Na,K-ATPase, Scaffolding Actin Reorganization

Science's STKE  01 Mar 2005:
Vol. 2005, Issue 273, pp. tw81
DOI: 10.1126/stke.2732005tw81

The Na,K-ATPase is best known for its role in ion homeostasis; however, it is emerging as a regulator of signaling processes as well. The pump is composed of two subunits, a catalytic, ion-transporting α subunit and a β subunit, which is required for proper targeting to the plasma membrane. Decreases in Na,K-ATPase have been associated with cancer and increased cell motility. Barwe et al. studied how the abundance of the noncatalytic β subunit affected cell motility of Maloney sarcoma virus-transformed Madin-Darby canine kidney (MSV-MDCK) cells. They compared cells expressing very low levels of the β subunit, which had previously been shown to have a high motility phenotype, with cells overexpressing the β subunit and found that motility was decreased and the formation of lamellipodia was stimulated in the overexpressing cells. The abundance of active Rac1 (a guanosine triphosphatase that regulates actin reorganization) was increased, and the activity of phosphatidylinositol 3-kinase (PI3K) was also increased in the cells overexpressing the β subunit. Each of these effects required the C-terminal domain of the β subunit, suggesting a protein-protein interaction. Using a glutathione pull-down assay, the authors found that annexin II interacted with the β subunit C-terminal domain, and this interaction was confirmed by coimmunoprecipitation assays. The interaction between the β subunit and annexin II was inhibited if cells were treated with a pharmacological inhibitor of PI3K. The catalytic α subunit was important as well. Although overexpression of the α subunit alone had no effect on motility, the α subunit interacted with the p85 subunit of PI3K, and the abundance of phosphorylated p85 correlated with the abundance of the α subunit. Inhibition of the ion transport activity of Na,K-ATPase with ouabain did not alter the suppression of motility caused by overexpression of the β subunit. The authors propose that the α subunit, when delivered to the plasma membrane by proper assembly with the β subunit, interacts with the p85 subunit, allowing PI3K to be active. Annexin II interacts with the lipid products of PI3K and interacts with the β subunit of the Na,K-ATPase and sequesters active Rac1, leading to actin reorganization, the formation of lamellipodia, and decreased motility.

S. P. Barwe, G. Anilkumar, S. Y. Moon, Y. Zheng, J. P. Whitelegge, S. A. Rajasekaran, A. K. Rajasekaran, Novel role for Na,K-ATPase in phosphatidylinositol 3-kinase signaling and suppression of cell motility. Mol. Biol. Cell 16, 1082-1094 (2005). [Abstract] [Full Text]