Component of the cleavage factor Im (CFIm) complex that functions as an activator of the pre-mRNA 3'-end cleavage and polyadenylation processing required for the maturation of pre-mRNA into functional mRNAs. CFIm contributes to the recruitment of multiprotein complexes on specific sequences on the pre-mRNA 3'-end, so called cleavage and polyadenylation signals (pA signals). Most pre-mRNAs contain multiple pA signals, resulting in alternative cleavage and polyadenylation (APA) producing mRNAs with variable 3'-end formation. The CFIm complex acts as a key regulator of cleavage and polyadenylation site choice during APA through its binding to 5'-UGUA-3' elements localized in the 3'-untranslated region (UTR) for a huge number of pre-mRNAs. NUDT21/CPSF5 activates indirectly the mRNA 3'-processing machinery by recruiting CPSF6 and/or CPSF7. Binds to 5'-UGUA-3' elements localized upstream of pA signals that act as enhancers of pre-mRNA 3'-end processing. The homodimer mediates simultaneous sequence-specific recognition of two 5'-UGUA-3' elements within the pre-mRNA (By similarity). Plays a role in somatic cell fate transitions and pluripotency by regulating widespread changes in gene expression through an APA-dependent function(PubMed:29249356). Binds to chromatin (PubMed:18032416). Binds to, but does not hydrolyze mono- and di-adenosine nucleotides (By similarity). Homodimer (via N- and C-terminus); binds RNA as homodimer. Component of the cleavage factor Im (CFIm) complex which is a heterotetramer composed of two subunits of NUDT21/CPSF5 and two subunits of CPSF6 or CPSF7 or a heterodimer of CPSF6 and CPSF7. The cleavage factor Im (CFIm) complex associates with the CPSF and CSTF complexes to promote the assembly of the core mRNA 3'-processing machinery. Interacts with CPSF6 (via the RRM domain); this interaction is direct and enhances binding to RNA. Interacts with CPSF7. Interacts with FIP1L1; this interaction occurs in a RNA sequence-specific manner. Interacts with PABPN1 (By similarity). Interacts (via N-terminus) with PAPOLA (via C-terminus); this interaction is direct and diminished by acetylation (PubMed:11716503). Interacts with SNRNP70 (By similarity). Interacts with VIRMA (By similarity). Shuttles between the nucleus and the cytoplasm in a transcription- and XPO1/CRM1-independent manner, most probably in complex with the cleavage factor Im complex (CFIm). In punctate subnuclear structures localized adjacent to nuclear speckles, called paraspeckles. Expressed in testis (PubMed:18032416). Expressed in male germ cells (at protein level) (PubMed:18032416). Up-regulated during spermatogenesis (PubMed:18032416). Acetylated mainly by p300/CBP, recruited to the complex by CPSF6. Acetylation decreases interaction with PAPAO. Deacetylated by the class I/II HDACs, HDAC1, HDAC3 and HDAC10, and by the class III HDACs, SIRT1 AND SIRT2. Belongs to the Nudix hydrolase family. CPSF5 subfamily. Lacks the conserved metal-binding residues in the NUDIX motif and is not expected to have hydrolase activity.