Tropomyosine

Tropomyosin is a protein found in muscle and other cells. In muscle cells, it works together with troponin and actin to regulate muscle contraction. Tropomyosin is a long, fibrous protein that wraps around the actin filaments, which are the thin strands involved in muscle contraction. When a muscle is relaxed, tropomyosin blocks the binding sites on actin where the myosin heads (part of the thick filaments) would attach. This prevents the myosin from interacting with actin, and thus prevents contraction.

When a muscle receives a signal to contract, calcium ions are released into the cell. These calcium ions bind to troponin, a complex of proteins associated with tropomyosin. This binding causes a conformational change in troponin, which in turn pulls tropomyosin away from the myosin-binding sites on actin. With the binding sites now exposed, the myosin heads can attach to actin, forming cross-bridges. The myosin heads then pivot, pulling the actin filaments and causing the muscle to contract.

In non-muscle cells, tropomyosin plays a role in maintaining cell shape and structure, and is involved in various cellular processes such as cell migration and the transport of vesicles. Different isoforms of tropomyosin exist, encoded by different genes or produced through alternative splicing, which allows them to perform specialized functions in different cell types and at different stages of development. Mutations in tropomyosin genes can lead to various muscle diseases, including cardiomyopathies and nemaline myopathy, highlighting its critical role in cellular function.