Effect Of Myelin Basic Protein On Membrane Lipid Monolayers Composed Of Lipids Molecules

Biomembrane is a supramolecular structure composed of phospholipids, protein, and polysaccharide and so on, which is essential components for cellular activities. At present, many clinical diseases are closely related with biomembrane, such as cancer. Myelin basic protein (MBP) is one of the main proteins in central nervous system myelin, it is believed to play an important role in the structure and function of myelin. In recent years, research data show that MBP is concerned with experimental allergic encephalomyelitis (EAE), multiple sclerosis (MS) and many other neurological diseases. MBP also has many features typical of soluble proteins, and has strong interacted with cell membrane lipid molecules, which caused the conformational change of protein. Therefore, it is of the utmost importance to understand the essence of the interactions that maintain the structural integrity of myelin sheath. In this article we have investigated the physical effect of different concentration of MBP on neutral lipids(DPPC、DPPE), negatively charged lipids(DPPS.DPPG) and unsaturated lipid(DOPC) monolayers by using Langmuir-Blodgett (LB) and atomic force microscope(AFM) techniques.1. The interaction between myelin basic protein (MBP) and neutral lipid molecules dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylethanolamine (DPPE). By analyzing the Langmuir monolayer surface pressure-mean molecular area (π-A) curves, the interaction between MBP and DPPC and DPPE, which are the different head group of lipid molecules, has been systematically studied, respectively. The results showed that:(1) With the MBP concentration in subphase increasing, the isotherms of DPPC and DPPE monolayer moved to lager average molecular area direction when the number of the lipid on the interface was constant; With the amount of lipid content increasing, the isotherms of monolayer also shifted to larger average molecular area when the content of protein remained the same, and the more the content of phospholipid molecules, the smaller variation was in isotherm area. (2) When the surface pressure of the monolayer reached to 10mN/m, one MBP molecule could combined with(127±3)DPPC and (93±3) DPPE molecules, respectively. As the surface pressure increased, the number of proteins interacted with two kinds of phospholipid molecules inserting the interface decreased. (3) The morphology of DPPC, DPPE on different concentrations of MBP monolayer was observed by atomic force microscopy (AFM). (4) When the surface pressure of phospholipid monolayer was 10mN/m, the π-T curves indicated that the presence of protein could make DPPC monolayer surface pressure decreased, and the higher the concentration of protein was, the more for decreasing values of the surface pressure. Then the DPPC was brought into the MBP subphase. (5) For DPPE monolayer, the protein combined with DPPE inserted the membrane interface, causing the increase of surface pressure, and the higher the protein concentration was, the greater change the pressure appeared. The results showed that phospholipid head groups played a key role in the interaction between protein molecules and lipid. This study has great significance in biology for understanding the structure and function of myelin in the central nervous system.2. The interaction between MBP and negatively charged cell membrane lipids diapalmitoylphosphatidylserine (DPPS) and dipalmitoylphosphatidylglycerol (DPPG). The effects of MBP with different concentrations on membrane lipid molecules DPPS and DPPG at air/water monolayer were also investigated through analyzing the data of surface pressure-area (π-A) isotherms. The experimental results showed that:great changes have taken place in DPPS、DPPG monolayer isotherms and molecular arrangement at the certain surface pressure on different concentrations of MBP. With the increasing concentration of MBP, the π-A isotherms of monolayers shifted to the direction of larger molecules. On the base of theory, we have calculated the mixing ratio of the interaction between protein and lipid, the area and quantity of protein molecules inserted in the lipid film. At the surface pressure of 10mN/m, a MBP molecule could interact with 58±4 DPPS molecules and 37±6 DPPG molecules. Using atomic force microscopy for DPPS, DPPG monolayer surface morphology was observed.It was found that, with the protein concentration increasing, lipid molecules in form of monolayer conformational change, and MBP molecules is easy to be inserted into the smaller limit area DPPG monolayer.3. The interaction between MBP and unsaturated lipid 1,2-dioleoy-sn-glycero-3-phosphocholine(DOPC) were studied. Thermodynamic behavior of mixed systems of MBP of different concentrations with DOPC has been investigated by analyzing the data of equilibrium surface pressure-area (π-A) isotherms. The results show that the addition of the DOPC into subphase containing different concentrations of MBP made great changes in condensation of membrane and the arrangement of molecule. Moreover, for a constant pressure, as the concentration increasing, the MBP molecular area in the lipid film is also increased,π-A curves changed regularly. On the basis of π-A isotherms, the mixing ratio could be calculated by the interaction between protein and lipid, and the area of protein molecules inserted in the lipid film. Ultimately, at the surface pressure of 10 mN/m, there is 1 MBP molecule for every 76±3 DOPC molecules. The morphology of MBP adsorbed on lipid films was studied by atomic force microscopy.