Two-state model of the thin filament based on Scheme 2 of Heeley et al. (2019) . In this model, the thin filament has two activity states, I and A. The linkage between structural transitions of thin filament complex and activity states are not detailed by Heeley et al. (2019). To avoid any assumptions about the structural transitions, the two activity states are shown as black and gray, respectively. The thin filament is predominantly in the I form in the absence of both calcium and myosin. The binding of either calcium (Ca2+) or a single strongly bound M will bias the system toward the A state, but neither is sufficient on its own to switch the system totally to the A form. See Table 2 for the fraction of the system on under different conditions.
