This domain is found at the N-terminus of 3'-5' exonucleases with HRDC domains, and also in putative exosome components .
Exosomes are nano-compartments that function in the degradation or processing of RNA (including mRNA, rRNA, snRNA and snoRNA) [PUBMED:15951817, PUBMED:17174896]. Exosomes occur in both archaea and eukaryotes, and have a similar overall structure to each other and to bacterial/organelle PNPases (polynucleotide phosphorylases; EC) [PUBMED:17084501], consisting of a barrel structure composed of a hexameric ring of PH domains that act as a degradation chamber, and an S1-domain/KH-domain containing cap that binds the RNA substrate (and sometimes accessory proteins) in order to regulate and restrict entry into the degradation chamber [PUBMED:16285927]. There are two types of exosomes in eukaryotes, cytoplasmic exosomes that are responsible for 3'-5' exoribonuclease degradation of mRNAs, and nuclear exosomes that degrade pre-mRNAs (such as nonsense transcripts) and degrade rRNAs, snRNAs and snoRNAs. Unstructured RNA substrates feed in through the pore made by the S1 domains, are degraded by the PH domain ring, and exit as nucleotides via the PH pore at the opposite end of the barrel [PUBMED:16713559, PUBMED:17380186].