X‐ray diffraction analysis of myelin lipid/proteolipid protein multilayers

Abstract

To examine the proposal that myelin proteolipid protein underlies the adhesion of neighboring membranes in central nervous system (CNS) myelin, we carried out X-ray diffraction studies on the structure and interactions of model bilayers composed of total myelin lipids plus proteolipid apoprotein (PLP). Total myelin lipids were used because their heterogeneity was expected to provide an appropriate environment for the integral membrane protein to achieve its native conformation and establish appropriate contacts with the apposed bilayer. We found that incorporation of PLP into myelin lipid bilayers, whether organized into multilamellar vesicles or oriented multilayers, did not appreciably affect the lamellar period, which ranged from 65-71 A. In oriented multilayers, the wide-angle spacing at approximately 4.8 A, which arises from the lateral packing of lipid chains and is perpendicular to the lamellar diffraction, was less oriented and weaker in bilayers containing the protein. These results indicate that PLP was incorporated into the bilayers and had a disordering effect on the hydrocarbon chains but did not extend into the spaces between bilayers. Bilayer profiles calculated from the lamellar diffraction to about 15 A spacing did not show any major changes in the distribution of electron density, suggesting that to moderate resolution, the protein was distributed uniformly across the width of the lipid bilayer. Periodicities measured from osmotically stressed multilamellar vesicles did not depend on the presence of PLP, indicating that the protein did not form stabilizing contacts between bilayers.(ABSTRACT TRUNCATED AT 250 WORDS)

Multilamellar packing of myelin modeled by lipid-bound MBP

“…The relative intensities I(h) (where h is the Bragg order) of the reflections was I(1) ӷ I(2) ӷ I(4) ӷ I(3) Х I(5). This intensity distribution is similar to that recorded from centrosymmetric bilayers of myelin lipid (Franks et al, 1982;Brown et al, 1989) but not to that recorded from CNS myelin, which consists of membranes that are paired (Kirschner and Caspar, 1972), and which retains its 150 -160 Å periodicity after the membranes are isolated (Karthigasan and Kirschner, 1988). The width of a single membrane from CNS myelin is 75-80 Å, and is thus in the same range as the membranes formed by lipid-bound MBP.…”

“…Larger periods of ϳ100 Å result, however, when the protein is assembled as oriented multibilayers with sulfatide plus cholesterol, and the arrays are equilibrated against water vapor (MacNaughtan et al, 1985). By comparison, for bovine CNS myelin lipid multibilayers equilibrated against water vapor, the period is ϳ60 Å (Franks et al, 1982), and for lipid extract from bovine CNS white matter (Brown et al, 1989) or for sulfatide/40 mol% cholesterol (MacNaughtan et al, 1985), the period of oriented multibilayers equilibrated against water vapor similarly is ϳ70 Å, whereas for multilamellar vesicles in excess water the period is ϳ100 Å. For oriented multibilayers of egg phosphatidylcholine and 40 mol% cholesterol, which is similar to the polar lipid/ cholesterol ratio in myelin, the period ranges from about 50 to 60 Å, depending on water content (Franks, 1976).…”

Preparation of a protein‐free total brain white matter lipid fraction: Characterization of liposomes

“…occurring within its lipid matrix (Carmona et al, 1987;Brown et al, 1989). By contrast, the functional role of myelin proteins remains unclear yet, even when several enzymes have been reported to occur in this membrane, some of them closely related to ionic transport processes (Norton and Cammer, 1984).…”

“…During the last few years model membranes, in particular liposomes, have been shown to be a powerful tool for studying protein-lipid interactions (Brown et al, 1989), as well as the participation of membrane proteins in the regulation of ion levels by means of reconstitution experiments (C6zar et al, 1987;Jones et al, 1988). For some membrane proteins, such as ion channels or transport proteins, function can only be meaningfully defined and assayed in a reconstituted membrane system.…”

Solubilization of Myelin Membranes by Detergents

“…Such effects may not be readily apparent or easily measurable in nonenzymatic proteins, like those of myelin, but the awareness of this possibility continues to stimulate the search for solvents sufficiently mild as to permit their isolation with a minimum impact on their structure. This is, however, still a matter of concern for studies of myelin protein function, and it is expected that future experiments using model systems may utilize PLP and other myelin proteins that have been extracted and purified using nondenaturing aqueous detergent solutions (Brown et al, 1989).…”