title: Identification of the Plasma Membrane Ca2+-ATPase and of Its Autoinhibitory Domain.
abstract: The effect of controlled proteolysis on the plasma membrane (PM)Ca2+-ATPase was studied at the molecular level in PM purified from radish (Raphanus sativus L.) seedlings. Two new methods for labeling the PM Ca2+-ATPase are described. The PM Ca2+-ATPase can be selectively labeled by treatment with micromolar fluorescein isothiocyanate (FITC), a strong inhibitor of enzyme activity. Both inhibition of activity and FITC binding to the PM Ca2+-ATPase are suppressed by millimolar MgITP. The PM Ca2+-ATPase maintains the capability to bind calmodulin also after sodium dodecyl sulfate gel electrophoresis and blotting; therefore, it can be conveniently identified by 125l-calmodulin overlay in the presence of calcium. With both methods a molecular mass of 133 kD can be calculated for the PM Ca2+-ATPase. FITC-labeled PM Ca2+-ATPase co-migrates with the phosphorylated intermediate of the enzyme[mdash]labeled by incubation with [[gamma]-32P]GTP in the presence of calcium[mdash]on acidic sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Controlled trypsin treatment of purified PM determines a reduction of the molecular mass of the PM Ca2+-ATPase from 133 to 118 kD parallel to the increase of enzyme activity. Only the 133-kD but not the 118-kD PM Ca2+-ATPase binds calmodulin. These results indicate that trypsin removes from the PM Ca2+-ATPase an autoinhibitory domain that contains the calmodulin-binding domain of the enzyme.
authors.affiliation: Centro di Studio del Consiglio Nazionale delle Ricerche per la Biologia Cellulare e Molecolare delle Piante, Dipartimento di Biologia, Universita di Milano, via G. Celoria, 26, 20133 Milano, Italy (F.R.-C., A.C.).
authors.forename: F.
authors.initials: F
authors.lastname: Rasi-Caldogno
authors.forename: A.
authors.initials: A
authors.lastname: Carnelli
authors.forename: M. I.
authors.initials: MI
authors.lastname: De Michelis