Myelin P₀‐Glycoprotein: Predicted Structure and Interactions of Extracellular Domain

Protein zero (P 0 ), a transmembrane glycoprotein, accounts for over 50% of the total protein in PNS myelin. The extracellular domain of P 0 (P 0 ‐ED) is similar to the immunoglobulin variable domain, carrying one acceptor sequence for N‐linked glycosylation. The x‐ray diffraction analysis of PNS myelin has demonstrated reversible transitions that depend on pH and ionic strength, resulting in three distinct structures characterized by widths of about 36 Å, 50 Å (native), and 90 Å between the extracellular surfaces of the membranes. In the current work, we considered the constraints imposed by these x‐ray diffraction data on the orientation of P 0 ‐ED, and we propose how this immunoglobulin‐like domain could be accommodated in the variable widths of the extracellular space between myelin membranes. The modeling made use of the finding that β‐strand predictions for P 0 ‐ED are virtually superimposable with those of the V H domain of the phosphocholine‐binding immunoglobulin M603 of mouse, which has a similar number of residues as P 0 ‐ED and a structure that has been solved crystallographically. The dimensions of P 0 ‐ED from the space‐filling model, developed using PC‐ based molecular modeling software, were found to be 44 Å× 25 Å× 23 Å. On the assumption that neither the shape nor the orientation of P 0 ‐ED changes appreciably, then the different widths at the extracellular apposition would easily accommodate P 0 ‐ED from apposed membranes if the molecules were oriented so that the β‐ strands were approximately perpendicular to the membrane surface. The apposed P 0 ‐EDs would fully overlap at the closest apposition of the membranes, partially overlap in the native state, and align end to end in the incompletely swollen state. The P 0 ‐ED regions analogous to the complementarity‐determining regions of immunoglobulins can account for the recognition of P 0 ‐ED from apposed membranes in the incompletely swollen state. Two of the faces of P 0 ‐ED that show charge complementarity could account for the homophilic interactions of P 0 ‐ED from apposed membranes in the native state. This association can be stabilized further by hydrophobic interactions. The N‐ linked nonasaccharide after energy minimization fit into a cavity, which was at the base of P 0 ‐ED and which was lined with three positively charged residues. Thus, the carbohydrate may help to maintain the orientation of P 0 at the membrane surface. Our model shows how the single immunoglobulin‐like domain of P 0 can account for distinct structural states of myelin membrane packing by homophilic interactions.