mRNA cap regulation in mammalian cell function and fate

Abstract

In this review we explore the regulation of mRNA cap formation and its impact on mammalian cells. The mRNA cap is a highly methylated modification of the 5' end of RNA pol II-transcribed RNA. It protects RNA from degradation, recruits complexes involved in RNA processing, export and translation initiation, and marks cellular mRNA as "self" to avoid recognition by the innate immune system. The mRNA cap can be viewed as a unique mark which selects RNA pol II transcripts for specific processing and translation. Over recent years, examples of regulation of mRNA cap formation have emerged, induced by oncogenes, developmental pathways and during the cell cycle. These signalling pathways regulate the rate and extent of mRNA cap formation, resulting in changes in gene expression, cell physiology and cell function.

Fig. 1

The mRNA cap. A predominant cap structure in mammalian cells is depicted. 7-Methylguanosine is linked to the first transcribed nucleotide via a 5′ to 5′ triphosphate bridge. The first transcribed nucleotide is methylated on the O-2 position of the ribose. Other methylations are also observed including first transcribed nucleotide adenosine methylation on position 6 and second transcribed nucleotide ribose O-2 methylation.

Fig. 2

mRNA cap function. mRNA cap formation initiates during transcription. The mRNA cap protects pre-mRNA from decay during transcription. CBC (cap binding complex) binds to the mRNA cap and recruits proteins which mediate splicing, polyadenylation and export into the cytoplasm. eIF4F (eukaryotic initiation factor 4F) binds to the cap and recruits the 40S ribosomal subunit, initiating translation. The mature mRNA cap inhibits the action of 5′-3′ exonucleases until it is removed by the decapping enzymes. Antiviral responses can be induced by incomplete caps, particularly those with tri-phosphate ends or lacking O-2 methylation on the first and second transcribed nucleotides.

Fig. 3

mRNA cap synthesis. The major capping reactions in mammalian cells are depicted. Other methylation events on first and second transcribed nucleotide are observed. Enzyme symbol, name and function in blue. SAM is s-adenosyl methionine. SAH is S-adenosyl homocysteine. RNMT, CMTR1, CMTR2 can all act on G(5′)ppp(5′)XpX; i.e. no prior methylation of the cap is required.

Fig. 4

Regulation of mRNA cap synthesis. Phospho-RNA pol II CTD (C-terminal domain) recruits RNGTT and activates guanylyltransferase activity, and recruits RNMT and CMTR1. c-Myc, E2F-1 and other transcription factors promote RNA pol II phosphorylation. In embryonic stern cells, the pluripotency-associated gene network represses ERK 1/2 activity; during differentiation loss of pluripotency is associated with increased ERK1/2 activity which phosphorylates RAM-S36 resulting in ubiquitin-dependent degradation. T cell activation resulting in upregulation of SAHH expression which hydrolyses SAH, the inhibitory product of methylation reactions. CDK1-cyclin B1 phosphorylates RNMT T77 which increases methyltransferase activity. During the innate immune response CMTR1 is upregulated.