Structure and mechanism of the RNA triphosphatase component of mammalian mRNA capping enzyme

The 5' capping of mammalian pre-mRNAs is initiated by RNA triphosphatase, a member of the cysteine phosphatase superfamily. Here we report the 1.65 An gstrom crystal structure of mouse RNA triphosphatase, which reveals a deep, positively charged active site pocket that can fit a 5' triphosphate end, Structural, bio-chemical and mutational results show that despite sharing a n HCxxxxxR(S/T) motif, a phosphoenzyme intermediate and a core alpha/beta - fold with other cysteine phosphatases, the mechanism of phosphoanhydride cl eavage by mammalian capping enzyme differs from that used by protein phosph atases to hydrolyze phosphomonoesters. The most significant difference is t he absence of a carboxylate general acid catalyst in RNA triphosphatase, Re sidues conserved uniquely among the RNA phosphatase subfamily are important for function in cap formation and are likely to play a role in substrate r ecognition.