Troponin
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 interract with a single molecule of tropomyosin, an actin filament and a Ca2+ ion respectfully. The ability of troponin 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.
References
Molecular Biology of the Cell, 4th edition
Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, and Peter Walter. 2002.