Substitutions mimicking deimination and phosphorylation of 18.5-kDa myelin basic protein exert local structural effects that subtly influence its global folding

• Myelin basic protein (MBP) is a multifunctional intrinsically-disordered protein.
• MBP populates three energetically distinct and reversible equilibrium conformations.
• MBP's equilibrium conformational states are chimeras of globule and coil structures.
• Charge reduction (deimination) shifts distribution but not lateral packing density.
• Charge reduction (deimination) favors compaction of expanded conformational states.
• Threonyl phosphorylation has only local conformational effects.

Intrinsically-disordered proteins (IDPs) present a complex interplay of conformational variability and multifunctionality, modulated by environment and post-translational modifications. The 18.5-kDa myelin basic protein (MBP) is essential to the formation of the myelin sheath of the central nervous system and is exemplary in this regard. We have recently demonstrated that the unmodified MBP-C1 component undergoes co-operative global conformational changes in increasing concentrations of trifluoroethanol, emulating the decreasing dielectric environment that the protein encounters upon adsorption to the oligodendrocyte membrane [K.A. Vassall et al., Journal of Molecular Biology, 427, 1977–1992, 2015]. Here, we extended this study to the pseudo-deiminated MBP-C8 charge component, one found in greater proportion in developing myelin and in multiple sclerosis. A similar tri-conformational distribution as for MBP-C1 was observed with slight differences in Gibbs free energy. A more dramatic difference was observed by cathepsin D digestion of the protein in both aqueous and membrane environments, which showed significantly greater accessibility of the F42–F43 cut site of MBP-C8, indicative of a global conformational change. In contrast, this modification caused little change in the protein's density of packing on myelin-mimetic membranes as ascertained by double electron–electron resonance spectroscopy [D.R. Kattnig et al., Biochimica et Biophysica Acta (Biomembranes), 1818, 2636–2647, 2012], or in its affinity for Ca2 +-CaM. Site-specific threonyl pseudo-phosphorylation at residues T92 and/or T95 did not appreciably affect any of the thermodynamic mechanisms of conformational transitions, susceptibility to cathepsin D, or affinity for Ca2 +-CaM, despite previously having been shown to affect local structure and disposition on the membrane surface.

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