Homomeric interactions of the MPZ Ig domain and their relation to Charcot-Marie-Tooth disease

Mutations in MPZ (myelin protein zero) can cause demyelinating early-onset Charcot-Marie-Tooth type 1B disease or later onset type 2I/J disease characterized by axonal degeneration, reflecting the diverse roles of MPZ in Schwann cells. MPZ holds apposing membranes of the myelin sheath together, with the adhesion role fulfilled by its extracellular immunoglobulin-like domain (Ig(MPZ)), which oligomerizes.Models for how the Ig(MPZ) might form oligomeric assemblies has been extrapolated from a protein crystal structure in which individual rat Ig(MPZ) subunits are packed together under artificial conditions, forming three weak interfaces. One interface organizes the Ig(MPZ) into tetramers, a second 'dimer' interface links tetramers together across the intraperiod line, and a third hydrophobic interface that mediates binding to lipid bilayers or the same hydrophobic surface on another Ig(MPZ) domain. Presently, there are no data confirming whether the proposed Ig(MPZ) 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 nuclear magnetic resonance spectroscopy and small angle X-ray scattering analysis of wild-type Ig(MPZ) as well as mutant forms with amino acid substitutions designed to interrupt its presumptive oligomerization interfaces. Here, we confirm the interface that mediates Ig(MPZ) tetramerization, but find that dimerization is mediated by a distinct interface that has yet to be identified. We next correlated different types of Charcot-Marie-Tooth disease symptoms to subregions within Ig(MPZ) tetramers. Variants causing axonal late-onset disease (CMT2I/J) map to surface residues of Ig(MPZ) proximal to the transmembrane domain. Variants causing early-onset demyelinating disease (CMT1B) segregate into two groups: one is described by variants that disrupt the stability of the Ig-fold itself and are largely located within the core of the Ig(MPZ) domain; whereas another describes a region on the surface of Ig(MPZ) tetramers, accessible to protein interactions. Computational docking studies predict that this latter disease-relevant subregion may potentially mediate dimerization of Ig(MPZ) tetramers.