You are a researcher who studies the molecular structure of the ryanodine recept

Question

You are a researcher who studies the molecular structure of the ryanodine receptor (RyR) proteins. You've discovered a mutation in the gene encoding RyR that is associated with early infant death. In order to study the mutation, you genetically engineer a laboratory mouse so that it expresses only this mutant form of the gene encoding the ryanodine receptor protein. The mutation interferes with the ability of the protein to open in response to calcium ion binding, but does not confer any other structural or functional changes. Write a paragraph describing what you would predict would happen to contraction of skeletal and cardiac muscle in the mouse, and explaining why the mutation could be fatal in infants.

Explanation / Answer

Skeletal muscle fibers are made up of proteins, majorly myosin and actin fibers. Myosin fibers are called thick filaments, and actin fibers are called thin filaments. Actin fibers contain myosin binding sites. Binding of myosin to the actin filament is the key for muscle contraction. In order to control the muscle contraction, myosin binding sites on the actin fibers are covered by two other proteins, namely troponin and tropomyosin. Tropomyosin covers the myosin binding sites on the actin, and troponin binds calcium. When muscle gets excited by a nerve, calcium is released into the sarcoplasm of the muscle. The calcium ions bind to the troponin which results in the conformational change in the troponin that leads to positional change in the tropomyosin. Thus, tropomyosin exposes myosin binding sites on the actin fibers, and facilities the binding of myosin to actin.

The mutation interferes with the ability of the protein to open in response to calcium ion binding. This kind of mutation results in defective troponin. When bound to calcium this defective protein cannot make any structural changes in tropomyosin. So, myosin binding sites are not exposed to initiate muscle contraction. This mutation causes muscle disfunctioning. This mutation is fatal in children because, heart muscle disfunctioning results in rapid death.

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