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 and therefore moves the tropomyosin away from the myosin binding site on actin. This means that the two filaments can form a cross bridge.
Troponin is thought to be specialised form another calcium ion binding protein, calmodulin, which is found in smooth muscle. The extra subunits, T and I, mean that the myosin heads are able to move towards the actin filaments very quickly for rapid contraction, This is needed in skeletal muscle for example.