In resting stage, binding sites for myosin on actin filaments are masked by

Options

A. Troponin
B. Light meromyosin
C. Heavy meromyosin
D. Calcium ions

Answer

To solve the question "In resting stage, binding sites for myosin on actin filaments are masked by," we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Muscle Structure**:
- Skeletal muscle fibers consist of two types of filaments: thick filaments (myosin) and thin filaments (actin).

2. **Role of Myosin and Actin**:
- Myosin is the thick filament, while actin is the thin filament. The contraction of muscle fibers occurs through the sliding of these filaments over each other.

3. **Components of Actin Filaments**:
- Actin filaments are primarily composed of two strands of filamentous actin (F-actin) that are helically wound together. These strands are made up of globular actin (G-actin) monomers.

4. **Presence of Tropomyosin and Troponin**:
- Alongside F-actin, tropomyosin runs along the length of the actin filament. Troponin is a complex of proteins that interacts with tropomyosin and actin.

5. **Resting State of Muscle**:
- In the resting state, the binding sites for myosin on the actin filaments are not exposed. This is because a subunit of troponin binds to tropomyosin, which in turn covers the active sites on actin, preventing myosin from binding.

6. **Identifying the Correct Answer**:
- The question asks what masks the binding sites for myosin on actin filaments during the resting stage. Based on the information, it is clear that troponin is responsible for masking these binding sites.

7. **Final Answer**:
- Therefore, the correct answer is **A. Troponin**.