Commonly referred to as Myosin or "thick filaments" (see sarcomere).
This is a type of motor protein that has an alpha helical twist at the end of its tail that causes it to wrap around another (or several other) Myosin II protein(s) to form a coiled coil of tails. Though usually referred to in isolated dimer form, it is commonly in an interwovern form found in the "thick filaments" of sarcomeres and diagramed below in limited ASCII art:

oooooo       oooooo
oooooo       oooooo

where the x's are the coiled coil of interwoven tails o's are the heads, if you count, you should see there are 12 distinct Myosin II dimers (each dimer has two heads and there are 6 facing one way and six facing another see sarcomere).

Myosin II is one of the most abundant types of Myosin motor protein in human (particularly muscle) cells. The two opposing sets of heads (see the o's in the diagram above) of thick filaments bind to actin filaments and walk toward the (+) end of the actin filament, using ATP as they walk (with out ATP, the myosin heads cannot unbind the actin, and the cell remains in the rigor state (which is why dead people are stiff when rigor moris sets in)). The binding itself in controlled by calcium.

See also muscle contraction, Myosin, and sarcomere.

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