Binding of hisactophilin I and II to lipid membranes is controlled by a pH-dependent myristoyl-histidine switch

The interaction of the two N-terminally myristoylated isoforms of Dictyostelium hisactophilin with lipid model membranes was investigated by means of the monolayer expansion method and high-sensitivity titration calorimetry. The two isoforms, hisactophilin I and hisactophilin II, were found to insert with their N-terminal myristoyl residue into an electrically neutral POPC monolayer corresponding in its lateral packing density to that of a lipid bilayer. The partition coefficient for this insertion process was Kp = (1.1 +/- 0.2) x 10(4) M-1. The area requirement of the protein in the lipid membrane was estimated as 44 +/- 6 A2 which corresponds to the cross sectional area of the myristoyl moiety with an additional small contribution from amino acid side chains. The interaction of hisactophilin I (hisactophilin II) with negatively charged membrane surfaces is modulated in a pH-dependent manner by charged amino acid residues clustered around the myristoyl moiety. The electrostatic binding site consists of three lysine (one arginine and two lysine), seven (nine) histidine, and four (four) glutamic acid residues and has an isoelectric point of 6.9 (7.1). For small unilamellar POPC/POPG (75/25 mole/mole) vesicles, an apparent binding constant, K(app) = (8 +/- 1) x 10(5) M-1, was measured at pH 6.0 by means of high-sensitivity titration calorimetry. Electrostatic interactions hence increase the binding constant by about 2 orders of magnitude compared to hydrophobic binding alone. With increasing pH, the electrostatic attraction decreases and turns into an electrostatic repulsion at pH < 7.0 +/- 0.1. The area occupied by the cluster of charged residues constituting the membrane binding region was 280 +/- 20 A2 as derived from monolayer measurements in close agreement with molecular modeling data derived from the NMR structure of hisactophilin I [Habazettl et al. (1992) Nature 359, 855-858].