CYTH-like mRNA triphosphatase (RTPase) component of the mRNA capping apparatus
This subgroup includes fungal and protozoal RTPases. RTPase catalyzes the first step in the mRNA cap formation process, the removal of the gamma-phosphate of triphosphate terminated pre-mRNA. This activity is metal-dependent. The 5'-end of the resulting mRNA diphosphate is subsequently capped with GMP by RNA guanylytransferase, and then further modified by one or more methyltransferases. The mRNA cap-forming activity is an essential step in mRNA processing. The RTPases are not conserved among eukarya. The structure and mechanism of this fungal RTPase domain group is different from that of higher eukaryotes. This subgroup belongs to the CYTH/triphosphate tunnel metalloenzyme (TTM)-like superfamily, whose enzymes have a unique active site located within an eight-stranded beta barrel. The RTPase domain of the mimivirus RTPase-GTase fusion mRNA capping enzyme also belongs to this subgroup.