Serine/arginine protein kinases have been conserved throughout evolution and are thought to play important roles in the regulation of mRNA processing, nuclear import, germline development, polyamine transport, and ion homeostasis. Human SRPK1, which was first identified as a kinase specific for the SR family of splicing factors, is located on chromosome 6p21.2-p21.3. We report here the cloning and characterization of SRPK1a, which is encoded by an alternatively processed transcript derived from the SRPK1 gene. SRPK1a contains an insertion of 171 amino acids at its NH 2 -terminal domain and is similar to SRPK1 in substrate specificity and subcellular localization. . Mammalian SRPK1 and SRPK2, which are highly related in sequence, kinase activity, and substrate specificity, were initially purified and cloned on the basis of their ability to phosphorylate members of the SR family of splicing factors in vitro and mediate splicing factor redistribution during the cell cycle (1-4). SR proteins themselves constitute a highly conserved protein family that is intimately involved in the regulation of pre-mRNA splicing and other steps of RNA metabolism (for reviews, see Refs. 5-7). Biochemical studies demonstrated that SR proteins are required at multiple steps in the assembly of the spliceosome, the dynamic RNA-protein complex that catalyzes intron removal (8 -10). Because RS domains are known to participate in protein-protein and protein-RNA interactions during spliceosome assembly, phosphorylation of these domains can modulate interactions involving SR proteins and is, therefore, essential for their function in constitutive splicing (3,11,12). Furthermore, phosphorylation of SR proteins leads to their release from nuclear speckles, in which they are concentrated to active sites of transcription in the nucleoplasm (1, 3, 13-15). Because changes in the intranuclear SR protein concentration play a critical role in determining which of the competing splice sites are selected, phosphorylation can also indirectly control alternative splice site selection (16 -20). Finally, it has been proposed that the formation of complexes between SF2/ASF and SRPKs may modulate the subcellular distribution of SF2/ASF (21).Yet, the lack both of authentic SR proteins in the yeast genome and of alternative mRNA splicing in yeast suggests that these kinases play roles in the regulation of cellular processes in addition to that of mRNA splicing. Indeed, genetic analyses have implicated Dsk1, which is the fission yeast homologue of SRPK1 in the regulation of chromosome segregation at the metaphase/anaphase transition (22). Furthermore, one of the endogenous substrates of Sky1p, in S. cerevisiae, is the RNA binding protein Npl3p, which has been implicated in mRNA transport (23). Sky1p was found to regulate nuclear import of Npl3p by promoting the interaction between Npl3p