(A, B) To determine whether cyclin I and Cdk5 coimmunoprecipitate in glomerular podocytes, cyclin I–null (–/–) podocytes were infected with either myc-tagged cyclin I or GFP. Reciprocal coimmunoprecipitation studies with either α-myc (A, lanes 1, 2) or α-Cdk5 (B, lanes 3, 4) antibodies revealed that cyclin I bound to endogenous Cdk5 in podocytes. (C) Histone H1 kinase activity was abundant in cyclin I–null podocytes infected with cyclin I–myc (lane 1). In contrast, kinase activity was not detected in cultured cyclin I–null podocytes infected with GFP (lane 2). (D) To prove that the cyclin I–associated histone H1 kinase activity shown in C was specifically due to the activation of Cdk5, WT (+/+) podocytes were transfected with siRNA. Cyclin I–associated histone H1 kinase activity was present in podocytes transfected with control siRNA (lane 1). In contrast, cyclin I–associated kinase activity was not detected in cells transfected with siRNA targeting Cdk5 (lane 2). As expected, an IP with a control preimmune IgY antibody showed no kinase activity (lane 3). Ctrl, control. (E and F) To determine whether cyclin I–Cdk5 was also active in tissues in vivo, kinase activity was measured in protein extracts from kidney glomeruli and brain of WT and cyclin I–null mice. Cdk5 was active in WT cultured podocytes, glomeruli, and brain tissue. In contrast, histone H1 kinase activity was not detected in corresponding tissues from cyclin I–null mice. Densitometric analysis (n = 3). Data shown represent mean + SD.