Capping of Eukaryotic Pre-mRNA
pre-mrna : primary transcript of an RNA polymerase.
transferase : it is the class of enzyme which involves transfer of specific functional group from one molecule to another.
S-Adenosylmethionine : is a physiological molecule formed from essential aa methionine in our body
Process of capping
Capping is simply addition of methylated guanine at the 5' end of nascent mRNA. It occurs during translation. The initial step of capping is catalysed by capping enzyme which is recruited by phosphorylated carboxyl-terminal tail domain (CTD) of RNA polymerase II. Capping enzyme is a two subunit protein. One subunit removes γ-phosphate of nascent RNA from 5' end. The other subunit transfer GMP moiety from GTP to 5'-diphosphate of nascent transcript. Then α and β-phosphate are also removed forming gaunosine 5'-5'-triphosphate structure.
In the final step, some enzymes transfer methyl group from S-adenosylmethionine to N7 position of Gaunine at 5' end. This is known as cap 0.
Some important points
Nucleotide that starts an RNA chain is usually purine
Cap 1 and Cap 2
Methyl group can also be added to second as well as third nucleoside of capped mRNA.
Cap1 - methyl group on N7 position of gaunine and 2'-OH position of second nucleotide, at 5'end.
Cap 2- methyl group prsesnt in both second and third nucleoside
In the above diagram, P is phosphate, N is ribonucleotide and s is ribosesugar
Functions of 5' Cap
- protection of mRNA from being digested by exonucleases.
- Transport of mRNA from nucleus to cytoplasm.
- Cap structure is recognized by protein factors to influence mRNA stability, splicing, export and translation.
- Binding of ribosome with mRNA
© 2020 Khanishka Sharma