Cardiomyocytes - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 207

Fig. 5.3 Sarcomere cross-section and myofilament arrangement. The inner region of the H zone
(in the M line and its vincinity) contains the rod part of the myosin filament (bare zone). Myosin
heads are located in the outer regions of the H zone and A band. Myosin filaments are surrounded
by actin filaments in the outer region of the A band. The I band (not represented) is composed only
by actin filaments.

5.3.2 Myosin Thick Filaments

15 nm) is composed of myosin 31 (Fig. 5.4 ). Myosin The thick filament (diameter ∼

filaments produce the A bands.
The myosin structure is commonly split into 2 regions, light (LMM) and heavy
(HMM) meromyosin. The former represents part of the tail and is responsible for the
assembling of myosin (Fig. 5.4 ). The latter contains the 2 heads and the remaining
part of the tail. Heavy meromyosin is further divided into globular fragment-1
(S1) and rod fragment-2 (S2). The latter contains binding sites for essential and
regulatory light chains (MLC1 and MLC2). The former (S1) has ATP hydrolysis
and actin-binding domains; it has helical arrangements around the filament axis.
The thick filament also contains M-line proteins myomesin and myosin-binding
protein-C (cardiac isoform cMyBPc or MyBPc3), or C-protein, that crosslink
myosins and the M-band part of titin. Myosin-binding protein-C maintains
the width of myosin-containing thick filament, as it restricts them to 200 to
400 molecules each.
Titin is involved in the connections between the ends of myosin filaments and
Z discs; it can span the entire sarcomere length. It controls both the length and
elasticity of the sarcomere.

31 The myosin molecule (length 1.55

m, diameter 12-15 nm) can be divided into 2 heavy chains

and 4 light chains [ 361 ].