Which one of the following is present intracellularly in muscle cells?
**Core Concept**
Troponin and tropomyosin are key regulatory proteins in the contractile apparatus of skeletal and cardiac muscle cells. Intracellular calcium regulates their movement, allowing muscle contraction to occur.
**Why the Correct Answer is Right**
Troponin is a complex of three subunits (troponin C, troponin I, and troponin T) that binds to tropomyosin in the muscle thin filament. Troponin C binds to calcium ions (Ca2+), causing a conformational change that shifts tropomyosin away from the myosin binding sites on the actin filaments. This allows muscle contraction to occur. Troponin is present in the cytosol of muscle cells and is integral to the regulation of muscle contraction.
**Why Each Wrong Option is Incorrect**
**Option A:** Actin filaments are present in the cytoskeleton of muscle cells but are not the correct answer, as the question asks for a regulatory protein.
**Option B:** Myosin is a motor protein involved in muscle contraction but is not the correct answer, as it is not present intracellularly in its regulatory form.
**Option C:** Titin is a giant protein that provides elasticity to muscle cells but is not the correct answer, as it is not directly involved in the regulation of muscle contraction.
**Clinical Pearl / High-Yield Fact**
Troponin is released into the bloodstream when cardiac muscle is damaged, making it a sensitive and specific biomarker for myocardial infarction (MI). Elevated troponin levels are diagnostic of MI, even in the absence of significant ECG changes.
**Correct Answer: C. Troponin**