Conformational choreography of myelin basic protein by solution NMR spectroscopy
Structural studies of proteins have long been guided by the 'structure defines function' paradigm of structural biology. There is now an overwhelming body of evidence that a protein's function is not dependent on a predetermined fold and that in fact intrinsic disorder is prevalent in nature and highly advantageous for a protein's function. Intrinsically disordered proteins (IDPs) are conformationally pliable and thus highly dependent on their local environment and various interactions for structural stability. In this context, this thesis is an investigation of the structure and function(s) of myelin basic protein (MBP) using solution-based nuclear magnetic resonance (NMR) spectroscopy. A rich repertoire of functions has been described for MBP including membrane adhesion, and as a hub connecting the oligodendrocyte membrane to the cytoskeleton. Specific interactions with calmodulin and SH3-domain proteins suggest a further multifunctionality. Structural studies of MBP are necessary to define its precise role(s) 'in vivo'. The structural behaviour of MBP in a variety of solvents, as a function of concentration, temperature and pH, was investigated. In these studies, the structure of a peptide representing an immunodominant epitope (in multiple sclerosis) was solved. The fragment formed an amphipathic [alpha]-helix in membrane mimetic conditions but was only partially helical in aqueous solution. A primary SH3 binding site in MBP was demonstrated to form a polyproline type II helix and phosphorylation affected the structure and membrane associations of this segment. Together, these studies highlighted the conformational plasticity of MBP. These investigations were extended to incorporate full-length MBP and the resonance assignments were completed in two solution conditions: aqueous (100 mM KC1) and membrane-mimetic (30% TFE-d2). The conformational dependence on environment of MBP was characterized with an analysis of secondary structure by chemical shift indexing and 15N spin relaxation measurements. Collectively, the data revealed three major segments of MBP with a propensity towards [alpha]-helicity that were stabilized by membrane-mimetic conditions and corresponded with bioinformatics predictions of secondary structure. Finally, chemical shift perturbation experiments demonstrated a dramatic conformational change in MBP upon association with calmodulin, which were consistent with the C-terminal segment of MBP being the primary binding site for calmodulin.