CDK5 increases the association between spinophilin and PP1 and decreases spinophilin expression. (A) HEK293 cells were transfected with spinophilin in the absence or presence of CDK5 and its activator, p35. Lysates and spinophilin immunoprecipitates were immunoblotted for spinophilin, PP1α, PP1γ1, Myc epitope, and Flag epitope. For lysates, 0.5% of the total was loaded for spinophilin and PP1 blots and 1.5% was loaded for Flag and Myc blots. (B) The association between spinophilin and PP1α or PP1γ1 was significantly increased by overexpression of CDK5/p35. (C) Spinophilin expression was decreased by CDK5/p35 overexpression. (D) PP1 expression was unchanged by CDK5/p35 overexpression. A 1-sample t-test comparing to a theoretical value of 1 was performed. *p < 0.05, **p < 0.01.