An analysis of the regions of the myelin basic protein that bind to phosphatidylcholine

Interaction of Myelin Basic Protein with Myelin-like Lipid Monolayers at Air-Water Interface

Interaction of myelin basic protein (MBP) and the cytoplasmic leaflets of the oligodendrocyte membrane is essential for the formation and compaction of the myelin sheath of the central nervous system and is altered aberrantly and implicated in the pathogenesis of neurodegenerative diseases like multiple sclerosis. To gain more detailed insights into this interaction, the adsorption of MBP to model lipid monolayers of similar composition to the myelin of the central nervous system was studied at the air-water interface with monolayer adsorption experiments. Measuring the surface pressure and the related maximum insertion pressure of MBP for different myelin-like lipid monolayers provided information about the specific role of each of the single lipids in the myelin. Depending on the ratio of negatively charged lipids to uncharged lipids and the distance between charges, the adsorption process was found to be determined by two counteracting effects: (i) protein incorporation, resulting in an increasing surface pressure and (ii) lipid condensation due to electrostatic interaction between the positively charged protein and negatively charged lipids, resulting in a decreasing surface pressure. Although electrostatic interactions led to high insertion pressures, the associated lipid condensation lowered the fluidity of the myelin-like monolayer.

Membrane Association Landscape of Myelin Basic Protein Portrays Formation of the Myelin Major Dense Line

Compact myelin comprises most of the dry weight of myelin, and its insulative nature is the basis for saltatory conduction of nerve impulses. The major dense line (MDL) is a 3-nm compartment between two cytoplasmic leaflets of stacked myelin membranes, mostly occupied by a myelin basic protein (MBP) phase. MBP is an abundant myelin protein involved in demyelinating diseases, such as multiple sclerosis. The association of MBP with lipid membranes has been studied for decades, but the MBP-driven formation of the MDL remains elusive at the biomolecular level. We employed complementary biophysical methods, including atomic force microscopy, cryo-electron microscopy, and neutron scattering, to investigate the formation of membrane stacks all the way from MBP binding onto a single membrane leaflet to the organisation of a stable MDL. Our results support the formation of an amorphous protein phase of MBP between two membrane bilayers and provide a molecular model for MDL formation during myelination, which is of importance when understanding myelin assembly and demyelinating conditions.

Terminal deletion mutants of myelin basic protein: new insights into self-association and phospholipid interactions

The 18.5kDa isoform of myelin basic protein (MBP) has strong and probably specific interactions with phosphoinositides that are of interest regarding this protein's function, and in effecting its two-dimensional crystallization for structural determination. We have designed and constructed truncation mutants of recombinant 18.5kDa murine myelin basic protein (rmMBP) lacking either the N- or C-terminal third, i.e. rmMBPDeltaN and rmMBPDeltaC, respectively. Both variants rmMBPDeltaC and rmMBPDeltaN generally had a reduced ability to aggregate lipid vesicles, compared to the whole protein, especially at lower protein/lipid ratios. Lipid vesicle cosedimentation showed that both truncated variants exhibited altered binding with phosphatidylinositol (PI). Incubation of these proteins under monolayers comprising PI and a nickel-chelating lipid yielded crystalline arrays of rmMBPDeltaC (but not rmMBPDeltaN) in the absence of high salt or osmolytes, which are required for crystallization of whole protein. This result suggests that the C-terminal segment of MBP is a significant source of conformational heterogeneity, and its removal will facilitate future planar or three-dimensional crystallization attempts. Incubation of rmMBPDeltaN and rmMBPDeltaC under monolayers comprising phosphatidylinositol-4-phosphate and a nickel-chelating lipid yielded tubular structures of opposite chirality, suggesting a synergistic effect of both termini of MBP in organizing myelin lipids.

Myelin basic protein component C1 in increasing concentrations can elicit fusion, aggregation, and fragmentation of myelin-like membranes

Myelin basic protein (MBP) is considered to have a primary role in the formation and maintenance of the myelin sheath. Many studies using artificial vesicle systems of simple lipid composition, and generally small size, have shown that MBP can elicit vesicle fusion, aggregation, or even fragmentation under different conditions. Here, we have studied the effects of increasing concentrations of bovine MBP charge isomer C1 (MBP/C1) on large unilamellar vesicles (LUVs) composed of phosphatidylcholine and phosphatidylserine (92:8 molar ratio), or with a lipid composition similar to that of the myelin membrane in vivo (Cyt-LUVs). Using absorbance spectrophotometry, fluorescence resonance energy transfer, dynamic light scattering and transmission electron microscopy, we have shown that vesicle aggregation and some vesicle fusion occurred upon addition of MBP/C1, and as the molar protein-lipid ratio increased. Fragmentation of Cyt-LUVs was observed at very high protein concentrations. These results showed that the phenomena of vesicle fusion, aggregation, and fragmentation can all be observed in one in vitro system, but were dependent on lipid composition and on the relative proportions of protein and lipid.

Analysis of the membrane-interacting domains of myelin basic protein by hydrophobic photolabeling

Myelin basic protein is a water soluble membrane protein which interacts with acidic lipids through some type of hydrophobic interaction in addition to electrostatic interactions. Here we show that it can be labeled from within the lipid bilayer when bound to acidic lipids with the hydrophobic photolabel 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine (TID) and by two lipid photolabels. The latter included one with the reactive group near the apolar/polar interface and one with the reactive group linked to an acyl chain to position it deeper in the bilayer. The regions of the protein which interact hydrophobically with lipid to the greatest extent were determined by cleaving the TID-labeled myelin basic protein (MBP) with cathepsin D into peptides 1-43, 44-89, and 90-170. All three peptides from lipid-bound protein were labeled much more than peptides from the protein labeled in solution. However, the peptide labeling pattern was similar for both environments. The two peptides in the N-terminal half were labeled similarly and about twice as much as the C-terminal peptide indicating that the N-terminal half interacts hydrophobically with lipid more than the C-terminal half. MBP can be modified post-translationally in vivo, including by deamidation, which may alter its interactions with lipid. However, deamidation had no effect on the TID labeling of MBP or on the labeling pattern of the cathepsin D peptides. The site of deamidation has been reported to be in the C-terminal half, and its lack of effect on hydrophobic interactions of MBP with lipid are consistent with the conclusion that the N-terminal half interacts hydrophobically more than the C-terminal half. Since other studies of the interaction of isolated N-terminal and C-terminal peptides with lipid also indicate that the N-terminal half interacts hydrophobically with lipid more than the C-terminal half, these results from photolabeling of the intact protein suggest that the N-terminal half of the intact protein interacts with lipid in a similar way as the isolated peptide. The similar behavior of the intact protein to that of its isolated peptides suggests that when the purified protein binds to acidic lipids, it is in a conformation which allows both halves of the protein to interact independently with the lipid bilayer. That is, it does not form a hydrophobic domain made up from different parts of the protein.

The structure and function of central nervous system myelin

Multiple sclerosis (MS) is characterized by the active degradation of central nervous system myelin, a multilamellar membrane system that insulates nerve axons. MS arises from complex interactions between genetic, immunological, infective, and biochemical mechanisms. Although the circumstances of MS etiology remain hypothetical, one persistent theme involves immune system recognition of myelin-specific antigens derived from myelin basic protein, the most abundant extrinsic myelin membrane protein, and/or another equally suitable myelin protein or lipid. Knowledge of the biochemical and physical-chemical properties of myelin proteins, and lipids, particularly their composition, organization, structure, and accessibility with respect to the compacted myelin multilayers, thus becomes central to understanding how and why these antigens become selected during the development of MS. This article focuses on the current understanding of the molecular basis of MS as it may relate to the protein and lipid components of myelin, which dictate myelin morphology on the basis of protein-lipid and lipid-lipid interactions, and the relationship, if any, between the protein/lipid components and the destruction of myelin in pathological situations.

Interaction of phospholipids with proteins and peptides. New advances 1990

1. The review deals with the recent achievements in the study of the various interactions of phospholipids with proteins and peptides. 2. The interactions are classified according to the hydrophobic, hydrophilic or mixed character of the interactive forces. 3. The effect of the interaction on the structure and biological activity of the interacting molecules is also discussed.