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. Binds to poly(A) and to poly(G) with high affinity. May protect the poly(A) tail from degradation. Monomer and homooligomer. Binds RNA as a monomer and oligomerizes when bound to poly(A) (By similarity). Shuttles between the nucleus and the cytoplasm but predominantly found in the nucleus. The RRM domain is essential for specific adenine bases recognition in the poly(A) tail but not sufficient for poly(A) binding.