5.3 Protein Interactions Modulated by Chemical Energy: Actin, Myosin, and
Molecular Motors
SUMMARY 5.3 Protein Interactions Modulated by Chemical Energy: Actin, Myosin,
and Molecular Motors
■ Protein-ligand interactions achieve a special degree of spatial and temporal organization
in motor proteins. Muscle contraction results from choreographed interactions between
myosin and actin, coupled to the hydrolysis of ATP by myosin.
■ Myosin consists of two heavy and four light chains, forming a fibrous coiled coil (tail)
domain and a globular (head) domain. Myosin molecules are organized into thick filaments,
which slide past thin filaments composed largely of actin. ATP hydrolysis in myosin is
coupled to a series of conformational changes in the myosin head, leading to dissociation of
myosin from one F-actin subunit and its eventual reassociation with another, farther along
the thin filament. The myosin thus slides along the actin filaments.
2+
■ Muscle contraction is stimulated by the release of Ca from the sarcoplasmic reticulum.
2+
The Ca binds to the protein troponin, leading to a
conformational change in a troponin-tropomyosin complex that triggers the cycle of actin-
myosin interactions.
Organisms move. Cells move. Organelles and macromolecules within cells move. Most of
these movements arise from the activity of a fascinating class of protein-based molecular
motors. Fueled by chemical energy, usually derived from ATP, large aggregates of motor
proteins undergo cyclic conformational changes that accumulate into a unified, directional
force—the tiny force that pulls apart chromosomes in a dividing cell, and the immense force
that levers a pouncing, quarter-ton jungle cat into the air.
The interactions among motor proteins, as you might predict, feature complementary
arrangements of ionic, hydrogen-bonding, and hydrophobic groups at protein binding sites.
In motor proteins, however, the resulting interactions achieve exceptionally high levels of
spatial and temporal organization.
Motor proteins underlie the migration of organelles along microtubules, the motion of
eukaryotic and bacterial flagella, the movement of some proteins along DNA, and the
contraction of muscles. Proteins called kinesins and dyneins move along microtubules in
cells, pulling along organelles or reorganizing chromosomes during cell division. An
interaction of dynein with microtubules brings about the motion of eukaryotic flagella and
cilia. Flagellar motion in bacteria involves a complex rotational motor at the base of the
flagellum (see Fig. 19-41). Helicases, polymerases, and other proteins move along DNA as
they carry out their functions in DNA metabolism (Chapter 25). Here, we focus on the well-
studied example of the contractile proteins of vertebrate skeletal muscle as a paradigm for
how proteins translate chemical energy into motion.
, The Major Proteins of Muscle Are Myosin and Actin
The contractile force of muscle is generated by the interaction of two proteins, myosin and
actin. These proteins are arranged in filaments that undergo transient interactions and slide
past each other to bring about contraction. Together, actin and myosin make up more than
80% of the protein mass of muscle.
Myosin (Mr 520,000) has six subunits: two heavy chains (each of Mr 220,000) and four light
chains (each of Mr 20,000). The heavy chains account for much of the overall structure. At
their carboxyl termini, they are arranged as extended α helices, wrapped around each other
in a fibrous, left-handed coiled coil similar to that of α-keratin (Fig. 5-27a).
At its amino terminus, each heavy chain has a large globular domain containing a site
where ATP is hydrolyzed. The light chains are associated with the globular domains. When
myosin is treated briefly with the protease trypsin, much of the fibrous tail is cleaved off,
dividing the protein into components called light and heavy meromyosin (Fig. 5-27b).
The globular domain—called myosin subfragment 1, or S1, or simply the myosin head group
—is liberated from heavy meromyosin by cleavage with papain, leaving myosin subfragment
2, or S2. The S1 fragment is the motor domain that makes muscle contraction possible. S1
fragments can be crystallized, and their overall structure, as determined by Ivan Rayment
and Hazel Holden, is shown in Figure 5-27c.
In muscle cells, molecules of myosin aggregate to form structures called thick filaments
(Fig. 5-28a). These rodlike structures are the core of the contractile unit. Within a thick
filament, several hundred myosin molecules are arranged with their fibrous “tails”
associated to form a long bipolar structure. The globular domains project from either end of
this structure, in regular stacked arrays.
The second major muscle protein, actin, is abundant in almost all eukaryotic cells. In
muscle, molecules of monomeric actin, called G-actin (globular actin; Mr 42,000), associate
to form a long polymer called F-actin (filamentous actin). The thin filament consists of F-
actin (Fig. 5-28b), along with the proteins troponin and tropomyosin (discussed below).
The filamentous parts of thin filaments assemble as successive monomeric actin molecules
add to one end. On addition, each monomer binds ATP, then hydrolyzes it to ADP, so every
actin molecule in the filament is complexed to ADP. This ATP hydrolysis by actin functions
only in the assembly of the filaments; it does not contribute directly to the energy expended
in muscle contraction. Each actin monomer in the thin filament can bind tightly and
specifically to one myosin head group (Fig. 5-28c).
Additional Proteins Organize the Thin and Thick Filaments into Ordered Structures
Skeletal muscle consists of parallel bundles of muscle fibers, each fiber a single, very
large, multinucleated cell, 20 to 100 μm in diameter, formed from many cells fused
together; a single fiber often spans the length of the muscle. Each fiber contains about
1,000 myofibrils, 2 μm in diameter, each consisting of a vast number of regularly arrayed
thick and thin filaments complexed to other proteins (Fig. 5-29).
A system of flat membranous vesicles called the sarcoplasmic reticulum surrounds each
myofibril. Examined under the electron microscope, muscle fibers reveal alternating regions
of high and low electron density, called the A bands and I bands (Fig. 5-29b, c). The A and
Molecular Motors
SUMMARY 5.3 Protein Interactions Modulated by Chemical Energy: Actin, Myosin,
and Molecular Motors
■ Protein-ligand interactions achieve a special degree of spatial and temporal organization
in motor proteins. Muscle contraction results from choreographed interactions between
myosin and actin, coupled to the hydrolysis of ATP by myosin.
■ Myosin consists of two heavy and four light chains, forming a fibrous coiled coil (tail)
domain and a globular (head) domain. Myosin molecules are organized into thick filaments,
which slide past thin filaments composed largely of actin. ATP hydrolysis in myosin is
coupled to a series of conformational changes in the myosin head, leading to dissociation of
myosin from one F-actin subunit and its eventual reassociation with another, farther along
the thin filament. The myosin thus slides along the actin filaments.
2+
■ Muscle contraction is stimulated by the release of Ca from the sarcoplasmic reticulum.
2+
The Ca binds to the protein troponin, leading to a
conformational change in a troponin-tropomyosin complex that triggers the cycle of actin-
myosin interactions.
Organisms move. Cells move. Organelles and macromolecules within cells move. Most of
these movements arise from the activity of a fascinating class of protein-based molecular
motors. Fueled by chemical energy, usually derived from ATP, large aggregates of motor
proteins undergo cyclic conformational changes that accumulate into a unified, directional
force—the tiny force that pulls apart chromosomes in a dividing cell, and the immense force
that levers a pouncing, quarter-ton jungle cat into the air.
The interactions among motor proteins, as you might predict, feature complementary
arrangements of ionic, hydrogen-bonding, and hydrophobic groups at protein binding sites.
In motor proteins, however, the resulting interactions achieve exceptionally high levels of
spatial and temporal organization.
Motor proteins underlie the migration of organelles along microtubules, the motion of
eukaryotic and bacterial flagella, the movement of some proteins along DNA, and the
contraction of muscles. Proteins called kinesins and dyneins move along microtubules in
cells, pulling along organelles or reorganizing chromosomes during cell division. An
interaction of dynein with microtubules brings about the motion of eukaryotic flagella and
cilia. Flagellar motion in bacteria involves a complex rotational motor at the base of the
flagellum (see Fig. 19-41). Helicases, polymerases, and other proteins move along DNA as
they carry out their functions in DNA metabolism (Chapter 25). Here, we focus on the well-
studied example of the contractile proteins of vertebrate skeletal muscle as a paradigm for
how proteins translate chemical energy into motion.
, The Major Proteins of Muscle Are Myosin and Actin
The contractile force of muscle is generated by the interaction of two proteins, myosin and
actin. These proteins are arranged in filaments that undergo transient interactions and slide
past each other to bring about contraction. Together, actin and myosin make up more than
80% of the protein mass of muscle.
Myosin (Mr 520,000) has six subunits: two heavy chains (each of Mr 220,000) and four light
chains (each of Mr 20,000). The heavy chains account for much of the overall structure. At
their carboxyl termini, they are arranged as extended α helices, wrapped around each other
in a fibrous, left-handed coiled coil similar to that of α-keratin (Fig. 5-27a).
At its amino terminus, each heavy chain has a large globular domain containing a site
where ATP is hydrolyzed. The light chains are associated with the globular domains. When
myosin is treated briefly with the protease trypsin, much of the fibrous tail is cleaved off,
dividing the protein into components called light and heavy meromyosin (Fig. 5-27b).
The globular domain—called myosin subfragment 1, or S1, or simply the myosin head group
—is liberated from heavy meromyosin by cleavage with papain, leaving myosin subfragment
2, or S2. The S1 fragment is the motor domain that makes muscle contraction possible. S1
fragments can be crystallized, and their overall structure, as determined by Ivan Rayment
and Hazel Holden, is shown in Figure 5-27c.
In muscle cells, molecules of myosin aggregate to form structures called thick filaments
(Fig. 5-28a). These rodlike structures are the core of the contractile unit. Within a thick
filament, several hundred myosin molecules are arranged with their fibrous “tails”
associated to form a long bipolar structure. The globular domains project from either end of
this structure, in regular stacked arrays.
The second major muscle protein, actin, is abundant in almost all eukaryotic cells. In
muscle, molecules of monomeric actin, called G-actin (globular actin; Mr 42,000), associate
to form a long polymer called F-actin (filamentous actin). The thin filament consists of F-
actin (Fig. 5-28b), along with the proteins troponin and tropomyosin (discussed below).
The filamentous parts of thin filaments assemble as successive monomeric actin molecules
add to one end. On addition, each monomer binds ATP, then hydrolyzes it to ADP, so every
actin molecule in the filament is complexed to ADP. This ATP hydrolysis by actin functions
only in the assembly of the filaments; it does not contribute directly to the energy expended
in muscle contraction. Each actin monomer in the thin filament can bind tightly and
specifically to one myosin head group (Fig. 5-28c).
Additional Proteins Organize the Thin and Thick Filaments into Ordered Structures
Skeletal muscle consists of parallel bundles of muscle fibers, each fiber a single, very
large, multinucleated cell, 20 to 100 μm in diameter, formed from many cells fused
together; a single fiber often spans the length of the muscle. Each fiber contains about
1,000 myofibrils, 2 μm in diameter, each consisting of a vast number of regularly arrayed
thick and thin filaments complexed to other proteins (Fig. 5-29).
A system of flat membranous vesicles called the sarcoplasmic reticulum surrounds each
myofibril. Examined under the electron microscope, muscle fibers reveal alternating regions
of high and low electron density, called the A bands and I bands (Fig. 5-29b, c). The A and
