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Actin-myosin
interaction and its regulation
Mixtures
of myosin and actin are used in the test tube to study the
relationship between the ATP breakdown reaction and the
interaction of myosin and actin. The ATPase reaction can
be followed by measuring the change in the amount of phosphate
present in the solution. The myosin-actin interaction also
changes the physical properties of the mixture. If the concentration
of ions in the solution is low, myosin molecules aggregate
into filaments. As myosin and actin interact in the presence
of ATP, they form a tight, compact gel mass; the process
is called superprecipitation. Actin and myosin interaction
can also be studied on muscle fibres whose membrane is destroyed
by glycerol treatment; these fibres still develop tension
when ATP is added. A form of ATP that is inactive unless
irradiated with a laser beam has been found to be useful
in the study of the precise time course underlying contraction.
If
troponin and tropomyosin are also present, however, the
actin and myosin do not interact, andATP is not broken down.
This inhibitory effect corresponds to the state of relaxation
in the intactmuscle. When calcium ions are added, they combine
with troponin, inhibition is released, actin and myosin
interact, and ATP is broken down. This corresponds to the
state of contraction in intact muscle. The exact mechanism
by which troponin, tropomyosin, and calcium ions regulate
the myosin-actin interaction is not fully agreed upon. In
the thin filament there are one troponin and one tropomyosin
molecule for every seven actin units. According to one view,
Ca2+binding to troponin, actually the TnC subunit, induces
a change in the position of tropomyosin, moving it away
from the site where myosin also binds (steric blocking).
Alternatively, the calcium-induced movement of tropomyosin
in turn induces changes in the structure of actin, permitting
its interaction with myosin (allosteric model). In smooth
muscles, Ca2+ activates anenzyme (kinase) that catalyzes
the transfer of phosphate from ATP to myosin, and the phosphorylated
form is then activated by actin.
A
somewhat different scheme of regulation operates in the
muscle of mollusks. As in vertebrate muscles, calcium ions
act as the initiator of contraction. The difference is that
the component that binds calcium ions in the molluscan muscle
is myosin rather than a componentof the actin-containing
thin filaments. The interaction of actin and myosin provides
a basis for molecular models of force generation and contraction
in living muscle.
Robert E. Davies
Nancy A. Curtin
John Gergely
