Troponin I

Troponin is an accessory protein found in thin filaments. Thin filaments are structural components of an important muscle unit called the sarcomere. These thin filaments play an important role in muscle contraction. Troponin consist of three sub-units: troponin T, I and  C. When the muscle is at rest the T and I sub-unit  binds to tropomyosin and actin resulting in an inhibitory process that prevents myosin heads from binding to actin. When Ca²+ concentration level is raised the Ca²+ ions bind to troponin C. This causes troponin I to release its hold on actin, which allows tropomyosin to move back to its normal position thus allowing myosin heads to bind to actin triggering contraction.

Alberts,Bruce etal (2002) Molecular biology of the cell 4th ed. (chapter 16 pg 962-965). New York, Garland Science.