CH2 - Cell Locomotion and Chemotaxis

Lecture 7

motility allows cells to

reach different parts of their environment where resources may be better

two major types of prokaryotic cell movement:

swimming and gliding

directed movement toward or away from a particular stimulus/signal =

taxes or taxis

flagella:

structures that assist in swimming in Bacteria (analogous structure in archaea called archaella)

flagella structure

long, thin appendages (15-20 nm wide) that act as tiny rotating machines that push/pull the cell through a liquid

flagella increase or decrease

rotational speed relative to strength of proton motive force

flagella arrangement on bacteria - polar

singular flagella attach to one or both ends of a cell

flagella arrangement on bacteria - lophotrichous

multiple flagella attached to one pole

flagella arrangement on bacteria - peritrichous

singular flagella attached through cell

flagella arrangement on bacteria - amphitrichous

multiple flagella attached to both poles of the cell

flagella are

helical with a constant distance between curves (called wavelength)

flagellar filament is composed of

many copies of the protein flagellin

flagellin

• affects the shape, wavelength, and direction of rotation of the flagellum

  • amino acid is highly conserved across bacterial species

flagellum motor consists of

a central rod that passes through a series of rings

gram negative bacteria has

• outer ring (L ring)

• P ring

• MS ring

• C ring

outer ring (L ring)

anchored in outer ring

P ring

anchored in peptidoglycan layer

MS ring

located in cytoplasmic membrane

C ring

located in cytoplasm

gram-postive bacteia flagella

only inner rings are present

flagella rotor

consists of central rod and L, P, C< and MS rings; together make up the basal body

flagella stator

consists of the motor proteins; functions to generate torque

protein turbine process:

• Protons are translocated thru the Stator Mot complex (~1200 protons/flagellum rotation) → exert electrostatic forces on helically arranged charges on the rotor proteins

• Alternating attractions between positive and negative charges on the rotor as protons flow thru cause the basal body to rotate; speed set by proton flow rate
  

flagella synthesis - assembly

• MS ring is made first and inserted into cytoplasmic membrane

• Other anchoring proteins and hook are made next

• Flagellin molecules synthesized in the cytoplasm pass thru 3-nm channel inside flagellar filament → added to end to form the flagellum (filament grows from the tip, not the base); cap assists in positioning flagellin molecules at growing end

archaeal flagellum analog =

archaellum

archaeallum

• half the diameter of bacterial

• provide motility by rotation

• composed of several different filament proteins

archaella are also capable of both

clockwise and counterpoise rotation

archralla rotation energy

ATP hydrolysis

surface motility

• cells are typically filamentous or rod-shaped

• slower and smoother than swimming

• requires surface contact and movement typically occurs away from colony

• can be aided by secretion of extracellular polysaccharides

twitching motility

repeated extension and retraction of type IV pili drag the cell along the surface

• seems in bacteria and archaea

• energy from ATP

gliding motility

• adhesion complexes or other specialized proteins located in the cytoplasmic and outer membranes

• energy from proton motive force

• found in bacteria but not archaea

taxis =

directed movement in response to chemical or physical gradients

chemotaxis

response to chemicals

phototaxis

response to light

aerotaxis

response to oxygen

osmotaxis

response to ionic strength

hydrotaxis

response to water

chemotaxis found in both

swimming and gliding bacteria species and in swimming archaea

chemotaxis in flagellated bacteria - in the absence of a chemical gradient, cells move in

a random fashion - “run and tumble” behavior

run

smooth forward motion, flagellar motor rotates counterclockwise

tumble

stops and jiggles, flagellar motor rotates clockwise, flagellar bundle comes apart

chemotaxis in Peritrichously flagellated bacteria - in the presence of a chemical attractant,

these moments become biased

• when moving towards attractant, runs become longer and tumbles occur less frequently

chemotaxis in Peritrichously flagellated bacteria - chemical gradient sensing

• cells sample the chemicals in their environment periodically and

• compare the concentration of a particular chemical with the concentration sensed a few moments earlier

  • thus, bacteria respond to temporal, not spatial, differences in chemical concentrations

chemotaxis in Peritrichously flagellated bacteria - attractants/repellants are sensed via

membrane proteins called chemoreceptors

chemotaxis in Peritrichously flagellated bacteria - sensory information fed through

cascade of proteins → affect flagellar motor rotational direction

chemotaxis is polarity flagellated bacteria - many can

reverse direction of flagellar rotation; avoiding tumbling

• some cells have only a single flagellum that can rotate in only one direction

measuring chemotaxis is the lab

• Inserting a capillary tube containing an attractant or a repellent in a medium of motile bacteria

• Gradient of attractant forms from the tip of the capillary tube into the surrounding medium (concentration gradually decreases with distance from the tip)

• Chemotactic bacteria move toward capillary tube, swarm
  around the open tip and some swimming up into the tube

• With repellants (bacteria move away from tube)

phototaxis

allows phototrophic organisms to optimize position to harvest light for photosynthesis

to measure photosynthesis

• spread light spectrum across a slide of motile phototrophic bacteria

• bacteria move to and accumulate at light wavelengths their photosynthetic pigments absorb

• with highly motile bacteria, entire colonies move in unison toward the light

scotophobotaxis

entering darkness cause cell to tumble, reverse direction, enter another run in order to head back toward the light