Effect of calcium on kinase activity of PSKR1. A Kinase activity (relative fluorescence units (RFU)) of the cytoplasmic domain of wild-type PSKR1 (residues 686–1008) was measured at increasing calcium concentrations buffered with EGTA and Mg2+. Kinase activity was significantly reduced at calcium concentrations greater than 0.01 μM (mean ± s.e.m., n = 3 independent experiments; P = 0.0008, one-way ANOVA, Tukey-Kramer multiple comparison test). The inset shows the SDS-PAGE analysis of the purified cytoplasmic domain of PSKR1 (5 μg). B Suppression of wild-type PSKR1 kinase activity by calcium. Kinase activity of wild-type PSKR1 (residues 686–1008) was determined when incubated in zero calcium and then subjected to 10 μM free calcium before 10 mM EDTA was added. Kinase activity was significantly reduced by the calcium treatment (mean ± s.e.m., n = 3; P = 0.0013 one-way ANOVA, Tukey-Kramer multiple comparison test).