The γ subunit of Na+, K+-ATPase: Role on ATPase activity and regulatory phosphorylation by PKA

In kidney, Na+, K+-ATPase is an oligomer (αβγ) with equimolar amounts of essential α and β subunits and one small hydrophobic FXYD protein (γ subunit). This report describes γ subunit as an activator of pig kidney outer medulla Na+, K+-ATPase in aqueous medium. The effects of γ subunit on Na+, K+-ATPase were dose-dependent and preincubation-dependent. Changes in αβ/γ stoichiometry did not alter Km1 for ATP, and slightly increased Km2, but Vmax was increased at both catalytic and regulatory sites. Hydroxylamine treatment of enzyme phosphorylated by ATP (E-P), in the presence of additional γ subunit, revealed that 52% of the E-P accumulation was not via acyl-phosphate formation. The γ subunit was phosphorylated by endogenous kinases and by commercial catalytic subunit of protein kinase A (PKA). Additionally, we demonstrated that PKA phosphorylation of γ subunit increased its capacity to stimulate ATP hydrolysis. These results suggest that γ subunit can act as an intrinsic Na+, K+-ATPase regulator in kidney.