Interactions of the fatty acid-binding protein ReP1-NCXSQ with lipidmembranes. Influence of the membrane electricfield on bindingand orientation

Abstract

The regulatory protein of the squid nerve sodium calcium exchanger (ReP1-NCXSQ) is a 15 kDa soluble, intracel-lular proteinthatregulates the activity ofthe Na+/Ca2+exchanger inthe squidaxon. It isa member of thecellularretinoic acid-binding proteins family and the fatty acid-binding proteins superfamily. It is composed of ten betastrands defining an inner cavity and a domain of two short alpha helix segments. In this work, we studied thebinding and orientation of ReP1-NCXSQ in anionic and zwitterionic lipid membranes using molecular dynamics(MD) simulations. Binding to lipid membranes was also measured byfiltration binding assay. ReP1-NCXSQ ac-quired an orientation in the anionic membranes with the positive end of the macrodipole pointing to the lipidmembrane. Potential of mean force calculations, in agreement with experimental measurements, showed thatthe binding to the anionic interfaces in low ionic strength was stronger than the binding to anionic interfacesin high ionic strength or to zwitterionic membranes. The results of MD showed that the electrostatic bindingcan be mediated not only by defined patches or domains of basic residues but also by a global asymmetric distri-bution of charges. A combination of dipole–electricfield interaction and local interactions determined theorientation of ReP1-NCXSQ in the interface.