Ca2+- and phospholipid-binding properties of Torpedo electric organ calelectrin

The Ca2+-regulated lipid-binding properties of the H- and L-forms of calelectrin present in the electric organ of Torpedo marmorata have been compared. Binding of H-calelectrin required Ca2+ in millimolar concentrations, whereas that of L-calelectrin occurred in the micromolar range. Dissociation of H-calelectrin previously bound to lipids in the presence of 2 mM Ca2+ took place only when the Ca2+ concentration was reduced to micromolar concentrations. Binding was most effective to acidic phospholipids such as phosphatidylserine. Both forms of calelectrin promoted the aggregation of membrane vesicles in the presence of Ca2+.Mg2+, Na+ and K+ inhibited the Ca2+-induced binding to phospholipid, decreasing in effectiveness in that order. Binding was also inhibited by high pH. The surface activity and hydrophobicity index showed that H-calelectrin is a hydrophilic molecule. It may represent a less active, more highly phosphorylated "down-regulated" form of L-calelectrin. The role of calcium in H-calelectrin binding to lipid appeared to be consistent with the formation of a ternary complex of the protein, an acidic lipid and Ca2+, rather than with a direct interaction of lipid with hydrophobic sequences in H-calelectrin whose accessibility is Ca2+-regulated.