Structural context of homomeric interactions in the Ig domain of the MPZ (P0) myelin adhesion protein and relation to Charcot-Marie-Tooth disease phenotype variants
Mutations in Myelin Protein Zero (MPZ) account for 5% of Charcot-Marie-Tooth cases and can cause demyelinating or axonal phenotypes, reflecting the diverse roles of MPZ in Schwann cells. MPZ holds the apposing membranes of the myelin sheath together, with the adhesion role fulfilled by the extracellular lmmunoglobulin-like domain (lgMPZ), which can oligomerize. Current knowledge for how the lgMPZ might form oligomeric assemblies involving 3 weakly-interacting interfaces has been extrapolated from a protein crystal structure in which individual rat lgMPZ subunits are packed together under artificial conditions. These interfaces include one that organizes the lgMPZ into tetramers, a dimer interface that could link tetramers together, and a third hydrophobic interface that could mediate binding to lipid bilayers or the same hydrophobic surface on another lgMPZ domain. There are at present no data confirming whether the proposed lgMPZ interfaces actually mediate oligomerization in solution, whether they are required for the adhesion activity of MPZ, whether they are important for myelination, or whether their loss results in disease. We performed NMR and SAXS analysis of wild-type lgMPZ as well as mutant forms with amino-acid substitutions designed to interrupt its presumptive oligomerization interfaces. Here, we confirm the interface that mediates lgMPZ tetramerization, but find that dimerization is mediated by a distinct interface that has yet to be identified. We next correlated CMT phenotypes to subregions within lgMPZ tetramers. Axonal late-onset disease phenotypes (CMT2l/J) map to surface residues of lgMPZ proximal to the transmembrane domain. Early-onset demyelinating disease phenotypes (CMT1B/Dejerine-Sottas syndrome) map to two groups: one is described by variants that disrupt the stability of the lg-fold itself and are largely located within the core of the lg domain; whereas another describes a surface on the distal outer surface of lgMPZ tetramers. Computational docking studies predict that this latter disease-relevant subregion may mediate dimerization of lgMPZ tetramers.