Necessary for the degradation of mRNAs, both in normal mRNA turnover and in nonsense-mediated mRNA decay. Removes the 7-methyl guanine cap structure from mRNA molecules, yielding a 5'-phosphorylated mRNA fragment and 7m-GDP. Contributes to the transactivation of target genes after stimulation by TGFB1 (By similarity). Essential for embryonic development (By similarity). a 5'-end (N(7)-methyl 5'-triphosphoguanosine)-ribonucleoside in mRNA + H2O = a 5'-end phospho-ribonucleoside in mRNA + 2 H(+) + N(7)-methyl-GDP Forms a complex with EDC3, DCP2, DDX6 and EDC4/HEDLS, within this complex directly interacts with EDC3. Part of a cytoplasmic complex containing proteins involved in mRNA decay, including XRN1 and LSM1. Interacts with DCP1B. Interacts with DCP2. Interacts with DDX17 in an RNA-independent manner. Interacts with PNRC2. Interacts with SMAD4. Interacts with UPF1. Interacts with ZC3HAV1. Interacts with ZFP36L1. Interacts with NBDY. Interacts with DHX34; the interaction is RNA-independent (By similarity). Co-localizes with NANOS3 in the processing bodies (By similarity). Predominantly cytoplasmic, in processing bodies (PB). Nuclear, after TGFB1 treatment. Translocation to the nucleus depends on interaction with SMAD4 (By similarity). (Microbial infection) Cleaved by porcine reproductive and respiratory syndrome virus serine protease nsp4 after Glu-238. The cleavage inhibits DCP1A function. Belongs to the DCP1 family.