Capping of the 5'-end of pre-mRNA

All mRNAs have a cap-structure at their 5'-end. The cap-structure consists of a 7-methylguanosine linked to the first nucleotide via a 5'-5' triphosphate bridge. Usually one or two methyl groups are present at specific positions. The cap-structure is added to the 5'-end of the pre-mRNA during transcription, apparently already when the RNA polymerase has polymerized 20-100 nucleotides. The enzym guanylyl transferase, which may first interact with the RNA polymerase, adds the 7-methylguanosine. The cap-structure seems to be involved in several aspects of pre-mRNA and mRNA metabolism, in most cases related to identifying the 5'-end of the RNA.

The cap-structure binds two proteins in the nucleus, the CBP 20 and CBP 80 proteins (CBP= Cap Binding Protein), to form a Cap Binding Complex, CBC. At least CBP 20 binds to the cap-structure during transcription. The CBC influences splicing of the pre-mRNA and it may be involved in transport of the pre-mRNA to the cytoplasm. The CBC is in addition, involved in the transport to the cytoplasm of some snRNPs. In the cytoplasm, the cap-structure is important for the initiation of translation. The eukaryotic translation initiation factor eIF4A binds directly to the cap-structure.

see initiation of translation »