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5' cap plays a role in translation initiation, pre-mRNA splicing, mRNA 3'-end processing, transport of mRNA from the nucleus to the cytoplasm, protecting mRNA from 5' exoribonucleolytic degradation, and translation.
The 5' cap on mRNA transcripts play a vital role in pre-mRNA splicing in higher eukaryotes through a nuclear cap binding protein complex (CBC). CBC consists of two subunits - CBP80 and CBP20, both of which are necessary for normal cap binding. The amino acid sequence of these subunits has been highly conserved through evolution; and hence, it is believed that their functions have been conserved as well. Ultimately, studies suggest that the binding of CBC to the 5' cap facilitates an early stage in the assembly of the splicing complex, thereby, illustrating another vital function for 5' capping.
Furthermore, by inactivating CEG1 (part of the capping enzyme), mRNA levels significantly declines. In consequence of this, pre-mRNA spicing, protein synthesis, and the accumulation of newly synthesized mRNA transcripts are all inhibited. It is believed that the 5' cap protects nascent mRNA strands from degradation by a 5' exoribonuclease, such as Xrn1. However, the removal of Xrn1 does not circumvent the requirement of the 5' cap for normal growth; specifically, the requirement of Ceg1. Hence, this implicates the variety of necessary functions that the 5' cap plays.
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