J. Biol. Chem., Vol. 263, Issue 30, 15485-15491, Oct, 1988
Calmodulin supports the force-generating function in desensitized muscle fibers
A Babu, G Orr and J Gulati
Albert Einstein College of Medicine, Bronx, New York 10461.
Externally added calmodulin (CaM) restored Ca2+ regulation for the tension
development by skeletal muscle fibers of hamster and rabbit desensitized by
the troponin C (TnC) extraction treatment. CaM produced this action by
combining with the TnC-denuded sites in the fiber. However, the binding
properties differed strikingly from TnC: unlike TnC, CaM binding required
the continued presence of Ca2+ and the bound portion was completely
released with EGTA in the physiological milieu. The maximal uptake was 1.7
g of CaM/kg of muscle in the present study. The apparent Ca2+ sensitivity
for force development with 200 micrograms/ml CaM in the solution was lower
than in the native fiber or in the TnC-loaded fiber. The apparent
association constant for CaM binding to the TnC-denuded sites was found as
4.9 x 10(5) M-1, and the extrapolated maximum force (Fmax) with CaM was
close to PO. The intrinsic CaM level in intact muscle was also measured and
was 18.6 mg/kg, amounting to about 1% of the total TnC or the CaM uptake by
TnC- denuded fibers. The intrinsic CaM was not dislodged by EDTA treatment,
indicating tight binding and suggesting that it exists in a separate pool
from the vacated TnC sites adsorbing externally added CaM. The stringent
Ca+ dependence of the CaM adsorption to TnC sites in the regulatory complex
in the fiber supports the view that the evolutionary replacement of
residues in the amino terminus helix portion of the "EF- hand" motif of
site IV may be critical for the functional specialization by TnC.
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