Protein Function : Necessary for the complete degradation of mRNAs, both in normal mRNA turnover and in nonsense-mediated mRNA decay. Removes the 7-methyl guanine cap structure from mRNA fragments shorter than 10 nucleotides that are produced by 3'->5' exosome-mediated mRNA decay. Releases m7GMP. Can also degrade m7GDP to m7GMP. Has no activity towards mRNA molecules longer than 25 nucleotides (By similarity).