Novel SR-rich-related Protein Clasp Specifically Interacts with Inactivated Clk4 and Induces the Exon EB Inclusion of Clk

We identified a novel serine/arginine (SR)-rich-related protein as a binding partner of Clk4 mutant lacking kinase activity (Clk4 K189R) in the two-hybrid screen and designated it Clasp (Clk4-associating SR-related protein). Northern blot analysis revealed that Clasp mRNA was highly expressed in brain, and in situ hybridization of a mouse brain sagittal section hybridized with antisense probes revealed that both Clasp and Clk4 mRNAs were expressed in the hippocampus, the cerebellum, and the olfactory bulb. Two forms of Clasp were produced by a frameshift following alternative splicing. The staining of an HA-tagged short form of Clasp (ClaspS) showed a nucleoplasmic pattern, while the long form of Clasp (ClaspL) was localized as nuclear dots. In vitro protein interaction assay demonstrated that Clk4 K189R was bound to Clasp while wild Clk4 was not. Overexpression of ClaspL promoted accumulation of Clk4 K189R in the nuclear dots and the exon EB inclusion from CR-1 and CR-2 pre-mRNA of Clk1. These data suggest that Clasp is a binding partner of Clk4 and may be involved in the regulation of the activity of Clk kinase family.