Myosin is a protein found in muscle tissue, forming the thick filaments of a sarcomere in a myfibril. Strictly speaking, it is a polypeptide with a molecular weight of 470-kD. During muscular contraction, the double globular head region of a myosin in the thick filaments interacts with actin in the thin filament and ATP (adenosine triphosphate). When the myofibril contracts, ATP binds to myosin, preventing its binding to actin. Proteolytic cleavage divides myosin into light meromyosin (LMM) and heavy meromyosin (HMM) fragments.
The orientation of myosin molecules in the thick filament allows the cross-bridge part of the molecule to oscillate back and forth by swiveling in the remainder. Immediately, following efferent stimulation of the muscle fiber, there is release of stored calcium ions into its myofibrils. As a result, the cross-bridges of the myosin latch onto actin in the thin filaments. Once they are attached, they slightly change their position, disconnect and swivel back to their former position, ready to latch on for another cycle of operation.
The muscle fiber contractile force is generated by the interaction of myosin and actin, which produces relative movement between these proteins. With actin forming the thin filament of myofibrils, they are both linked to the organelles, cytoskeleton, and the cell surface. As a result, relative movement of these proteins generates weak contractile force at the expense of adenosine triphosphate.
Below, the myosin in the relaxed and contracting phase in the thick filament.
Electron micrograph of part of a sarcomere, showing the cross bridges of thick filaments (extracted from a rabbit psoas muscle)
