The flattened face of type II beta phosphatidylinositol phosphate kinase binds acidic phospholipid membranes

Type II beta phosphatidylinositol phosphate kinase is a representative phosphatidylinositol phosphate kinase that is active against membrane-bound substrates. The structure of the enzyme contains a flattened basic face that spans the crystallographic dimer interface and is adjacent to the active site. Analytical ultracentrifugation shows that phosphatidylinositol phosphate kinase is a dimer in solution. Modeling suggested that the flattened face binds to acidic phospholipids by electrostatic interactions. The enzyme binds to acidic vesicles containing phosphatidylserine, phosphatidic acid, or phosphoinositides mixed with phosphatidylcholine, but not to neutral phosphatidylcholine vesicles. Binding to acidic vesicles is abolished in the presence of 1.0 M NaCl, consistent with an essential electrostatic contribution to the free energy of binding. The +14 charge on the flattened face of the dimer was reduced to +2 in the triple mutant Lys72Glu/Lys76Glu/Lys78Glu. The mutation has no effect on dimerization, but reduces the apparent KA for 25% phosphatidylserine/75% phosphatidylcholine mixed vesicles by 16-fold. The reduction in the level of binding can be ascribed to a loss of electrostatic interactions based on the finite difference solution to the Poisson-Boltzmann equation. The mutant reduces catalytic activity toward phosphatidylinositol 5-phosphate by approximately 50-fold. The wild-type enzyme binds half-maximally to phosphatidylinositol 4,5-bisphosphate-containing vesicles at a mole fraction of 0.3% in a phosphatidylcholine background, as compared to a 22% mole fraction in phosphatidylserine. The binding to phosphatidylinositol 4,5-bisphosphate-containing membranes is less sensitive to salt and to the triple mutation than binding to phosphatidylserine-containing membranes, suggesting that at least part of phosphatidylinositol 4,5-bisphosphate's interaction with the enzyme is independent of the flattened face. It is concluded that the flattened face of type II beta phosphatidylinositol phosphate kinase binds to membranes through nonspecific interactions, and that this interaction is essential for efficient catalysis.