Troponin - Tropomyosin

Actin-linked Regulatory System - Level of Intracellular Ca2+

Troponin/tropomyosin - these two proteins interact to control the attach-ment of crossbridges to actin. When intracellular calcium is low (<10-8M), tropomyosin blocks all actin sites along one turn of the actin helix. However when intracellular calcium is elevated (>10-7M), calcium binds to troponin which then alters the position of tropomyosin on the actin filament. Once tropomyosin has moved, the actin sites are exposed allowing crossbridges to attach.