The thin actin filament is a dimeric polymer of G-actin sub-units arranged like two strings
of beads twisted together.
Attached to the actin chain of the thin filament, are the proteins troponin (Tn) and tropomyosin.
A tropomyosin molecule runs along each actin chain, bound to the actin. Each tropomyosin sub-
unit covers about 7 G-actin sub-units.
The troponin molecule has three sub-units: TnT that binds to tropomyosin near the ends of the
tropomyosin sub-units; TnI that binds to the actin; and TnC that binds to the TnI and TnT sub-
units, and which also has a strong affinity for Ca2+ at four binding sites.
The thick filaments, at either end, have sticking off at regular intervals,
the fibrillar necks and globular, ATPase heads of the myosin subunits -
the polymerized tails of which make the backbone of the filament.
The heads at the opposite ends of the thick filament stick off in opposite
directions leaving a bare middle region (pseudo H-zone) on the filament.
Each successive myosin head is staggered so that six rows of heads stick
off in six directions (60o intervals) around the filament.
Each myosin filament is surrounded by six actin filaments and each actin
filament by three myosins.
