Studies of the interaction of troponin I with proteins of the I-filament and calmodulin

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Abstract
All lysine residues in native troponin I from rabbit fast-twitch skeletal muscle reacted with methyl acetimidate and ethyl acetimidate. The reactivity of lysine-18 of troponin I to acetimidate was much diminished when the troponin I was complexed in the presence of Ca2+ with troponin C alone or in the whole troponin complex. In the presence of EGTA [ethyleneglycol-bis(.beta.-aminoethylether)-N,N,N'',N''-tetraacetic acid], lysine-18 of troponin I in the troponin I-troponin C complex was more reactive to acetimidate than it was in the presence of Ca2+. No masking of lysine residues could be detected when troponin I interacted with calmodulin or actin. Sedimentation-equilibrium studies indicated that the complex of troponin I with calmodulin was more readily dissociated in the absence of Ca2+ than was its complex with troponin C under otherwise identical conditions. The nature of the involvement of the N-terminal region of troponin I is a major difference between its modes of interaction with calmodulin and with troponin C.