Now, what about step number two? Step number two is the binding of that calcium to a special protein found on our actin filament, on the thin filament known as troponin. So we have this protein that is shown by these three globular pink regions. This is troponin. A tropomycin basically is this other blue protein that extends all the way around our actin filament. And our troponin is actually bound to our tropomycin.