Structural bases for the Charcot-Marie-Tooth disease induced by single amino acid substitutions of myelin protein zero
Myelin protein zero (MPZ or P0) is a major transmembrane protein expressed in peripheral compact myelin and functions to glue membranes to form multiple layered membranes characteristic of myelin. Intermembrane adhesion is mediated by homophilic interactions between the extracellular domains (ECDs) of MPZ molecules. Single amino acid substitutions in an ECD cause demyelinating neuropathy, known as Charcot-Marie-Tooth disease (CMT); however, the mechanisms by which such substitutions induce the disease are not well understood. To address this issue, we constructed a novel assay to evaluate the membrane-stacking activity of ECD using ECD-immobilized nanodiscs. Using this novel "nanomyelin" system, we found that octameric (8-meric) ECDs with a stacked-rings-like configuration are responsible for membrane adhesion. Two inter-ECD interactions, cis and head-to-head, are essential to constituting the 8-mer and, consequently, to gluing the membranes. This result was further reinforced by the observation that the CMT-related N87H substitution at the cis interface abolished membrane-adhesion activity. In contrast, the CMT-related D32G and E68V variants of ECD retained membrane-stacking activity, whereas their thermal stability was reduced compared to that of the WT. Reduced thermal stability may lead to impairment of the long-term stability of ECD and the layered membranes of myelin.