Component of the cleavage and polyadenylation specificity factor (CPSF) complex that plays a key role in pre-mRNA 3'-end formation, recognizing the AAUAAA signal sequence and interacting with poly(A) polymerase and other factors to bring about cleavage and poly(A) addition. FIP1L1 contributes to poly(A) site recognition and stimulates poly(A) addition. Binds to U-rich RNA sequence elements surrounding the poly(A) site. May act to tether poly(A) polymerase to the CPSF complex (By similarity). Component of the cleavage and polyadenylation specificity factor (CPSF) complex, composed of CPSF1, CPSF2, CPSF3, CPSF4 and FIP1L1. Found in a complex with CPSF1, FIP1L1 and PAPOLA. Interacts with CPSF1, CPSF4, CSTF2 and CSTF3 (By similarity). Interacts with AHCYL1 (when phosphorylated); the interaction is direct and associates AHCYL1 with the CPSF complex and RNA (PubMed:19224921). Interacts with PAPOLA; the interaction seems to be increased by the interaction with AHCYL1 (PubMed:19224921). Interacts with NUDT21/CPSF5; this interaction occurs in a RNA sequence-specific manner. Interacts (preferentially via unphosphorylated form and Arg/Glu/Asp-rich domain) with CPSF6 (via Arg/Ser-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. Interacts (preferentially via unphosphorylated form and Arg/Asp/Glu-rich domain) with CPSF7 (via Arg/Ser-rich domain); this interaction mediates, at least in part, the interaction between the CFIm and CPSF complexes and may be inhibited by CPSF7 hyper-phosphorylation (By similarity). Belongs to the FIP1 family.