The beta chain of mRNA capping enzyme has triphosphatase activity. The function of the capping enzyme also depends on the guanylyltransferase activity conferred by the alpha chain (see PF01331)
The mRNA capping enzyme in yeast is composed of two subunits, alpha and beta. The alpha subunit has guanylyltransferase activity, whilst the beta subunit is an RNA 5'-triphosphatase (RTPase) [PUBMED:9345280]. This entry represents a domain found in the mRNA capping enzyme beta subunit Cet1. 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 and this subgroup includes fungal and protozoal RTPases. The RTPase domain of the mimivirus RTPase-GTase fusion mRNA capping enzyme also belongs to this subgroup. The domain is related to the CYTH domain. Cet1 has a novel active site fold whereby an eight-strand beta-barrel forms a topologically closed tunnel known as the
Yamada-Okabe T, Mio T, Matsui M, Kashima Y, Arisawa M, Yamada-Okabe H; , FEBS Lett 1998;435:49-54.: Isolation and characterization of the Candida albicans gene for mRNA 5'-triphosphatase: association of mRNA 5'-triphosphatase and mRNA 5'-guanylyltransferase activities is essential for the function of mRNA 5'-capping enzyme in vivo. PUBMED:9755857 EPMC:9755857 .