The cytoplasmic tail of myelin protein zero induces morphological changes in lipid membranes
The major myelin protein expressed by the peripheral nervous system Schwann cells is protein zero (P0), representing 50% of the total protein content in myelin. This 30-kDa integral membrane protein consists of an immunoglobulin (Ig)-like domain, a transmembrane helix, and a 69-residue C-terminal cytoplasmic tail (P0ct). The basic residues in P0ct contribute to the tight packing of the myelin lipid bilayers, and alterations in the tail affect how P0 functions as an adhesion molecule necessary for the stability of compact myelin. Several neurodegenerative neuropathies are related to P0, including the more common Charcot-Marie-Tooth disease (CMT) and Dejerine-Sottas syndrome (DSS), but also rare cases of motor and sensory polyneuropathy. We find that high P0ct concentrations affect the membrane properties of bicelles and induce a lamellar-to-inverted hexagonal phase transition, which causes the bicelles to fuse into long, protein-containing filament-like structures. These structures likely reflect the formation of semi-crystalline lipid domains of potential relevance for myelination. Not only is P0ct important for stacking lipid membranes, but time-lapse fluorescence microscopy shows that it might affect membrane properties during myelination. We further describe recombinant production and low-resolution structural characterization of full-length human P0. Our findings shed light on P0ct effects on membrane properties, and with successful purification of full-length P0, we have new tools to study in vitro the role P0 has in myelin formation and maintenance.