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J. Biol. Chem., Vol. 261, Issue 2, 608-613, 01, 1986
Z Grabarek, PC Leavis and J Gergely
Residues 89-100 of troponin C (C89-100) and 96-116 of troponin I (I96- 116)
interact with each other in the troponin complex (Dalgarno, D.C., Grand,
R.J.A., Levine, B.A. Moir, A., J.G., Scott, G.M.M., and Perry, S.V. (1982)
FEBS Lett. 150, 54-58) and are necessary for the Ca2+ sensitivity of
actomyosin ATPase (Syska, H., Wilkinson, J.M., Grand, R.J.A., and Perry,
S.V. (1976) Biochem. J. 153, 375-387 and Grabarek, Z., Drabikowski, W.,
Leavis, P.C., Rosenfeld, S.S., and Gergely, J. (1981) J. Biol. Chem. 256,
13121-13127). We have studied Ca2+-induced changes in the region C89-100 by
monitoring the fluorescence of troponin C (TnC) labeled at Cys-98 with 5-
(iodoacetamidoethyl)aminonaphthalene-1-sulfonic acid. Equilibrium titration
of the labeled TnC with Ca2+ indicates that the probe is sensitive to
binding to both classes of sites in free TnC as well as in its complex with
TnI. When Mg2 X TnC is mixed with Ca2+ in a stopped flow apparatus, there
is a rapid fluorescence increase related to Ca2+ binding to the unoccupied
sites I and II followed by a slower increase (k = 9.9 s-1) that represents
Mg2+-Ca2+ exchange at sites III and IV. In the TnC X TnI complex, the fast
phase is much larger and the Mg2+- Ca2+ exchange at sites III and IV
results in a small decrease rather than an increase in the fluorescence of
the probe. The possibility is discussed that the fast change in the
environment of Cys-98 upon Ca2+ binding to sites I and II may be
instrumental in triggering activation of the thin filament by facilitating
a contact between C89-100 and I96- 116.
Calcium binding to the low affinity sites in troponin C induces conformational changes in the high affinity domain. A possible route of information transfer in activation of muscle contraction
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