Flexibility of the Myelin Scaffolding Protein Periaxin

Abstract

Rapid nerve impulse conduction is necessary for a correctly functioning vertebrate nervous system. This is enabled by myelin, an insulating lipid-rich structure that encloses selected axonal segments. The insulation arises from compact myelin, which is composed of tightly stacked lipid bilayers devoid of cytosolic content. Non-compact myelin is richer in cytosol, has a role in myelin maintenance, and resides mostly under the inner- and outermost myelin membranes. In the peripheral nervous system, myelin is produced by Schwann cells, which in addition to their outermost abaxonal membrane, are surrounded by a carbohydrate-rich basal lamina that is connected to the abaxonal membrane via dystroglycans. These penetrate the membrane and connect to a network of intramyelin proteins, including periaxin (PRX) - a scaffolding molecule for other proteins within this network that links the abaxonal membrane to the cytoskeleton. Both PRX isoforms, S- and L-PRX, contain a folded 100-amino acid PDZ-like domain. Additionally, the longer isoform contains a ∼1300-residue tail, which - based on sequence analysis - can be divided into several distinct regions that are predicted to be disordered. This tail is a protein scaffolding unit, which is crucial for the correct morphology and stability of peripheral myelin, as point mutations in the tail region result in Charcot-Marie-Tooth disease (CMT) - an incurable demyelinating neuropathy. We characterized L-PRX using different biophysical methods, including X-ray crystallography, NMR spectroscopy, synchrotron-radiation circular dichroism (SRCD) spectroscopy, and small-angle X-ray scattering (SAXS). The crystal structure is an intertwined, domain-swapped PDZ-like homodimer with potential membrane interactions, as suggested by detergent titrations. Different parts of the tail are very flexible and functional in vitro. Quantitative studies regarding the affinities and structural implications of these interactions are underway. Our study will contribute to understanding the structure-function relationships of myelin proteins in health and disease.