Splice factor regulating alternative splice site selection for certain mRNA precursors. Mediates regulation of pre-mRNA splicing in a PKA-dependent manner. Monomer. Component of the spliceosome. Interacts with ZRANB2 and SFRS1/ASF through its Arg/Ser-rich domain. Interacts with RI and RII subunits of PKA. Widely expressed. Found in heart, brain, lung, liver, skeletal muscle, kidney and pancreas. Expressed in activated B-cells and placenta. Expressed in all cell lines tested including Jurkat-TAg, U-937 and HEK293 cells. RI-alpha binding site, predicted to form an amphipathic helix, could participate in protein-protein interactions with a complementary surface on the R-subunit dimer. The gene coding for this protein is located in the pseudoautosomal region 1 (PAR1) of X and Y chromosomes. May be produced at very low levels due to a premature stop codon in the mRNA, leading to nonsense-mediated mRNA decay. Was originally thought to be a cell surface protein involved in B-cell activation.