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. Stimulates poly (A) polymerase (PAPOLA) conferring processivity on the poly (A) tail elongation reaction and controls also the poly (A) tail length. Increases the affinity of poly (A) polymerase for RNA. 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. Binds to poly (A) and to poly (G) with high affinity. May protect the poly (A) tail from degradation.