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. It can functionally substitute for U2AF1 in constitutive splicing and enhancer-dependent splicing. 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. Shows a preference for AGC or AGA (PubMed:11739736, PubMed:16819553, PubMed:18460468). Alternative splicing of U2AF1L4 may play a role in connecting the circadian rhythm to changing external cues: may provide a circadian buffering system in central and periphery clocks that allows synchronized adaption to clock-resetting stimuli in order to prevent potentially pathogenic desynchronization (PubMed:24837677). Interacts with GFI1, U2AF2 and C1QBP. Isoform 3 interacts with PER1. Interaction with C1QBP is required for the nuclear translocation. Displays active nucleo-cytoplasmic shuttling. Circadian and light-inducible alternative splicing, which occurs at least in brain and liver. Ubiquitous. Highly expressed in the brain. Up-regulated in response to T-cell activation (at protein level) (PubMed:16819553). Circadian alternative splicing switch accounts for rhythmic isoform expression. A circadian splicing switch produces isoform 3 in the brain cerebellum and liver (at protein level). Isoform 3 expression is regulated with the circadian rhythm but is also quickly increased upon light exposure (4-8 hours after light exposure). Expression of isoform 3 changes approximately 5-fold across a period of 24 hours, with concomitant changes in isoform 1, resulting in total U2af1l4 remaining constant (PubMed:24837677). 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. Isoform 3 contains a C-terminus domain with homology to Drosophila TIM (THD domain). Isoform 3 interacts with Per1 and specifically down-regulates its expression, probably by facilitating its recruitment to the proteasome. The interaction with PER1 depends on the presence of the THD domain, but the THD domain is not directly involved in the interaction. Isoform 3 is rapidly degraded by a proteasome-mediated degradation pathway. Mutant mice show defects in circadian gene expression and jet lag phenotype, but the master clock is not strongly affected. Per2 expression is rhythmic, but Per1 expression becomes nearly arrhythmic as well as Per1 target genes, such as Dbp. No splicing alteration was observed in the brain. Mutant mice show a faster adaptation to experimental jet lag, which is consistent with the circadian splicing switch providing a buffering system against sudden light changes. Cytoplasmic and nuclear. Displays active nucleo-cytoplasmic shuttling. Shows a circadian expression at the mRNA level. Produced by exon 7 skipping. Cytoplasm (PubMed:16819553). Shows a circadian expression at the mRNA level. Produced by exons 6 and 7 skipping. Produced by a circadian splicing switch in brain and liver. Contains a C-terminus domain with homology to Drosophila TIM (THD). Cytoplasmic. Shows a circadian expression at the mRNA and protein levels. Expression is quickly increased upon light exposure. Has a strongly reduced half-life compared to other isoforms. Produced by usage of an alternative 3' splice site in exon 8. 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 (PubMed:11739736). Existence of splicing isoforms of U2af1l4 in human and rat is not yet proven.