RNA exonuclease that binds to the 3'-end of histone mRNAs and degrades them, suggesting that it plays an essential role in histone mRNA decay after replication. A 2' and 3'-hydroxyl groups at the last nucleotide of the histone 3'-end is required for efficient degradation of RNA substrates. Also able to degrade the 3'-overhangs of short interfering RNAs (siRNAs) in vitro, suggesting a possible role as regulator of RNA interference (RNAi). Binds with high affinity to the 3' side of the stem-loop structure and to the downstream cleavage product (DCP) of histone pre-mRNAs. Requires for binding the 5'-ACCCA-3' sequence present in stem-loop structure. Able to bind other mRNAs. Required for 5.8S rRNA 3'-end processing. Also binds to 5.8s ribosomal RNA (By similarity). Binds 2 magnesium ions per subunit. Although it can bind simultaneously with SLBP to the 3'-end of histone mRNA, the presence of SLBP prevents the exonuclease activity. Identified in a histone pre-mRNA complex, at least composed of ERI1, LSM11, SLBP, SNRPB, SYNCRIP and YBX1. Binds to 40S and 60S ribosomal subunits and to 80S assembled ribosomes. Interacts in a cooperative manner with SLBP to the mature 3'-end of histone mRNAs. Found in a ternary complex with SLBP and the stem-loop structure of the 3'-end of histone mRNAs (By similarity). The SAP domain is necessary for binding to the stem-loop structure of histone mRNAs and to form the ternary complex with SLBP, but not for 3'-end histone mRNA exonuclease activity.