Deimination of myelin basic protein. 1. Effect of deimination of arginyl residues of myelin basic protein on its structure and susceptibility to digestion by cathepsin D

The effect of deimination of arginyl residues in bovine myelin basic protei n (MBP) on its susceptibility to digestion by cathepsin D has been studied. Using bovine component 1 (C-1) of MBP, the most unmodified of the componen ts with all 18 arginyl residues intact, we have generated a number of citru llinated forms by treatment of the protein with purified peptidylarginine d eiminase (PAD) in vitro. We obtained species containing 0-9.9 mol of citrul line/mol of MBP. These various species were digested with cathepsin D, a me talloproteinase which cleaves proteins at Phe-Phe linkages. The rate of dig estion compared to component 1 was only slightly affected when 2.7 or 3.8 m ol of citrulline/mol of MBP was present. With 7.0 mol of citrulline/mol of MBP, a large increase in the rate of digestion occurred. No further increas e was observed with 9.9 mol of citrulline/mol of MBP. The immunodominant pe ptide 43-88 (bovine sequence) was released slowly when 2.7 and 3.8 mol of c itrulline/mol of MBP was present, but it was released rapidly when 7.0 mol of citrulline/mol of MBP was present. The dramatic change in digestion with 7.0 mol of citrulline/mol of MBP or more could be explained by a change in three-dimensional structure. Molecular dynamics simulation showed that inc reasing the number of citrullinyl residues above 7 mol/mol of MBP generated a more open structure, consistent with experimental observations in the li terature. We conclude that PAD, which deiminates arginyl residues in protei ns, decreases both the charge and compact structure of MBP. This structural change allows better access of the Phe-Phe linkages to cathepsin D. This s cheme represents an effective way of generating the immunodominant peptide which sensitizes T-cells for the autoimmune response in demyelinating disea se.