps. Sm-class snRNAs are found with 5′-trimethylguanosine caps, while Lsm-class snRNAs are found with 5′-monomethylphosphate caps.<ref name="matera2007">{{cite journal | vauthors = Matera AG, Terns RM, Terns MP | title = Non-coding RNAs: lessons from the small nuclear and small nucleolar RNAs | journal = Nature Reviews. Molecular Cell Biology | volume = 8 | issue = 3 | pages = 209–220 | date = March 2007 | pmid = 17318225 | doi = 10.1038/nrm2124 | s2cid = 30268055 }}</ref>

In [[bacteria]], and potentially also in higher organisms, some RNAs are capped with [[nicotinamide adenine dinucleotide|NAD<sup>+</sup>]], [[NADH]], or [[coenzyme A|3′-dephospho-coenzyme A]].<ref name = "PMID27383794">{{cite journal | vauthors = Bird JG, Zhang Y, Tian Y, Panova N, Barvík I, Greene L, Liu M, Buckley B, Krásný L, Lee JK, Kaplan CD, Ebright RH, Nickels BE | title = The mechanism of RNA 5′ capping with NAD+, NADH and desphospho-CoA | journal = Nature | volume = 535 | issue = 7612 | pages = 444–447 | date = July 2016 | pmid = 27383794 | pmc = 4961592 | doi = 10.1038/nature18622 | bibcode = 2016Natur.535..444B }}</ref><ref>{{cite journal | vauthors = Cahová H, Winz ML, Höfer K, Nübel G, Jäschke A | title = NAD captureSeq indicates NAD as a bacterial cap for a subset of regulatory RNAs | journal = Nature | volume = 519 | issue = 7543 | pages = 374–377 | date = March 2015 | pmid = 25533955 | doi = 10.1038/nature14020 | bibcode = 2015Natur.519..374C | s2cid = 4446837 }}</ref>

In all organisms, mRNA molecules can be decapped in a process known as [[messenger RNA decapping]]. This is usually followed by degradation of the mRNA.