Troponin complexes consist of three subunits - troponin T (also known as TnT or TNNT), troponin I (TnI or TNNI) and troponin C (TnC or TNNC). Troponin is therefore heterotrimeric, with each of the different subunits providing specific binding sites, allowing each troponin to interact with a single molecule of tropomyosin, an actin filament and a Ca2+ ion respectfully. The ability of the troponin C subunit to bind Ca2+ allows it to undergo a conformational change and therefore moves the tropomyosin away from the myosin binding site on actin. This means that the two filaments can form an actin-myosin cross bridge.
Troponin is thought to be a specialised form of another calcium ion binding protein, calmodulin, which is found in smooth muscle. The extra subunits, T and I, aid to propagate the movement of the myosin heads towards the actin filaments at great speed for rapid contraction, This characteristic is necessary in a number of processes, one example being action of the skeletal muscle.