Myelin basic protein-lipid complex: an atomic force microscopy study

Myelin basic protein (MBP) efficiently bound Dilauroyl-L alpha-phosphatidic acid (DLPA) monolayers at the air water interface. Langmuir-Blodgett (LB) films were prepared from these monolayers and the hydrophilic surface of th e upper layer was inspected by atomic force microscopy (AFM) in a water sol ution. The topography images of nominally four-layer films of DLPA in the a bsence of MBP revealed regions of different thickness corresponding to diff erent numbers of lipid bimolecular layers. This morphology is characteristi c of those lipid films which reorganise spontaneously when kept under an aq ueous solution. The DLPA films containing MBP did not reorganise; their thi cknesses were uniform and surfaces were usually covered by disordered clust ers of protein molecules. Quasi-ordered arrays of smaller particles were ob served in films with small amounts of bound protein. The surface charge den sities of DLPA and DLPA-MPA films were investigated performing force-distan ce measurements by using tips modified with surface carboxyl groups. The cu rves obtained on pure lipid films showed repulsive forces well described in terms of double layer forces. Repulsive forces were not observed in the pr esence of a uniform MBP overlayer indicating that MBP neutralised most of t he lipid charge. (C) 2000 Elsevier Science B.V. All rights reserved.