Involved in the 3'-end formation of mRNA precursors (pre-mRNA) by the addition of a poly(A) tail of 200-250 nt to the upstream cleavage product (By similarity). Stimulates poly(A) polymerase (PAPOLA) conferring processivity on the poly(A) tail elongation reaction and controls also the poly(A) tail length (By similarity). Increases the affinity of poly(A) polymerase for RNA (By similarity). Is also present at various stages of mRNA metabolism including nucleocytoplasmic trafficking and nonsense-mediated decay (NMD) of mRNA. Cooperates with SKIP to synergistically activate E-box-mediated transcription through MYOD1 and may regulate the expression of muscle-specific genes (PubMed:11371506). Binds to poly(A) and to poly(G) with high affinity (By similarity). May protect the poly(A) tail from degradation (By similarity). Subunit of the trimeric poly(A) tail exosome targeting (PAXT) complex, a complex that directs a subset of long and polyadenylated poly(A) RNAs for exosomal degradation. The RNA exosome is fundamental for the degradation of RNA in eukaryotic nuclei. Substrate targeting is facilitated by its cofactor MTREX, which links to RNA-binding protein adapters (PubMed:27871484). May interact with SETX (PubMed:21700224). Monomer and homooligomer. Binds RNA as a monomer and oligomerizes when bound to poly(A). Interacts with PAPOLA, but only in presence of oligo(A) RNA. Interacts with transportin. Identified in a IGF2BP1-dependent mRNP granule complex containing untranslated mRNAs. Association in a ternary complex with CPSF4 and influenza A virus NS1 blocks pre-mRNAs processing, thereby preventing nuclear export of host cell mRNAs. Associates in a single complex with SKIP and MYOD1 and interacts with SKIP in differentiated myocytes. Interacts with NUDT21/CPSF5. Interacts (via RRM domain and C-terminal arginine-rich region) with ZFP36 (via hypophosphorylated form); this interaction occurs in the nucleus in a RNA-independent manner, decreases in presence of single-stranded poly(A) RNA-oligomer and in a p38-dependent-manner and may down-regulated RNA poly(A) polymerase activity (By similarity). Component of the poly(A) tail exosome targeting (PAXT) complex made of accessory factors, such as PABPN1, ZFC3H1 and MTREX (PubMed:27871484). Interacts with ZFC3H1 in a RNase-insensitive manner (PubMed:27871484). Interacts with FRG1 (PubMed:17103222). Localized in cytoplasmic mRNP granules containing untranslated mRNAs. Shuttles between the nucleus and the cytoplasm but predominantly found in the nucleus (PubMed:10688363). Its nuclear import may involve the nucleocytoplasmic transport receptor transportin and a RAN-GTP-sensitive import mechanism (By similarity). Is exported to the cytoplasm by a carrier-mediated pathway that is independent of mRNA traffic. Colocalizes with SKIP and poly(A) RNA in nuclear speckles (By similarity). Intranuclear filamentous inclusions or 'aggregates' are detected in the myocytes of patients; these inclusions contain PABPN1, ubiquitin, subunits of the proteasome and poly(A) RNA. Ubiquitous. The RRM domain is essential for specific adenine bases recognition in the poly(A) tail but not sufficient for poly(A) binding. Arginine dimethylation is asymmetric and involves PRMT1 and PRMT3. It does not influence the RNA binding properties (By similarity). The poly-Ala region of PABPN1 is polymorphic (6-7 repeats) in the population and is expanded to 8-13 repeats in OPMD patients. Compound heterozygotes for (GCG)9 mutation and a (GCG)7 allele result in earlier onset and more severe clinical manifestations of the disease. The disease is caused by variants affecting the gene represented in this entry. The association of the expanded polyalanine mutations together with the capability to oligomerize may induce intranuclear inclusions and cell death. Expanded polyalanine mutations may either result from unequal crossing over during germ cell homologous recombination or from DNA slippage. The pathogenic mechanisms mediated by polyalanine expansion mutations may be either a general disruption of cellular RNA metabolism due to the trapping by the inclusions of PABPN1, mRNAs and/or nuclear proteins, resulting in the induction of cell death; or may change the normal muscle cell differentiation. May be due to a competing donor splice site. Extended N-terminus.