ACTIN-FILAMENTS STIMULATE THE NA+-K+-ATPASE

In this report, the functional role of actin on Na+-K+-adenosinetriphosphatase (Na+-K+-ATPase) activity was explored. The Na+- and K+-dependent, ouabain-sensitive ATP hydrolysis mediated by rat kidney Na+-K+-ATPase increased by 74% in the presence of previously unpolymerized actin (24 mu M), whereas addition of polymerized actin was without effect. Addition of actin was associated instead with an increase in the affinity of the Na+-K+-ATPase for Na+ but not other enzymatic substrates. A maximal stimulatory effect (296%) was observed either at an Na+-K+-ATPase:actin ratio of 1:50,000 or at lower ratios (1:625) by shifting from the E(2) (K+ selective) to the E(1) (Na+ selective) conformation of the enzyme. Immunoblotting of actin to the purified Na+-K+-ATPase suggested that this interaction may be linked to binding of actin to the enzyme, which was further supported by sequence analysis indicating putative actin-binding domains in the alpha-subunit of the enzyme. The interaction between actin and the Na+-K+-ATPase may imply a novel functional role of the cytoskeleton in the control of ion transport.