|In Vitro Kinase Assay and Kinetic Analysis
||Kinase activity of Clks and SRPKs is assayed in a reaction mixture, containing 200 mM Tris-HCl (pH 7.5), 12.5 mM MgCl2, 8 mM dithiothreitol, 4 mM EGTA, 1–20 μM ATP, 1 μCi of [γ-32P]ATP, 1 μg of synthetic peptide of SF2/ASF RS domain (NH2-RSPSYGRSRSRSRSRSRSRSRSNSRSRSY-OH), and 0.1–1 μg of purified kinases in a final volume of 40 μl. cAMP-dependent protein kinase activity was assayed in a reaction mixture containing 80 mM Tris-HCl (pH 7.5), 12.5 mM MgCl2, 8 mM dithiothreitol, 4 mM EGTA, 10 μM ATP, 1 μCi of [γ-32P]ATP, 5 μg of histone H1 (Sigma), and 1 μg of catalytic subunit of rat cAMP-dependent protein kinase purified. Protein kinase C activity is assayed in a reaction mixture containing 200 mM Tris-HCl (pH 7.5), 12.5 mM MgCl2, 1 mM CaCl2, 80 μg/ml phosphatidylserine, 8 μg/ml diolein, 10 μM ATP, 1 μCi of [γ-32P]ATP, 5 μg of histone H1, and 2 μl of partially purified rat protein kinase C. The final concentration of Me2SO is adjusted to 1% regardless of inhibitor concentration. The reaction mixture is incubated at 30 or 25 °C for mammalian or Xenopus recombinant proteins, respectively, for 10 min, and a half-portion is spotted on P81 phosphocellulose membrane (Whatman). The kinase assay conditions, including the incubation period and concentration of kinases and substrates, are optimized to maintain the linearity during incubation. The membrane is washed with 5% phosphoric acid solution (SF2/ASF RS domain) or 5% trichloroacetic solution (histone H1) at least over 15 min. The radioactivity is measured using a liquid scintillation counter. The net radioactivity is deduced by subtracting the background count from the reaction mixture without kinase, and the data are expressed as the percentage to the control sample containing the solvent.