se shown to play a role in splicing.16 The mammalian homolog of prp4 has been shown to interact with premRNA splicing factors PRP6 and Suppressor-of-White-Apricot and copurify with the U5 snRNP.17 Furthermore, PRP4K has been shown to be a key regulator of U4/U6U5 tri-snRNP assembly through the phosphorylation of PRP6 and PRP31.18 In humans, pre-mRNA splicing and the expression of the splicing kinases are perturbed in cancer. In this review, we will discuss the conserved roles of these kinases in pre-mRNA splicing and their emerging roles in tumorigenesis and treatment response. 280 Nucleus Volume 6 Issue 4 Splicing kinases in spliceosomal assembly and mRNA splicing Transcribed pre-mRNA must be spliced to remove introns prior to nuclear export and translation. This process is carried out by the spliceosome; a large macromolecular machine composed of 5 small nuclear ribonucleic proteins and numerous protein cofactors.19 Spliceosome assembly is a complex, multistep process as illustrated in www.tandfonline.com Nucleus 281 releases the RS domain and promotes RRM binding with the U1snRNP, promoting spliceosomal assembly.21 Therefore, stepwise phosphorylation mediated by SRPK1 and CLK1 is essential in the regulation of splicing. Once splicing is complete hyper-phosphorylated SR proteins bound to mRNA are dephosphorylated by nuclear phosphatases and 2A ) and either recycled to the cytoplasm as a chaperone for mRNA export41,42 where they also play a role in regulating translation of specific transcripts43 or re-phosphorylated and returned to speckles to await the next round of splicing. Thus, the splicing kinases connect the cell biology of splicing speckles domains to the biochemistry of pre-mRNA splicing by providing a mechanism for the shuttling of splicing factors to and from these domains to sites of splicing through reversible phosphorylation events. Regulation of SR Protein Phosphorylation SR-protein phosphorylation by the splicing kinases plays an essential role in splice site selection, subcellular localization, mRNA transport and translation, as described above. Regulation of PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19839222 these phosphorylation events is mediated in part by the distinct cellular localization and substrate specificity of the splicing kinases. For example, the RS domain of SR-protein SRSF1 contains an N-terminal stretch of Arg-Ser repeats and a C-terminal stretch of Ser-Pro repeats. Cytoplasmic SRPK1 phosphorylates RS1 at multiple sites using a directional and 
processive mechanism. An acidic docking grove distal to the active site of SRPK1 binds RS1 leading to the “priming” phosphorylation of a single site in RS2. This site then binds a basic site within SRPK1 which serves to advance the RS repeat sequence through the docking grove and toward the kinase domain resulting in sequential phosphorylation events.44 The phosphorylation of SRSF1 enhances the interaction with TRNSR allowing it to transport SRSF1 into the nucleus where it assembles into nuclear speckles.40,44 Nuclear CLK1 then phosphorylates RS2 causing SRSF1 dispersion from speckles and changes in alternative splicing.14,45 CLK1 substrate specificity has been shown to be mediated, at least in part, by LOXO 101 chemical information nature and extent of CLK1 autophosphorylation. It has been shown that CLK1 specificity for SRSF1 phosphorylation is sensitive to Tyr, but not Ser/Thr autophospharylation whereas its specificity for SC35 displays the opposite pattern.46 In this example, sequential phosphorylation of SRSF1 by SRPK1 and CLK1 is reg