The regulatory protein (s)' involved in masking and unmasking the active sites of actin are

Options

A. Troponim
B. Tropomyosin
C. Both 1 & 2
D. Actin and myosin

Answer

### Step-by-Step Solution

1. **Identify the Proteins Involved**: The question asks about the regulatory proteins that mask and unmask the active sites of actin. The main proteins involved in this process are Troponin and Tropomyosin.

2. **Understand the Structure of Actin Filaments**: Actin filaments consist of two strands of F-actin (filamentous actin) that are helically wound around each other. Alongside these, Tropomyosin runs parallel to the F-actin strands.

3. **Role of Tropomyosin**: Tropomyosin is a regulatory protein that covers the active sites on the actin filaments. This prevents myosin from binding to actin when the muscle is relaxed.

4. **Role of Troponin**: Troponin is another regulatory protein that is attached to Tropomyosin. It plays a crucial role in the contraction mechanism by masking the active sites on actin.

5. **Mechanism of Unmasking**: When calcium ions are released into the muscle cell, they bind to Troponin. This binding causes a conformational change that moves Tropomyosin away from the active sites on actin, allowing myosin to bind and initiate muscle contraction.

6. **Conclusion**: Therefore, the regulatory proteins involved in masking and unmasking the active sites of actin are both Troponin and Tropomyosin.

### Final Answer
The regulatory proteins involved in masking and unmasking the active sites of actin are Troponin and Tropomyosin.