The 14-3-3 protein translates the NA+,K+-ATPase α1-subunit phosphorylation signal into binding and activation of phosphoinositide 3-kinase during endocytosis

Clathrin-dependent endocytosis of Na+, K+-ATPase molecules in response to G protein-coupled receptor signals is triggered by phosphorylation of the alpha-subunit and the binding of phosphoinositide 3-kinase. In this study, we describe a molecular mechanism linking phosphorylation of Na+, K+-ATPase alpha-subunit to binding and activation of phosphoinositide 3-kinase. Co-immunoprecipitation studies, as well as experiments using confocal microscopy, revealed that dopamine favored the association of 14-3-3 protein with the basolateral plasma membrane and its co-localization with the Na+, K+-ATPase alpha-subunit. The functional relevance of this interaction was established in opossum kidney cells expressing a 14-3-3 dominant negative mutant, where dopamine failed to decrease Na+, K+-ATPase activity and to promote its endocytosis. The phosphorylated Ser-18 residue within the alpha-subunit N terminus is critical for 14-3-3 binding. Activation of phosphoinositide 3-kinase by dopamine during Na+, K+-ATPase endocytosis requires the binding of the kinase to a proline-rich domain within the alpha-subunit, and this effect was blocked by the presence of a 14-3-3 dominant negative mutant. Thus, the 14-3-3 protein represents a critical linking mechanism for recruiting phosphoinositide 3-kinase to the site of Na+, K+-ATPase endocytosis.