SECTION 21.1

Muscle Systems

461

FIGURE 21-6

Major structural features of the myosin molecule. The myosin rod, extending from the C terminus to cleavage site 2,

consists of two a-helical polypeptides coiled around each other. Cleavage by trypsin at site 1, which is located in the

hinge region, yields light meromyosin (LMM) and heavy meromyosin (HMM). Digestion of HMM with papain cleaves

HMM at site 2 (swivel region), releasing two Si fragments (myosin heads) and one S

2

fragment (myosin arm) for each

molecule of HMM. The locations of the light chains (R = regulatory; E = essential) are indicated roughly in this figure.

The actual locations are shown in Figure 21-7.

FIGURE 21-7

The head domain of myosin shown in its relation to the actin filament. The NH

2

-terminal end of the myosin heavy chain

is in the globular head. The light chains bind to the neck region of the MHC. In this figure, the orientation of the myosin

to the actin is that of the rigor bond, i.e., at the end of the power stroke. [From M. Irving and G. Piazzesi, Motions of

myosin heads that drive muscle contraction.

N ew s P hysiol. Sci.

12(6), 249-254 (December 1997).]