The α-Subunit of the Na+/K+-ATPase has Catalytic Activity Independent of the ß-subunit
A functional Na+/K+-ATPase enzyme consists of two polypeptides: a large catalytic α-subunit and a smaller glycosylated ß-subunit. Although all catalytic properties of the Na+/K+-ATPase are associated with the α-subunit, it has never been determined if this subunit has activity independent of the ß-subunit. Using the baculovirus expression system we have previously demonstrated that when expressed alone, the rodent α-subunit exists in a stable conformation unassociated with other proteins in the infected Sf-9 cell (3). Here we show that the α-subunit has catalytic activity independent of the ß-subunit. This ATPase activity is dependent on Mg2+, does not require Na+ or K+, and is not inhibited by ouabain. However, the independent α-subunit ATPase activity is inhibited by EGTA, vanadate and increasing ionic strength. The inhibition of the ATPase activity by EGTA is not abolished by the addition of Ca2+ suggesting that EGTA inhibits activity in a manner other than its ion-binding abilities. Phosphorylated intermediates of the independent α-subunit were acid-stable and alkaline-labile, as well as sensitive to hydroxylamine, strongly suggesting the presence of an acylphosphate and thus phosphorylation at an aspartate residue. The physiological role, if any, of this independent α-subunit activity is unknown.
KeywordsATPase Activity High Ionic Strength Aspartate Residue Baculovirus Expression System Dependent Phosphorylation
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