Interaction of Myelin Basic Protein with Phospholipid Monolayers: Mechanism of Protein Penetration

The myelin basic protein (MBP) is the second most abundant protein in the myelin sheath of the central nervous system and is believed to be important for the compactness and integrity of the membrane. We investigated the mechanism of the interaction of lipid-free MBP with phospholipid monolayers at the air/water interface; in particular, we studied the process of MBP adsorption onto monolayers made up either of neutral dipalmitoylphosphatidylcholine (DPPC) or of negatively charged dipalmitoylphosphatidylserine (DPPS) monolayers. They are natural constituents of the myelin membrane, and sharing an identical hydrophobic chain, they differ only in headgroup composition. The MBP-lipid interaction is investigated for the first time by means of nullellipsometric measurements, monitoring in real time the effect of adsorbed molecules in the insoluble monolayer at different monolayer conditions, such as surface pressure and molecular area. The different behavior of monolayer thickness and surface pressure confirmed the hypothesis of a different interaction mechanism of MBP with the two kind of lipids. While in the presence of neutral DPPC the protein seems to penetrate among the lipid domains, in the case of negatively charged DPPS the electrostatic interaction appears to be the driving force, because protein intimately associates with the headgroups and binds to the Langmuir layer as a specific lipid-protein complex. Results with DPPS were confirmed by FTIR spectroscopy measurements, performed after transferring phospholipid multilayers onto a solid substrate by the Langmuir-Schaefer method.