FLAGELLAR MOTORS

Objectives

• Functions of bacterial flagella.• How flagellar motors work to bring about

locomotion!!!

Bacteria Swim by Rotating Their Flagella

• Bacteria swim by rotating flagella that protrude from their surfaces.

• When the flagella rotate in a counter clockwise direction, the separate flagella form a bundle that very efficiently propels the bacterium through solution.

Bacterial flagella in a bundle

Bacterial flagella• Bacterial flagella are polymers approximately 15 nm in

diameter and as much as 15 μm in length, composed of a protein called flagellin.

• Each flagellum has a hollow core.• At its base, each flagellum has a rotory motor (200-1000rpm).• Three parts: Basal body, hook and filament

Deriving forces of flageller motion

• ATP is not used for flagellar motion.• The necessary free energy is derived from either the

proton gradient or sodium ion gradient that exists across the plasma membrane.

• Hence, either proton motive force or sodium motive force facilitates flagellar motion.

• Flagella do not rotate at a constant speed but instead can increase or decrease their rotational speed in relation to the strength of the proton motive force.

Proton motive force

• The bacterial flagellum is driven by a rotary engine. The engine is powered by proton motive force, i.e., by the flow of protons (hydrogen ions) across the bacterial cell membrane due to a concentration gradient set up by the cell's metabolism

Sodium motive force• In some bacteria especially marine bacteria,

ion channels eject sodium ions from the cell. This leads to formation of sodium ion gradient. This gradient can also be used as intermediate energy storage for flagellar rotation.

• This gradient is a form of sodium motive force which assists in flagellar rotation.

Flagellar Motor

• Five components crucial to motor function have been identified.

• MotA and MotB are membrane proteins.• Approximately 11 MotA - MotB pairs form a ring

around the base of the flagellum. • The proteins FliG, FliM, and FliN are part of a disc-

like structure called the MS (membrane and supramembrane) ring, with approximately 30 FliG subunits coming together to form the ring.

Flagellar Motion

• The MotA - MotB pair and FliG combine to create a proton channel that drives rotation of the flagellum.

• Each MotA - MotB pair is conjectured to form a structure that has two half-channels; FliG serves as the rotating proton carrier

• A proton from the periplasmic space passes into the outer half channel and is transferred to an FliG subunit.

• The MS ring rotates, rotating the flagellum with it and allowing the proton to pass into the inner half-channel and into the cell.

Flagellar Motion

Flagella

Sensory Role of Flagella: Bacterial Chemotaxis

• In the presence of a gradient of certain substances such as glucose, bacteria swim preferentially toward the direction of the higher concentration of the substance. Such compounds are referred to as chemoattractants (positive chemotaxis).

• Bacteria also swim preferentially away from potentially harmful compounds such as phenol, a chemorepellant (negative chemotaxis).

• The process of moving in specific directions in response to environmental cues is called chemotaxis.

Bacterial Chemotaxis• The bacteria swim in one

direction for some length of time, senses the concentration of chemo-attractants or chemo-repellants , tumble briefly, and then set off in a new direction.

• Tumbling is caused by an abrupt reversal of the flagellar motor, which disperses the flagellar bundle for some time.

Flagella changing direction