Motility and Chemotaxis

Exam 1

Swimmin Motility

flagellum-mediated movement in liquid

• common to bacteria and archaea

• random bias walk (chemotaxis)

Swarming motility

flagellum-mediated motility across surfaces

• bacteria often employ a different set of flagella than the flagella used for swimming

Twitching motility

requires cell contact, jerky movement, requires Type IV pili

Gliding motility

specialized smooth movement

• Occurs in many bacterial species

• different mechanisms (we will focus on one)

• mechanisms not as well characterized as flagellum-mediated motility

• can involve rotary motors, cytoskeletal proteins, or jets of polysaccharide (propulsion)

Type IV pilus machine-medated twitching motility

• Occurs in some Gram negative and positive bacteria

• Cells use Type IV pilus as a grappling hook to pull cell forward

• Involves assembly and disassembly of pilus

• Usually involved in social motility

• Process is powered by ATP hydrolysis

• Type IV pili have duel purpose - also used in DNA uptake and transfer (sex pilus)

Gliding motility in Flavobacterium johnsoniae

Molecular rack and pinion system (stationary rotary motor and mobile track)

• Rotary motor and track are achored to PG layer

• Track and associated mobile tread are spirally wound around cell

• The rotary motor drives a pinion that engages mobile tread (rack) that slides along the track

• Adhesin SprB is attached to tread and moves with it

• Cell is pulled forward and rotates as moves forward

Various flagellation patterns found in bacteria

• Peritrichous: lots along cell surface

• Amphitrichous: on both cell poles

• Single polar

• Lophotrichous: multiple at one cell pole

• Periplasmic flagella: internal

Swimmin and Swarming in Vibrio parahaemolyticus

• Cells make only polar flagella when grown in liquid medium (swimming)

• Cells synthesize lateral flagella when grown on agar medium, which are used for swarming (movement across surface)

• Components of the polar and lateral flagella are encoded by separate sets of genes

• Polar flagellum is a mechanosensor that senses when the cell is ona surface - increased load on the flagellar motor is presumably a signla to activate transcription of the genes that encode components of the lateral flagella

The bacterial flagellum uses a bi-directional rotary motor

• Stator is stationary component of rotary motor - MotA/MotB or PomA/PomB

• Rotor is the rotating component of the rotary motor - C-ring and MS-ring

• Drive shaft (rod) transfers torque to devise output (filament) - torque is force that causes an object to rotate around an axis

• Rotation rates for bacterial flagella are typically a coupld hundred revolutions per second

• Bushing/bearing maintains balance and coaxial orientation of rod during rotating

• Hook functions as a universal joint - allows torque to be transferred from rod to filament even when their axes are not aligned

• Filament acts as propeller - regid helical rod that pushes against the surrounding medium as it rotates

Stator

stationary component of rotary motor - MotA/MotB or PomA/PomB

Rotor

rotating component of the rotary motor – C-ring and MS-ring

Bushing/bearing (LP-ring)

maintains balance and coaxial orientation of rod during rotation

• only in Gram -

Hook

universal joint - allows torque to be transferred from rod to filament even when their axes are not aligned (often perpendicular to each other)

Filament

propeller - rigid helical rod that pushes against the surrounding medium as it rotates

What maintain the orientation and balance betwee the LP-ring and distal rod?

electrostatic interactions

• acidic amino acids repel distal rod

• basic amino acids attract distal rod

Stators are Powered by ________

Ion motive force

• MotA/MotB complexes use H+ gradient; PomA/PomB complexes usually use Na+

• B component is anchored in PG layer

Campylobacter jejuni MotA/MotB stator

• Subunit stoichiometry is 5 MotA subunits and 2 MotB subunits

• Upon recruitment to the motor, the stator unit undergoes a conformation change from plugged (inactive) to unplugged (active) state - unplugging allows for ion flow across the membrane

• Stator forms a proton channel in membrane. Proton flow through the channel results in rotation of MotA pentamer - torque is transferred to the rotor

How does the flagellar motor change the direction of rotation?

• MotA engages FliG within the C-ring (forms rotor along with MS-ring)

• C-ring undergoes conformational change when binds CheY-phosphate. Results in C-ring interacting with different sides of the stator.

• In the absence of CheY-phosphate, C-ring interacts with MotA subunits that are proximal to the central axis of the flagellar motor, resulting in CCW rotation of the rotor

• In the presence of CheY-phosphate , C-ring interacts with MotA subunits that are distal to the central axis of the flagellar motor, resulting in CW rotation of the rotor

Bacterial flagellar motors dffer in complexity

• Flagellar motors of different species share core strucutres, but some motors have additional feautres

• Number of stators varies among species

• Some bacterial have a membrane sheath surrounding the hook and filaments

Archaeal flagellum

Flagellin proteins are not like bacterial ones

• filament isn’t hollow

• assembly proteins look like Type IV pili

Chemotaxis

• Prokaryotes use a complex signaling system to travel towards attractants and away from repellents

• Constantly sensing concentration gradients for travel (chemoreceptors), adapt to concentrations

• Biased random “walk” consiting of tumbles (random direction change) and runs (directional movement)

• Not all bacterial species display runs and tumbles

Direction Depends on Flagellar Rotation - Run

CCW rotation of flagella; peritrichous flagella coalesce to form propulsive bundle during run

Direction Depends on Flagellar Rotation - Tumble

clockwise rotation of one or more flagella causes propulsive bundle to fall apart, resulting in tumbling of cell

Chemotaxis is a directed, but random, movement

Biased random walk

• Bacteria sense concentration gradients of attracts and repellents through chemoreceptors known as methyl-accepting chemotaxis proteins (MCPs)

E. coli has 4 MCPs

Tar

Tsr

Trg

Tap

Tar

taxis towards aspartate and maltose, away form nickel and cobalt

Tsr

taxis towards serine, away from leucine, indole and weak acids

Trg

taxis towards galactose and ribose

Tap

taxis towards dipeptides

How is the direction of flagellar rotation controlled?

•CheY is phosphorylated by CheA in response to subtle changes in environmental quality

•CheY~P interacts with flagellar motor to control direction of rotation

•Activity of CheA is modulated by MCPs – binding attractant to MCPs inhibits CheA activity

•Methylation status of MCPs affects CheA activity (CheR adds methyl groups, CheB~P removes methyl groups)

•High [attractant:methylation] ratio = low CheA activity; low [attractant:methylation] ratio = high CheA activity (adaptive response – prevents saturation of the sensory system)

CheY-P levels low

Run

CheY-P levels reach a threshold and trigger…

tumble (switch in direction of rotation)