RNA-binding protein that function as a pre-mRNA splicing factor. Plays a critical role in both constitutive and enhancer-dependent splicing by mediating protein-protein interactions and protein-RNA interactions required for accurate 3'-splice site selection. Acts by enhancing the binding of U2AF2 to weak pyrimidine tracts. Also participates in the regulation of alternative pre-mRNA splicing. Activates exon 5 skipping of PTPRC during T-cell activation; an event reversed by GFI1. Binds to RNA at the AG dinucleotide at the 3'-splice site (By similarity). Shows a preference for AGC or AGA (By similarity). Interacts with GFI1, U2AF2 and C1QBP. Interaction with C1QBP is required for the nuclear translocation. Displays active nucleo-cytoplasmic shuttling. Isoform 2 is widely expressed. Isoform 3 is highly expressed in heart, brain and lung, lower expressed in thymus and much lower expressed in peripheral blood leukocytes. The second zinc finger in necessary for interaction with GFI1 and for alternative pre-mRNA splicing events. The region 162-220 is essential for the nuclear import of the protein in spite of the absence of a nuclear localization signal (NLS). This region is essential for the interaction with C1QBP, interaction which is required for the nuclear translocation. This region may be involved in the localization in nuclear dot-like structures and it also confers the ability of nucleo-cytoplasmic shuttling. Belongs to the splicing factor SR family. Orthologs of U2AF1L4 do not appear to exist in lower eukaryotes, Drosophila, C. elegans, plants, or vertebrates such as Xenopus or zebrafish. Existence of circadian and light-inducible alternative splicing of U2AF1L4 similar to the mouse in human and rat is not yet proven.