Final answer:
In a resting state, tropomyosin covers the actin subunit binding site, preventing myosin binding and muscle contraction until calcium ions bind to troponin, causing conformational changes that expose the binding sites.
Step-by-step explanation:
In a resting state, tropomyosin covers the actin subunit binding site. Tropomyosin is a fibrous protein that binds along the actin filament, effectively blocking the myosin binding sites on the actin subunit. This prevents muscle contraction until the muscle receives a signal to contract. At this point, the troponin complex, which includes troponin T (tropomyosin-binding troponin) and troponin I (inhibitory troponin), maintains the position of tropomyosin on the actin subunit. Upon the binding of calcium ions (Ca++) to troponin C, a conformational change occurs, which in turn moves the tropomyosin out of the way, thereby exposing the binding sites to allow the myosin heads to attach and initiate muscle contraction.