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5’ Capping
Introduction
(15081560-089)
The five-prime capping of pre-RNA is a part of a series of processes which occurs as soon as the
transcription begins and goes all the way until the RNA assumes its final form
...
Many of the RNA molecules in bacteria and virtually all RNA
molecules in Eukaryotes are processed to some degree after synthesis
...
The three major steps involved are following:
Modifications at Ends of RNA
1
...
2
...
Modifications within the RNA
The primary transcript (newly synthesized RNA) for a eukaryotic mRNA contains both coding
segments (exons) and non-coding segments (introns)
...
Splicing- In this process the introns are removed from the primary transcript and the exons
are joined to form a continuous sequence that specifies a functional polypeptide
...
These
catalytic RNAs are called ribozymes
...
Definition of 5’ Capping
“The process of addition of a specially altered nucleotide (i
...
, methylated GTP) on the 5’ end
of some primary transcript (e
...
pre-mRNA) through an unusual 5’,5’ triphosphate linkage is
called the 5’ capping or mRNA capping”
7-Methylguanosine (m7G)
7-Methylguanosine (m7G) is a modified purine nucleoside
...
This guanosine is methylated on the 7 position directly after capping in vivo by a
methyltransferase
...
Capping Process
(15081560-065)
The 5’ cap is formed by condensation of a
molecule of GTP with the triphosphate at the 5’
end of the transcript
...
The
guanine
is
subsequently methylated at N-7 (catalyzed by
guanine-7-methyltransferase), and additional
methyl groups are often added at the 2’
hydroxyls of the first and second nucleotides
adjacent to the cap
...
(Figure 1)
...
Methyl groups are
often found at the 2’ position of the first and
second nucleotides
...
(adoHcy is S-adenosylhomocysteine)
...
All of the capping enzymes, and through them the
5’ end of the transcript itself, are associated with the
RNA polymerase II CTD until the cap is synthesized
...
(Figure 3)
...
The cap remains
tethered to the CTD through an association
with the cap-binding complex (CBC)
...
Regulation of nuclear export
Nuclear export of RNA is regulated by the cap binding complex (CBC), which binds exclusively
to 7-methylguanylate-capped RNA
...
Once in the cytoplasm after the pioneer round of translation, the CBC is replaced by the
translation factors eIF4E and eIF4G of the eIF4F complex
...
2
...
Second, the CBC and eIF4E/eIF4G block the access of decapping enzymes to the cap
...
3
...
The 5’ capping is also involved in the binding of the mRNA to the ribosomes to initiate translation
...
eIF4E has a significantly higher
affinity for the 7-methylguanosine cap than the unmethylated guanosine cap, and this has been
hypothesized to prevent free cellular GTP from interfering with translation initiation
...
4
...
The mechanism of 5′ proximal intron excision promotion is not well understood, but the 7methylguanylate cap appears to loop around and interact with the spliceosome in the splicing
process, promoting intron excision
...
Stability
Methylation of the mRNA guanosine cap also has the potential to indirectly regulate mRNA
stability
...
Once the guanosine
cap is methylated, it is no longer a substrate for the guanylyltransferase, and, since cap methylation
appears to be irreversible, it indirectly stabilizes the RNA