| F-actin is a polymer of G-actin subunits; in muscle there are about twice as many actin as myosin chains
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| Actin is composed of 42 kDa subunits, known as G-actin (globular), but polymerizes spontaneously into a filamentous array (F-actin). The G-actin subunits polymerize in a head-to-tail manner, and two polymer chains coil around one another to form the F-actin myofilament (see Fig. 19.3). The F-actin chains extend in opposite directions from the Z-line, overlapping with the myosin chains extending from the M-line. There are approximately twice as many actin as myosin chains in muscle, yielding an array in which each myosin molecule is associated with six actin molecules and each actin with three myosin molecules (see Fig. 19.1 for a cross-sectional view).
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