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chapter
21
Muscle and Nonmuscle Contractile Systems
heterodimer. The proportions of
a-
and /J-chains present,
and which
a-
and /3-chain genes are expressed, varies
with the type of muscle fiber. In large mammals, car-
diac tropomyosin is predominantly a homodimer of the
same a-tropomyosin found in slow-twitch skeletal mus-
cle. Fast-twitch fibers have primarily the a-/S heterodimer.
Tropomyosin is phosphorylated at specific sites in some
fiber types. The molecule is rigid and insoluble, and binds
to F-actin along the grooves formed by the spiraling of the
filament. The tropomyosins are arranged end-to-end, are
about 40 nm long, and each one spans seven G-actin units.
Tropomyosin normally inhibits interaction between actin
and myosin by interfering with a projection on the myosin
head distal to the actin binding site.
Near one end of each tropomyosin is a molecule of tro-
ponin (M.W. ~76,000), a trimer of noncovalently bound
subunits called troponins C, I, and T (Tn C, Tn I, and Tn T).
Tn T binds to the C-terminal region of tropomyosin and
to Tn C, Tn I, and actin. Tn I binds to Tn C in a Ca2+-
dependent manner, and to actin, and is thought to deter-
mine the position of the tropomyosin-troponin complex
on the actin filament. Tn C is structurally and function-
ally similar to calmodulin (Chapters 15 and 30). Each
Tn C molecule can cooperatively bind up to four Ca2+
ions (three in cardiac Tn C). Under physiological resting
conditions, typically two of these sites would be occu-
pied by Mg2+ and two would be unoccupied. Binding of
Ca2+ to Tn C (during the calcium spike induced by de-
polarization of the cell) reverses the tropomyosin inhibi-
tion of actin-myosin binding. Tn I is a 179 to 216 amino
acid polypeptide (M.W. ~23,000), with a preponderance
of basic amino acids in the actin binding region. It is so
named because Tn I binding to actin can inhibit contraction
even in the absence of tropomyosin. Tn T is an elongated
259 amino acid polypeptide (M.W. ~37,000).
In vivo,
about half of these amino acids are charged (with negative
charges near the N-terminus and positive charges near the
C-terminus).
Thick Myofilaments
Thick myofilaments are made primarily of myosin.
Myofibrillar myosin is one member of the multi-gene
myosin II sub-family in the myosin super-family. Of the
15 types of myosin that have been described, at least 7
occur in mammalian species. Of these seven, three are
fairly well characterized. Myosin I has a single heavy
chain and one to three (usually one) light chain, which is
calmodulin. Myosin V has a heavy chain dimer, and car-
ries six light chains, which are usually calmodulin. Both
types I and V are motors that carry organelles and vesicles
along the actin filaments of the cytoskeleton and probably
also perform other functions. Myosin II is a large hexam-
eric protein (M.W. ~475,000) consisting of two myosin
heavy chains (MHCs; M.W. ~ 200,000) and four myosin
light chains (MLCs; M.W. ~ 16,000 to 20,000). These are
non-covalently linked. The size and properties of the light
chains vary by tissue.
About half the amino acids of the MHCs form a-helices
which twist together in a coiled-coil to form a rigid, insol-
uble tail. The N-terminal halves of the chains form discrete
globular heads consisting of 839 to 850 amino acids. To
each head are bound two different MLCs, as discussed
below. The head of the myosin molecule has a site which
binds to a complementary site on actin and, about 3.5 nm
away, an ATP binding site which has a high ATPase activ-
ity only when myosin is bound to actin. The affinity of the
binding to actin is regulated by events at the ATP binding
site.
Papain readily cleaves myosin at site 2 (Figure 21-6),
releasing the heads, called
Si,
fragments and an almost
complete tail. Brief exposure to trypsin or chymotrypsin
cleaves myosin at site 1, yielding two large fragments
called
heavy meromyosin
(HMM; M.W. ~350,000) and
light meromyosin
(LMM; M.W. ~ 125,000). LMM forms
the core of thick filaments and is responsible for the
low solubility of myosin under physiological conditions.
HMM comprises the heads attached by short neck re-
gions to an arm region. Digestion of HMM by papain
releases the Si fragments and an S
2
fragment that corre-
sponds to the arm. There is probably a relatively flexible
region of the molecule at site 1, and the S
2
fragments
normally have little charge and thus little interaction with
LMM. These features allow the heads to project outward
from the filament to a distance where actin-myosin inter-
actions would not be influenced by the properties of LMM.
Sequence information and imaging techniques have led to
a detailed model of this complex structure in relation to
F-actin (Figure 21-7). The light chains bind to the neck
region, presumably stiffening it and permitting it to act as
a lever arm amplifying movement that occurs in the head
during the binding of myosin to actin and the subsequent
binding and hydrolysis of ATP.
Several nomenclatures have been used for the light
chains. Each head is said to have one “essential” and
one “regulatory” light chain. The term “essential” per-
sists despite the fact that myosin devoid of light chains
retains actin-activated ATPase activity. Phosphorylation
of, or Ca2+ binding to, the regulatory chains can alter the
behavior of myosin greatly. The MLCs have also been
named according to method of preparation. Light chains
released by exposure to alkali are called alkali light chains
and those released by exposure to the solvent DTNB [5,5'-
dithiobis(2-nitrobenzoate)] are called DTNB light chains.
