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The mRNA Cap (Guanine-N7) Methyltransferase is involved in the capping of mRNA. This enzyme catalyses the methyl transfer reaction on the G(5')pppR cap at the C-terminal domain of a mRNA.

Ecm1 was isolated of Encephalitozoon cuniculi. This organism belongs to the phylum Microsporidia, in the kingdom of Fungi. E. cuniculi is an obligate intracellular parasite which is able to infect many mammals. An other particularity of this organism is that it had one of the smallest eukaryotic genome. Ecm1 is also the smallest cap methyltranferase identified.

 

In eukaryotic cells, the synthezise of mRNA is followed by a process of maturation. The best-known modifications are the polyadenylation, the splicing and the capping. The cap is important to the traduction initiation ; it allows the mRNA to be recognized by initiation factors. It is also necessary to keep the stability of the mRNA.

The capping is therefore an important step of the maturation. It consists in adding a 7-methylguanosine on the first nucleotide of the mRNA with a diphosphate bonding.  

The capping involved many steps: First the γ-phosphate from the 5′ end of the nascent RNA is removed.  Then a GMP is transfered from GTP to the 5′-diphosphate creating the guanosine 5′-5′-triphosphate. After that, a methyl group is transfered from S-adenosylmethionine to the N7 position of the guanine and an other is transfered to 2′ oxygen of riboses at the 5′ end of the mRNA.

Ecm1 catalyses the transfer of the methyl group  from S-adenosylmethionine to the N7 position of the guanine. This enzyme is a key enzyme in this capping process. {{clear}}

[[Image:Cap.png]][[Image:Step_cap.png]] {{clear}}

The structure of ECM1 is similar to the structure of the whole family of cap methyltransferase.  

We can indentify structural repeatings characteristic of the family : amongst others alpha helices or beta-sheets can be found.  

• The <scene name='60/604481/Segment_1/1'>first segment</scene> from the aminoacids 41 to 62 and from 165 to 292 : carrys the <scene name='60/604481/Helices_segment_1/1'>alpha helices A, F, H, E, I and G</scene>. The helices G, H and I are the <scene name='60/604481/Helices_c_terminal/1'>C-terminal helices</scene> from the aminoacid 249 to 284. This segment carrys also the <scene name='60/604481/Sheets_segment_1/1'>bêta-sheets from 5 to 11</scene>.

• The <scene name='60/604481/Segment_2/1'>second segment</scene> from the aminoacids 63 to 164 : carrys the <scene name='60/604481/Helices_segment_2/1'>alpha helices B, C, D</scene> and the <scene name='60/604481/Sheets_segment_2/1'>beta-sheets from 1 to 4</scene>. <ref name="Fabrega C, Hausmann S, Shen V, Shuman S, Lima CD. Structure and mechanism of mRNA cap (guanine-N7) methyltransferase. Mol Cell. 2004 Jan 16;13(1):77-89. ">PMID:14731396</ref>