In the resting state , which protein masks the active binding sites of actin filaments ?

Options

A. troponin
B. tropomyosin
C. Myosin
D. dynein

Answer

To solve the question regarding which protein masks the active binding sites of actin filaments in the resting state, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Context**:
- In muscle physiology, actin filaments play a crucial role in muscle contraction. However, in the resting state, certain proteins regulate the interaction between actin and myosin.

2. **Identifying the Proteins**:
- The question provides several options: Troponin, Tropomyosin, Myosin, and Dynein. We need to analyze each one to determine which one masks the active sites on actin.

3. **Analyzing Each Option**:
- **Troponin**: This protein is part of the troponin complex that binds to tropomyosin and actin. In the resting state, troponin, along with tropomyosin, prevents myosin from binding to actin by covering the active sites on actin filaments.
- **Tropomyosin**: This protein runs along the length of actin filaments and, in conjunction with troponin, covers the active binding sites on actin in the resting state.
- **Myosin**: This is a motor protein that interacts with actin during muscle contraction but does not mask the binding sites.
- **Dynein**: This is a motor protein involved in cellular transport along microtubules and is not related to actin binding sites.

4. **Conclusion**:
- The protein that primarily masks the active binding sites of actin filaments in the resting state is **Tropomyosin**. Troponin plays a regulatory role but does not directly mask the binding sites.

### Final Answer:
The correct answer is **Tropomyosin**.