Membrane adhesion in peripheral myelin: good and bad wraps with protein P0  Daniel A Kirschner, Hideyo Inouye, Raul A Saavedra  Structure  Volume 4, Issue 11, Pages 1239-1244 (November 1996) DOI: 10.1016/S0969-2126(96)00132-3

Figure 1 The complex structure of a myelinated fiber illustrates a variety of adhesive contacts. (a) Internodal compact myelin (rabbit) from the CNS (left) and PNS (right); the dense lines are the ‘major dense lines’ corresponding to the apposition of cytoplasmic surfaces, and the single (left) or double (right) lines that are lighter are the ‘intraperiod lines’, corresponding to the apposition of extracellular surfaces. Scale bar=500 å. (b) Node of Ranvier from cross-sectioned PNS fiber (mouse). The subaxolemmal density, the radially arranged nodal processes, and the basal lamina are visible; scale bar=5000 å. (c) Paranodal terminal loops with axoglial junctions from PNS (shark, left; mouse, right). The periodic, dense transverse bands (arrow heads), linking the myelin terminal loops with the axolemma are shown; scale bar=500 å. (d) Radial component junctions (arrow heads) of cross-sectioned CNS myelin (frog, left; mouse, middle); scale bar=500 å. Right, highly magnified image of these junctional units in human myelin, showing globular domains at cytoplasmic and extracellular appositions, and fine filamentous arms within the bilayer [2]. (e) Schmidt-Lanterman incisure from PNS myelin (lungfish) shows arrayed islands of cytoplasm within the compact internodal myelin; scale bar=500 å. (Figure reproduced from [3] with permission.) Structure 1996 4, 1239-1244DOI: (10.1016/S0969-2126(96)00132-3)

Figure 2 Ribbon diagram of P0-ED based on the crystallographic structure of Shapiro et al. [15]. The protein folding resembles that of an immunoglobulin variable domain. The C terminus, which is disordered in the crystal, extends further than shown by five residues. The β strands (lettered A–G) correspond closely to those (numbered 1–10) predicted by Wells et al. [12] (i.e. A, A′≈1; B≈2; C≈3; C′≈4; C″≈5; D≈6; E≈7; F≈8; G≈9,10). (Figure reproduced from [15] with permission.) Structure 1996 4, 1239-1244DOI: (10.1016/S0969-2126(96)00132-3)

Figure 3 Proposed orientations of P0-ED at the membrane surface. (a) Interpretation of the crystallographic data [15]. View of the proposed tetramer of P0-ED molecules shown as Cα backbones, with the fourfold axis vertical. The C termini (without their five terminal residues) are at the base of the molecule, near the membrane surface which is perpendicular to the fourfold axis. The Trp28 side chains at the apices of the molecules are hypothesized to intercalate with the apposed membrane surface. The long axis of the individual molecule (protomer) is tilted about 45° relative to the fourfold axis. (Reproduced from [15] with permission.) (b,c) P0-ED molecules, shown as space-filling models, projecting from apposed membranes, and oriented to account for the different spacings at the extracellular apposition in intact myelin [12]. The long axis of the molecule is vertical, perpendicular to the plane of the membrane surface represented by the horizontal lines. (b) Packing in native myelin showing lateral interactions between P0-ED on left (from upper membrane) and P0-ED on right (from lower membrane). (c) Packing of P0-EDs in partially swollen myelin, where the intermembrane separation is 90 å. The end-to-end homophilic interaction is characterized by both hydrophobic interactions, including Trp28, and electrostatic complementarity, involving Glu27 and Lys79. Transition between the swollen and native states may occur by reciprocal sliding of the P0-EDs from apposed membranes past one another. (Figure reproduced from [12] with permission.) Structure 1996 4, 1239-1244DOI: (10.1016/S0969-2126(96)00132-3)

Figure 4 Space-filling model of P0-ED. The domain is shown projecting from one half of the lipid bilayer. In this view, the carbohydrate, which is proposed to help stabilize this orientation of the molecule, is shown in green at the base of the protein adjacent to the membrane surface. The aromatic side chain of Trp28, in yellow, can be seen at the very apex of the molecule. Negatively and positively charged amino acid residues are indicated by blue and orange, respectively. (Figure reproduced from [26] with permission.) Structure 1996 4, 1239-1244DOI: (10.1016/S0969-2126(96)00132-3)