Describe the structure of contractile proteins with a diagram.

(N/A) Contractile proteins consist of actin (thin) filaments and myosin (thick) filaments.
$1$. Actin Filaments:
- Each actin filament is composed of two '$F$' (filamentous) actins helically wound to each other.
- Each '$F$' actin is a polymer of monomeric '$G$' (globular) actins.
- Two filaments of another protein,tropomyosin,also run close to the '$F$' actins throughout their length.
- $A$ complex protein,troponin,is distributed at regular intervals on the tropomyosin.
- In the resting state,a subunit of troponin masks the active binding sites for myosin on the actin filaments.
$2$. Myosin Filaments:
- Each myosin filament is a polymerised protein consisting of many monomeric proteins called meromyosins.
- Each meromyosin has two important parts: a globular head with a short arm and a tail.
- The head and short arm are called heavy meromyosin $(HMM)$,and the tail is called light meromyosin $(LMM)$.
- The $HMM$ component projects outwards at regular distances and angles from the surface of a polymerised myosin filament and is known as the cross-arm.
- The globular head is an active $ATPase$ enzyme and has binding sites for $ATP$ and active sites for actin.