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 (PubMed:9659921, PubMed:8626397, PubMed:14690600, PubMed:29276085). 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) (PubMed:9659921, PubMed:8626397, PubMed:14690600). Most pre-mRNAs contain multiple pA signals, resulting in alternative cleavage and polyadenylation (APA) producing mRNAs with variable 3'-end formation (PubMed:23187700, PubMed:29276085). 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 (PubMed:20695905, PubMed:29276085). CPSF6 enhances NUDT21/CPSF5 binding to 5'-UGUA-3' elements localized upstream of pA signals and promotes RNA looping, and hence activates directly the mRNA 3'-processing machinery (PubMed:15169763, PubMed:29276085, PubMed:21295486). Plays a role in mRNA export (PubMed:19864460). (Microbial infection) Binds HIV-1 capsid-nucleocapsid (HIV-1 CA-NC) complexes and might thereby promote the integration of the virus in the nucleus of dividing cells (in vitro). 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 (PubMed:9659921, PubMed:8626397, PubMed:14561889, PubMed:20695905, PubMed:23187700). The cleavage factor Im (CFIm) complex associates with the CPSF and CSTF complexes to promote the assembly of the core mRNA 3'-processing machinery (PubMed:29276085). Associates with the exon junction complex (EJC) (PubMed:19864460). Associates with the 80S ribosome particle (PubMed:19864460). Interacts (via the RRM domain) with NUDT21/CPSF5; this interaction is direct and enhances binding to RNA (PubMed:14561889, PubMed:15169763, PubMed:19864460, PubMed:29276085, PubMed:21295486). Interacts (via Arg/Ser-rich domain) with FIP1L1 (preferentially via unphosphorylated form and Arg/Glu/Asp-rich domain); this interaction mediates, at least in part, the interaction between the CFIm and CPSF complexes and may be inhibited by CPSF6 hyper-phosphorylation (PubMed:29276085). Interacts (via N-terminus) with NXF1; this interaction is direct (PubMed:19864460). Interacts with SRSF3 (PubMed:15169763). Interacts with SRSF7 (PubMed:15169763). Interacts with SNRNP70 (PubMed:14561889). Interacts with TRA2B/SFRS10 (PubMed:15169763). Interacts with UPF1 (PubMed:19864460). Interacts with UPF3B (PubMed:19864460). Interacts with VIRMA (PubMed:29507755). Interacts (via Arg/Ser-rich domain) with TNPO3; promoting nuclear import of CPSF6 independently of its phosphorylation status (PubMed:30916345). Interacts with YTHDC1 (By similarity). (Microbial infection) Interacts (via C-terminus) with HIV-1 capsid protein p24 (CA). 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) (PubMed:19864460). Colocalizes with PSPC1 in punctate subnuclear structures often located adjacent to nuclear speckles, called paraspeckles, and corresponding to interchromatin granules-associated zones (IGAZs) (PubMed:17267687). Distribution in speckles and paraspeckles varies during the cell cycle (PubMed:17267687). Associates at sites of active transcription on nascent perichromatin fibrils (PFs) and perichromatin granules (PubMed:17267687). Nuclear import is mediated via interaction with TNPO3 independently of CPSF6 phosphorylation status (PubMed:30916345). Contains an Arg/Ser-rich domain composed of arginine-serine dipeptide repeats within the C-terminal region that is necessary and sufficient for activating mRNA 3'-processing and alternative polyadenylation (APA). Phosphorylated (PubMed:29276085). Phosphorylated in the Arg/Ser-rich domain by SRPK1, in vitro (PubMed:29276085). Symmetrically dimethylated on arginine residues in the GAR motif by PRMT5 in a WDR77- and CLNS1A-dependent manner (PubMed:20562214). Asymmetrically dimethylated on arginine residues in the GAR motif by PRMT1 (PubMed:20562214). Belongs to the RRM CPSF6/7 family.