Myosin II is a motor protein, which means it converts chemical energy into a mechanical response. Myosin II is a member of the myosin superfamily, and like its relatives, myosin II “walks” along tracks made of actin (learn about actin here).
Myosin II is categorized as a muscle myosin because it is responsible for generating the force that contracts your muscles, including your smooth muscles and your heart. Many myosin II’s stick together to form filaments which allows them to work effectively as a team. These myosin filaments pull on actin filaments to accomplish contraction.
Myosin II is made of two identical parts. Each part is made of about 2,000 amino acids (protein pieces) which form a head (S1), a neck (S2), and a tail.
Myosin II’s heads contain two very important features: a site for collecting energy (because just as your walking costs calories, myosin’s walking does too!) and a site for binding to actin.
Myosin II also has two necks which bend when it walks or pulls. Some other proteins bind to myosin II’s necks and give it more specific instructions for its work (regulatory proteins).
Myosin II’s tail wraps around the tails of other myosin II’s to make the myosin filament.
In muscle, the force generated in a contraction is related to how many myosin II heads are participating in the work–not all of them work at exactly the same time. (You could say that they take shifts.) When a myosin II head is at work or turned “on,” it is free to bind to actin. When it is turned “off,” the head bends back and binds to a portion of its tail.
There are scientists (like my lab mates and I) who want to know how myosin II knows when to turn off and when to show up to work. We believe that if we understand this process, then we can manipulate how much a muscle contracts by changing how many heads are at work. This could be of great benefit to treating heart diseases that are caused by hearts contracting too much or not enough.
Julia Migliore 
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