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>> Many species of bacteria propel
themselves through their environment
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by spinning helical, motorized flagella.
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Rhodobacter cells have one flagellum each,
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whereas E. coli cells have multiple
flagella that rotate in bundles.
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Each flagellum consists of a helical
filament that is 20 nanometers wide
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and up to 15 microns long, and spins
on the order of 100 times per second.
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These animations show a series of
schematized and speculative models
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about how bacterial flagella
might function and assemble.
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Just outside of the cell wall, the filament
is connected to a flexible, rotating hook.
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The filament, the hook, and a structure called
the basal body, located below the cell surface,
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make up the three parts of the flagellum.
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The basal body consists of a rod and a series
of rings embedded in the inner membrane,
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the peptidoglycan layer and the outer membrane.
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Some of the rings make up the flagellar motor,
which can be divided into two major parts;
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the stator, which is attached to the
peptidoglycan layer, and as its name implies,
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remains stationary, and the
rotor, which rotates.
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The motor derives its power from a
proton gradient across the membrane.
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In this example, a high concentration
of protons exists outside,
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and a low concentration exists inside the cell.
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The protons flow through the interface
between two types of proteins called MotA
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and MotB that make up the stator.
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Mutational studies suggest
that a conserved aspartic acid
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in MotB functions in proton conductance.
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Each stator contains two MotB proteins,
and therefore also contains two
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of these important aspartic acids.
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Although the molecular mechanism
of rotation is not known,
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one possible model describes protons
moving through the channels in the stators,
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and binding to the aspartic
acid in the MotB proteins.
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This binding causes a conformational
change in mode A proteins,
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resulting in the first power stroke
that moves the rotor incrementally.
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At the end of the first power stroke, the
two protons are released into the cytoplasm.
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The proton loss causes a
second conformational change
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that drives the second power stroke,
once again engaging the rotor.
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Although the mechanism for motor
function is not yet certain,
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many details of flagella
assembly have been determined.
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Flagella begin their assembly with
structures in the inner membrane.
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Twenty-six subunits of an integral
membrane protein called FliF come together
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in the plasma membrane to form the MS ring.
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The FliG proteins assemble under the MS ring.
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FliG, along with FliM and FliN
proteins make up the rotor.
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Flagellar proteins destined to be
part of the extracellular portion
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of the flagellum are exported from the cell
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by a flagellum specific export
pathway, and assembled at the center.
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MotA and MotB form the stationary part
of the flagellar motor: the stator.
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Both are integral membrane proteins, but MotB is
also anchored to the rigid peptidoglycan layer,
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keeping the stator proteins fixed in place.
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The subunits of the rod portion of the
rotor move up through the hollow cylinder
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in the assembly, and assisted by KAT proteins,
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build up the rod in a proximal
to distal fashion.
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Another set of rings, called L and P rings, are
found in gram negative bacteria such as E. coli.
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They penetrate the outer membrane,
forming a bearing for the rod.
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As the rod cap is exposed outside the L-ring,
it dissociates and is replaced by a hook cap
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that guides the assembly of the hook proteins.
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After the hook is assembled,
the hook cap dissociates,
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and a series of junction proteins assemble
between the hook and future filaments.
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Finally, yet another cap is built,
and filament proteins assemble.
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Like the rod and hook proteins, they travel
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through the hollow channel inside
the filament to reach the distal end.
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The cap rotates, which causes the
subunits to build in a helical fashion.
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A complete filament can consist
of 20,000 to 30,000 subunits.
