The study demonstrates that the cooperative activation of actin-S1 ATPase correlates with the apparent cooperative unit size, driven by strong end-to-end interactions due to C-terminal sequence similarities in specific tropomyosin variants.
Tropomyosin (Tm) bound to actin induces cooperative activation of actomyosin subfragment 1 (actin-S1) ATPase, observed as a sigmoid ATPase vs S1 dependence. The activation is much steeper for gizzard muscle Tm (GTm) than for rabbit skeletal Tm (RSTm). To investigate if this greater cooperativity is due to increased communication between GTms along the thin filament, we studied effects of S1 binding on the state of actin-Tm using the fluorescence of pyrene-labeled Tm. Kinetic and equilibrium studies provided values for n, the apparent cooperative unit size Geeves, M. A., and Lehrer, S. S. (1994) Biophys. J. 67, 273. We report comparative studies of Tm-actin-S1 ATPase with values of n using GTm, RSTm, and 5aTm, a 1/7 shorter nonmuscle Tm from rat fibroblast cells Pittenger, M. F., et al. (1994) Curr. Opin. Cell Biol., 6, 96. 5aTm and GTm produce similar cooperative activation of actin-S1 ATPase and have similar n values that are 2-fold greater than RSTm, indicating a correlation between ATPase activation and n value. This appears to be due to the similarity of the C-terminal amino acid sequences of 5a and GTm which produce strong end-to-end interactions. The results are discussed in terms of a continuous flexible Tm strand on the actin filament.
Lehrer et al. (Sat,) studied this question.
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