cells Review Flexible Players within the Sheaths: The Intrinsically Disordered Proteins of Myelin in Health and Disease Arne Raasakka 1 and Petri Kursula 1,2,* 1 Department of Biomedicine, University of Bergen, Jonas Lies vei 91, NO-5009 Bergen, Norway;
[email protected] 2 Faculty of Biochemistry and Molecular Medicine & Biocenter Oulu, University of Oulu, Aapistie 7A, FI-90220 Oulu, Finland * Correspondence:
[email protected] Received: 30 January 2020; Accepted: 16 February 2020; Published: 18 February 2020 Abstract: Myelin ensheathes selected axonal segments within the nervous system, resulting primarily in nerve impulse acceleration, as well as mechanical and trophic support for neurons. In the central and peripheral nervous systems, various proteins that contribute to the formation and stability of myelin are present, which also harbor pathophysiological roles in myelin disease. Many myelin proteins have common attributes, including small size, hydrophobic segments, multifunctionality, longevity, and regions of intrinsic disorder. With recent advances in protein biophysical characterization and bioinformatics, it has become evident that intrinsically disordered proteins (IDPs) are abundant in myelin, and their flexible nature enables multifunctionality. Here, we review known myelin IDPs, their conservation, molecular characteristics and functions, and their disease relevance, along with open questions and speculations. We place emphasis on classifying the molecular details of IDPs in myelin, and we correlate these with their various functions, including susceptibility to post-translational modifications, function in protein–protein and protein–membrane interactions, as well as their role as extended entropic chains. We discuss how myelin pathology can relate to IDPs and which molecular factors are potentially involved. Keywords: myelin; intrinsically disordered protein; multiple sclerosis; peripheral neuropathies; myelination; protein folding; protein–membrane interaction; protein–protein interaction 1.