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Interaction of the 18.5-kD isoform of myelin basic protein with Ca²⁺-calmodulin: Effects of deimination assessed by intrinsic Trp fluorescence spectroscopy, dynamic light scattering, and circular dichroism

¹ Department of Molecular Biology and Genetics, and Biophysics Interdepartmental Group, and
² Department of Physics, and Biophysics Interdepartmental Group, University of Guelph, Guelph, Ontario N1G 2W1, Canada
³ Photon Technology International, London, Ontario N6E 2S8, Canada

Reprint requests to: George Harauz, Department of Molecular Biology and Genetics, University of Guelph, 50 Stone Road East, Guelph, Ontario N1G 2W1, Canada; e-mail: gharauz{at}uoguelph.ca; fax: (519) 837-2075.

The effects of deimination (conversion of arginyl to citrullinyl residues) of myelin basic protein (MBP) on its binding to calmodulin (CaM) have been examined. Four species of MBP were investigated: unmodified recombinant murine MBP (rmMBP-Cit₀), an engineered protein with six quasi-citrullinyl (i.e., glutaminyl) residues per molecule (rmMBP-qCit₆), human component C1 (hMBP-Cit₀), and human component C8 (hMBP-Cit₆), both obtained from a patient with multiple sclerosis (MS). Both rmMBP-Cit₀ and hMBP-Cit₀ bound CaM in a Ca²⁺-dependent manner and primarily in a 1:1 stoichiometry, which was verified by dynamic light scattering. Circular dichroic spectroscopy was unable to detect any changes in secondary structure in MBP upon CaM-binding. Inherent Trp fluorescence spectroscopy and a single-site binding model were used to determine the dissociation constants: K_d = 144 ± 76 nM for rmMBP-Cit₀, and K_d = 42 ± 15 nM for hMBP-Cit₀. For rmMBP-qCit₆ and hMBP-Cit₆, the changes in fluorescence were suggestive of a two-site interaction, although the dissociation constants could not be accurately determined. These results can be explained by a local conformational change induced in MBP by deimination, exposing a second binding site with a weaker association with CaM, or by the existence of several conformers of deiminated MBP. Titration with the collisional quencher acrylamide, and steady-state and lifetime measurements of the fluorescence at 340 nm, showed both dynamic and static components to the quenching, and differences between the unmodified and deiminated proteins that were also consistent with a local conformational change due to deimination.

Keywords: Myelin basic protein; calmodulin; multiple sclerosis; deimination; citrulline; intrinsic Trp fluorescence; fluorescence lifetime; dynamic light scattering; circular dichroism

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