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Anatomy
particles by remodeling and/or exchange of subunits with the cell body cytoplasm, with a concurrent decrease in number. In this phase, the precursors of the flagellar structures that make up the
cargos are also loaded onto the particles. In phase II, the particles are transported from the base
to the distal end of the flagellum by a heterometric kinesin II with a velocity of about 2 μm s −1 . In
phase III, which occurs at the distal end of the flagellum, anterograde particles are remodeled into
retrograde particles with a concurrent increase in number, probably upon or after unloading their
cargo. Finally, in phase IV, retrograde particles are transported by a cytoplasmic flagellar dynein
from the distal end back to the basal body region of the flagellum, with a velocity of about 3 μm s −1 ,
higher than that of anterograde particles.
How a Flagellar Motor Works
Movement can arise by shape change of permanently linked elements, by reversible interactions
causing movement of elements relative to each other, by reversible assembly and disassembly,
etc. all of which need energy input. We know that such changes can occur in proteins, the most
likely molecules serving these locomotory functions in real movement system. But what drives
and controls these changes? In principle, the problem is not difficult. Altering the ionic milieu,
changing chemically or electrically its environment can in turn alter the tertiary and/or quaternary structure of a protein. In most control systems, if not all, a change in the environment brings
about a change in the properties of the motor, acting either directly or indirectly on the component
of the motor. We need only two proteins to make a motor using the sliding filaments mechanism,
that is, a globular protein (such as tubulin) and an anchor protein (such as the dynein–dynactin
complex). If the globular protein can polymerize, we can assemble it into a linear polymer that
can be attached via the anchor protein to another structure some distance away. The transformation of chemical energy into mechanical work depends on a conformational change of the anchor
protein, which uses the hydrolysis of ATP in ADP. Provided the anchor protein repeats the conformational change upon each monomer of the globular protein in turn, the “boat” can be hauled
“hand over hand” toward the distant anchorage. Provided some kind of metachrony regulates
adjacent motor molecules, we can link our small movements in a temporal series to amplify the
amount of movement that can be achieved. Each step costs hydrolysis of one ATP molecule per
anchor protein. The simplest and most obvious solution is either to have more than one anchor
protein, or to have a dimer, working out-of-phase, being careful not to detach before the new
attachment is formed. For instance, most (but not all) microtubular motors (dyneins, kinesins)
work as dimers whose subunits walk along microtubule walls just like human legs walk on a
surface. Once we have two hands to pull on the rope, we can indeed move hand over hand; the
one-armed man cannot do more than pull once. The flagellum movements are due to the transient
interaction between two anchored microtubules, coupled to a linkage control. The generation
of sliding of adjacent doublets by flagellar dynein is combined to the resisting forces localized
near the active sliding rows of dyneins. During the cycle of binding/release obtained by dynein
conformational change coupled to ATP hydrolysis, chemical energy is converted into mechanical
energy used for sliding. Owing to their regular spacing every 24 nm along the axoneme, several
adjacent dyneins participate to this local sliding, and their functioning proceeds by local waves
that propagate step by step all the way along the flagella. The postulated regulator has therefore
to trigger the functioning of the different dyneins alternatively along the length as well as around
the section of the axoneme, but its molecular nature remains unknown.
The model of Lindemann (1994) accounts for wave generation and propagation, regulated by
geometrical constraints. This model, the so-called “geometric clutch,” is based on the way a cylinder with nine generatrix (the nine outer doublets of the axoneme) changes form when submitted
to bending. In the zone of curvature, the doublets located outside the curvature are brought apart,
while the doublets located inside the curvature come closer to each other. This makes the corresponding dynein molecules efficient for sliding. In contrast, dynein molecules located outside
the curvature are too far for binding to adjacent microtubules: no active sliding can occur in this
Anatomy
particles by remodeling and/or exchange of subunits with the cell body cytoplasm, with a concurrent decrease in number. In this phase, the precursors of the flagellar structures that make up the
cargos are also loaded onto the particles. In phase II, the particles are transported from the base
to the distal end of the flagellum by a heterometric kinesin II with a velocity of about 2 μm s −1 . In
phase III, which occurs at the distal end of the flagellum, anterograde particles are remodeled into
retrograde particles with a concurrent increase in number, probably upon or after unloading their
cargo. Finally, in phase IV, retrograde particles are transported by a cytoplasmic flagellar dynein
from the distal end back to the basal body region of the flagellum, with a velocity of about 3 μm s −1 ,
higher than that of anterograde particles.
How a Flagellar Motor Works
Movement can arise by shape change of permanently linked elements, by reversible interactions
causing movement of elements relative to each other, by reversible assembly and disassembly,
etc. all of which need energy input. We know that such changes can occur in proteins, the most
likely molecules serving these locomotory functions in real movement system. But what drives
and controls these changes? In principle, the problem is not difficult. Altering the ionic milieu,
changing chemically or electrically its environment can in turn alter the tertiary and/or quaternary structure of a protein. In most control systems, if not all, a change in the environment brings
about a change in the properties of the motor, acting either directly or indirectly on the component
of the motor. We need only two proteins to make a motor using the sliding filaments mechanism,
that is, a globular protein (such as tubulin) and an anchor protein (such as the dynein–dynactin
complex). If the globular protein can polymerize, we can assemble it into a linear polymer that
can be attached via the anchor protein to another structure some distance away. The transformation of chemical energy into mechanical work depends on a conformational change of the anchor
protein, which uses the hydrolysis of ATP in ADP. Provided the anchor protein repeats the conformational change upon each monomer of the globular protein in turn, the “boat” can be hauled
“hand over hand” toward the distant anchorage. Provided some kind of metachrony regulates
adjacent motor molecules, we can link our small movements in a temporal series to amplify the
amount of movement that can be achieved. Each step costs hydrolysis of one ATP molecule per
anchor protein. The simplest and most obvious solution is either to have more than one anchor
protein, or to have a dimer, working out-of-phase, being careful not to detach before the new
attachment is formed. For instance, most (but not all) microtubular motors (dyneins, kinesins)
work as dimers whose subunits walk along microtubule walls just like human legs walk on a
surface. Once we have two hands to pull on the rope, we can indeed move hand over hand; the
one-armed man cannot do more than pull once. The flagellum movements are due to the transient
interaction between two anchored microtubules, coupled to a linkage control. The generation
of sliding of adjacent doublets by flagellar dynein is combined to the resisting forces localized
near the active sliding rows of dyneins. During the cycle of binding/release obtained by dynein
conformational change coupled to ATP hydrolysis, chemical energy is converted into mechanical
energy used for sliding. Owing to their regular spacing every 24 nm along the axoneme, several
adjacent dyneins participate to this local sliding, and their functioning proceeds by local waves
that propagate step by step all the way along the flagella. The postulated regulator has therefore
to trigger the functioning of the different dyneins alternatively along the length as well as around
the section of the axoneme, but its molecular nature remains unknown.
The model of Lindemann (1994) accounts for wave generation and propagation, regulated by
geometrical constraints. This model, the so-called “geometric clutch,” is based on the way a cylinder with nine generatrix (the nine outer doublets of the axoneme) changes form when submitted
to bending. In the zone of curvature, the doublets located outside the curvature are brought apart,
while the doublets located inside the curvature come closer to each other. This makes the corresponding dynein molecules efficient for sliding. In contrast, dynein molecules located outside
the curvature are too far for binding to adjacent microtubules: no active sliding can occur in this