In eukaryotes, the nascent mRNA needs to be processed and transported to the cytoplasm for translation to occur. The processing steps includes 5’ capping, 3’ polyadenylation and RNA splicing. RNA splicing is an essential posttranscriptional modification which increases protein diversity and regulates the development and growth of cells. The splicing events include the removal of introns from the precursor mRNA and occur at the spliceosome. Splicing factors (SFs) are proteins involved in the splicing event and can regulate alternative splice site by the binding to specific cis-acting sequence elements and promote or disrupt spliceosome assembly. Serine-arginine (SR) proteins are one family of SFs, which is essential for controlling many aspects of mRNA splicing as well as other RNA processing events. SR splicing factors (SRSFs) have a modular structure consisting of one or two RNA-recognition motifs (RRMs) and a COOH-terminal arginine / serine-rich domain (RS domain), and they stay in nuclear speckles, from which they are released when not actively involved in transcript processing. SR proteins can also play roles in lots of post-splicing events, including mRNA export , translation regulation , and genomic stability .
Previous study has shown that SR proteins undergo multiple rounds of phosphorylation and dephosphorylation in spliceosome assembly , and the phosphorylation of SRSFs has been shown to be a prerequisite for spliceosome assembly and splicing . Serine-arginine protein kinases (SRPKs) are a novel subfamily of serine-threonine kinases, which specifically phosphorylate serine residues in the RS domains of SR proteins, affecting SR protein localization and mRNA splicing .
The first study  that established the role of the SRPK in SR protein function in vivo showed that when the unique SRPK of S. cerevisiae (Sky1) was deleted, the interaction of SR proteins was prevented, and the SR proteins couldn’t translocate into the nucleus. Zhong et al.  have shown that when SRPKs enter the nucleus, phosphorylation of SR proteins is increased, demonstrating the nuclear action of SRPKs on SR proteins. Another study performed by Jiang et al.  showed that when SRPK2 is phosphorylated by Akt in neuronal cells, it enters the nucleus and is able to phosphorylate the SR splicing factor SC35. Koizumi’s study  suggests that the formation of complexes between SRSFs and SRPKs, and the phosphorylation state of SRSFs, may play regulatory roles in the assembly and localization of this splicing factor.
The first found SRPK was named SRPK1 . SRPK1 can bind and phosphorylate serine residues in the RS domain, and is required for nuclear localization of SRSF1. In humans, SRSF1s are phosphorylated in the cytoplasm by SRPK1 and are subsequently transported to the nucleus by transportin-SR2, which specifically interacts with phosphorylated RS domains . The cloning of SRPK2 was reported almost at the same time...