Decapping enzyme required for the removal of the 5'-end m7GpppN cap tethered to viral and host mRNAs to allow their decay in cells. May therefore accelerate viral and cellular mRNA turnover to eliminate competing host mRNAs and allow stage-specific synthesis of viral proteins. Acceleration of the turnover of cellular transcripts may even promote the shutoff of host protein synthesis. In addition to the mRNA cap, g5R also efficiently hydrolyzes diphosphoinositol polyphosphates. Down-regulation of the level of PP-InsP5 (diphosphoinositol pentakisphosphate) may play a role in viral manipulation of the cellular secretory pathway, a step necessary for the formation of virions. Binds viral and cellular poly(A) mRNAs, thereby decreasing both types of mRNAs. diphospho-myo-inositol polyphosphate + H2O = myo-inositol polyphosphate + phosphate. Interacts with host RPL23A. Accumulates at the periphery of the viral factories. Belongs to the Nudix hydrolase family. DIPP subfamily.