Troponin and tropomyosin are the main proteins responsible for calcium regulation of actin-myosin interactions. Tropomyosin is a rod-shaped molecule that binds in the groove of the actin helix, and prevents the attachment of the myosin fiber heads. The troponin complex consists of three polypeptide subunits, troponin T, troponin I, and troponin C. Troponin T (for tropomyosin binding unit) attaches the troponin complex to tropomyosin which results in tropomyosin being situated in the myosin-binding site on the actin molecule. Troponin I (for inhibitory subunit) binds to actin and troponin T, and this complex inhibits actin-myosin binding even in the presence of calcium. Troponin C (for calcium-binding subunit) binds to troponin T and troponin I, and makes the complex dependent upon calcium. With calcium present, troponin C binds up to 4 molecules and changes the position of the troponin and tropomyosin complex, so that myosin is able to bind to the actin filament. Troponin I is present in three different isoforms in mammals, a slow-twitch skeletal muscle-specific form, a fast-twitch muscle-specific form, and a cardiac muscle-specific form.
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Protein Aliases: cardiac muscle; TNNC1; troponin I; troponin I type 1 (skeletal, slow); troponin I, skeletal, slow 1; Troponin I, slow isoform; Troponin I, slow skeletal muscle; Troponin I, slow-twitch isoform
Gene Aliases: 2700018B22Rik; AI747285; SSTNI; TNN1; TNNI1