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

Abstract

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 K m1 for ATP, and slightly increased K m2, but V max 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.