5’terminal m7G cap

of messenger RNA:

structure and function

By Sukanya Varape

Roll No. 1385

Why the cap was evolved only in Eukaryotes?

Loss of Shine- Dulgarno base pairing in eukaryotes which is a means of

directing the ribosome to the mRNA

Eukaryote use ‘cap’ as an alternative signal for directing translational

machinery to the mRNA

Appearance of 5’exorionuclease only in eukaryotes

Existence of capping enzymes only in eukaryotes

Specificity of Guanylytransferase

Structure of 7-

methyl

guanosine

cap

5’ to 5’ triphosphate bridge( phosphoanhydride bonds)

Methyl group on N-7 position

Co-transcriptional capping

Carboxy-terminal

domain

~30 nucleotide

Addition of guanine in

opposite direction

Structure and mechanism of the RNA triphosphatase enzymes

Mechanically and structurally they are diverse

Two families

The divalent-cation-dependent RNA triphosphatase-

Found in protozoa, eukaryotic viruses and fungi

The metal-independent class of triphosphatase enzyme-

Found in metazoa and plants

Structure and mechanism of the RNA

guanylyltransferase enzymes

Lysine side chain

It have two structural domains

N-terminal nucleotidyl transferae

C-terminal OB fold domain

It resembles DNA and RNA ligases

Structure and mechanism of the mRNA cap

(guanine-N7) methyltransferase

It’s structure has revealed that, enzyme contains

separate ligand- binding pocket for AdoMet(methyl

donor) and cap guanine(methyl acceptor)

Types of caps

The exception

RNA dependent RNA polymerase of Vesicular Stomatitis

Virus(VSV) incorporates GDP into the cap structure

Functions of m7G cap

Essential for the growth of eukaryotic cells

Marks the translational start site

Protects mRNA from degradation by 5’exoribonucleases

Methyl group facilitate translation

Participate in mRNA splicing and enhances its accuracy

Binds to nuclear cap binding complex to facilitate mRNA transport to

cytoplasm

Specificity of Gunylytransferase ensures that the caps are added only to the

intact transcripts and not to endolyticaly cleaved once.

Up regulators:

1.Myc

2.E2F1-cell proliferation transcription factor

3.Importin alpha

Down regulators:

1.Cdc34-ubiquitin-conjugating enzyme

2.SAHH-S-adenosylhomocystine hydrolyase

1.Specificity

2.Potency

mammalian cells contain a surveillance machinery anchored by the DXO and

Xrn2 factors that will rapidly degrade incorrectly capped pre-mRNAs

Possible Target proteins:

1. Viral TAT protein

2. Mammalian PMIT enzyme

mRNAs with short poly-A tails are captured poorly by oligo-dT based methods

Recombinant human cap-binding protein eIF4E was used to capture mRNAs

on the basis of the 5’ cap structure

With mRNA capture by the 5’ 7-methyl guanosine cap provides a more

complete view of the transcriptome and can be used to assay changes in

mRNA poly-A tail length on a genome-wide scale.

Based preparation and sequencing of concatamers of DNA tags deriving from

the initial 20 nucleotides from 5’ end mRNAs CAGE

Allows high-throughout gene expression analysis and the profiling of

transcriptional start points (TSP), including promoter usage Analysis

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295–302

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2013, Volume 8, Issue 10

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