Structure of contractile proteins

Each thin (actin) filament is made of two F (filamentous) actins twisted round each other in a helix; each F actin is a polymer of G (globular) actin monomers.

Two filaments of tropomyosin run close to the F actins along the whole length.

Troponin, a complex protein, sits at regular intervals on the tropomyosin. At rest one subunit of troponin covers the sites on actin where myosin would bind.

Each thick (myosin) filament is a polymer of many monomers called meromyosins.

A meromyosin has a globular head with a short arm, the heavy meromyosin (HMM), and a tail, the light meromyosin (LMM).

The heads and short arms stick out from the thick filament at regular distances and angles as cross arms.

The globular head is an active ATPase enzyme, with binding sites for ATP and active sites for actin.