ZAK specifically downregulates the activities of RhoGDIβ as a consequence of decreasing the amount of membrane-associated Rac1. (A) ZAK decreases the levels of membrane-associated Rac1 induced by RhoGDIβ. Control H9c2 and transfected H9c2 cells were collected and fractionated into membrane (Mem.) and cytosolic (Cyto.) fractions by centrifugation. (B) SiRNA knockdown of ZAK restores the effects of RhoGDIβ in ZAK- and RhoGDIβ-expressing cells (GRi1 and GRi2) upon Rac1 membrane association. (C) RhoGDIβ inhibits ZAK functions upon the induction of ANF mRNA expression. (D) The effect of ZAK-RhoGDIβ and RhoGDIβ on total amount of Rac1expression.