Characterization of a P-type Ca(2+)-ATPase from Flavobacterium odoratum.

Abstract

In most bacterial cell types studied, low intracellular free calcium is maintained by a variety of secondary exchangers which utilize transmembrane ion gradients. Prokaryotic calcium ATPases appear to be extremely uncommon, and none have been reported in Gram-negative organisms. We demonstrate ATP-dependent calcium uptake in everted membrane vesicles of Flavobacterium odoratum, a common Gram-negative soil and water bacterium. Calcium is transported with an apparent initial rate of 10 nmol/min mg of protein. It is inhibited by 20 microM orthovanadate, a specific P-type ATPase inhibitor, but significantly, it is unaffected by the addition of N-ethylmaleimide, N,N-dicyclohexylcarbodiimide, valinomycin, or nigericin. Because the Ca(2+)-ATPase makes up a high proportion of the total ATPase activity it is easily detected by a soluble ATP hydrolysis assay, with an initial rate for calcium-dependent ATPase activity in vesicles of 25-40 nmol/min.mg at pH 7.8 and 25 degrees C. The calcium-dependent activity is preferentially solubilized by the detergent C12E8 and can be precipitated at 55-80% ammonium sulfate in a fraction free of other contaminating ATPase activities. This partially purified fraction is enriched 15-fold and demonstrates an apparent Km for calcium of 2 microM, and for ATP of 130 microM. The IC50 for vanadate is 1.6 microM. These values are similar to those obtained for the eukaryotic sarcoplasmic reticulum calcium ATPase. The enzyme is rapidly phosphorylated by [gamma-32P]ATP in a calcium-dependent, vanadate-inhibitable manner. The phosphorylated species migrates with an apparent molecular mass of 60 kDa by NaDodSO4-polyacrylamide gel electrophoresis, and the phosphoryl group is sensitive to alkaline conditions, a characteristic of the acylphosphate linkage found in ATPases. These data demonstrate that the majority of calcium transport in F. odoratum is facilitated by a P-type ATPase.