Finally, using S-adenosyl-methionine as a substrate, an RNA methyltransferase catalyzes the transfer of a methyl group to the N-7 position of the guanine to produce the characteristic m7GpppRNA cap structure. In humans, a bifunctional RNA capping ICG-001 enzyme catalyzes both the RTase and GTase reactions through distinct domains, while a separate polypeptide mediates the subsequent N-7 methylation. The importance of the cap structure for RNA metabolism is highlighted by genetic analyses in Saccharomyces cerevisiae that showed that the triphosphatase, guanylyltransferase and methyltransferase components of the capping apparatus are essential for cell growth. Nascent mRNA capping is a rapid, dynamic, and regulated cotranscriptional process that is subjected to quality control. Transcription initiation is associated with the RNA polymerase II carboxy-terminal domain Ser 5 phosphorylation, which recruits the capping apparatus. Nascent mRNAs are rapidly capped, followed by RNA Pol II CTD Ser 2 phosphorylation, HCE dissociation and mRNA elongation. Messenger RNA capping represents a quality control checkpoint as uncapped RNA are degraded by the Xrn2 59R39 exonuclease in order to avoid generation of uncapped mRNA which are not likely to be translated. Uncapped mRNAs are not recognized by the initiation factor eIF4E and are degraded by the 59R39 Xrn1. Given that the RNA Pol II synthesizes 10�C30 bases per second, the entire fate of an unsuccessfully capped mRNA can be sealed within few seconds, stressing the importance of rapid and efficient mRNA capping. The rate-limiting activity of the capping apparatus is the twostep ping-pong GTase activity. A general mechanism for phosphoryltransfer involving conformational changes between an open and closed form of the enzyme has been previously solved based on various GTases crystal structures. The first step of the reaction is initiated by the binding of GTP to the open form of the enzyme followed by the closure of the C-terminal oligomerbinding fold domain and the N-terminal nucleotidyl transferase domain. This closure is stabilized by interactions between the bound nucleotide and residues from both NT and OB fold domain. Once in the catalytically active close conformation, the GTP substrate is hydrolyzed to produce the enzyme-GMP covalent intermediate. Interactions between the bound 945595-80-2 structure guanylate and the OB fold domain are disrupted upon GTP hydrolysis, which leads to the reopening of the enzyme concomitant with the release of pyrophosphate. The open conformation exposes the RNA-binding site, thereby allowing the subsequent transfer of the GMP moiety onto the acceptor RNA.