Hisactophilin-mediated binding of actin to lipid lamellae: A neutron reflectivity study of protein membrane coupling

The neutron reflectivity technique is applied to determine the adsorptive interaction of the 13.5-kDa actin-binding protein hisactophilin from Dictyostelium discoideum with lipid monolayers at a lateral pressure of 21 mN/m ≤ π ≤ 25 mN/m at the air-water interface. We compare binding of natural hisactophilin exhibiting a myristic acid chain membrane anchor at the N-terminus (DIC-HIS) and a fatty acid-deficient genetic product expressed in Escherichia coli (EC-HIS). It is demonstrated that only the natural hisactophilin DIC-HIS is capable of mediating the strong binding of monomeric actin to the monolayer, where it forms a layer of about 40 Å thickness corresponding to the average diameter of actin monomers. Monolayers composed of pure dimyristoyl phosphatidylcholine with fully deuterated hydrocarbon tails and headgroup (DMPC-d67) and 1:1 mixtures of this lipid with chain deuterated dimyristoyl phosphatidylglycerol (DMPG-d54) are studied on subphases consisting either of fully deuterated buffer (D₂O) or of a 9:1 H₂O/D₂O buffer that matches the scattering length density of air (CMA buffer). The reflectivity data are analyzed in terms of layer models, consisting of one to three layers, depending on the contrast of the buffer and the system. We show that both protein species bind tightly to negatively charged 1:1 DMPC-d67/DMPG-d54 monolayers, thereby forming a thin and most probably monomolecular protein layer of 12-15 Å thickness. We find that the natural protein (DIC-HIS) partially penetrates into the lipid monolayer, in contrast to chain-deficient species (EC-HIS), which forms only an adsorbed layer. The coverage of the monolayer with DIC-HIS strongly depends on the presence of anionic DMPG in the monolayer. At a bulk protein concentration of 1.5 μg/ml, the molar ratio of bound protein to lipid is about 1:45 for the 1:1 lipid mixture but only 1:420 for the pure DMPC.