Chemotaxis

What’s Chemotaxis?

• Behaviour of cells moving towards attractants and away from toxins

What causes Chemotaxis?

• Occurs when cells sense a change in concentration of a chemical over time rather than the absolute conformation of the chemical stimulus

What directs direction of flagellar rotation?

• Sensed information

How does pseudomonus aeruginosa move?

• It moves towards damaged epithelial cells to scavenge nutrients

What does the mechanism of Chemotaxis depend on?

• It depends on a signal cascade of multiple proteins

• Sensory proteins sense attractants/ repellants and interact with cytoplasmic sensor kinases which form methyl accepting proteins (MCPs)

What do methyl-accepting Chemotaxis proteins (MCPs) do?

• Thousands of them cluster to form hexagonal arrays known as chemoreceptors which are located in the cytoplasmic membrane and/ or cytoplasm

What kind of chemoreceptors does Vibrio sp contain?

• Transmembrane and cytoplasmic chemoreceptors

What kind of chemoreceptors does E.coli have?

• Only transmembrane chemoreceptors

How many transmembrane chemoreceptors are in e.coli and explain them

• Ecoli has 5 different MCPs that are specific for certain compounds

  • Eg: the Tar MCP senses the attractants aspartate and maltose, and the repellents cobalts and nickel

How do MCPs bind attractants and repellents?

• It binds to them directly

What is CheA for Chemotaxis?

• Sensor kinase

What is Che Y in Chemotaxis?

• Response regulator

What happens when MCP binds to and attractant or repellant initially?

• It triggers interactions with the cytoplasmic proteins CheA and CheW

What happens when MCPs binds to an attractant OR release a repellant?

• Coupling protein CheW is inactive and auto phosphorylation of CheA is inhibited

• No binding of CheY so it can’t bid to the motor so counterclockwise flagella rotation continues (cells move in a run; swim smoothly)

What happens when MCP binds a repellent OR release an attractant?

• Conformational change is induced and CheA is autophosphorylated to become CheA-P

• CheA-P then phosphorylates CheY to become CheY-P which leads to clockwise flagellar rotation and tumbling cells (which means they move randomly)

• CheZ dephosphorylates CheY and returns the cells to “runs” (more ordered?)

What happens when there’s an increase in attractant concentration?

• Less CheY-P

• Promotes runs because tumbles get repressed

What happens when theres an increase in repellant concentration?

• More CheY-P

• Promotes runs as tumbles get surpressed

What happens once stimulus has been responded to?

• Sensory system needs to reset to await further signals → adaption

What does varying methylation of MCPs do?

• It allows adaption to sensory signals

What do fully methylated MCPs do?

• They no longer respond to attractants but are sensitive to repellents

What do unmethylated MCPs do?

• They respond strongly to attractants but are insensitive to repellents

Explain the control of methylation

• Chemotaxis protein CheR methylates MCPs

• Chemotaxis protein CheB-P demethylate MCPs

What occurs when there’s a high level of attractants?

• Rate of autophosphorylated CheA is low, which leads to unphosphorylated CheY and CheB which causes the cell to swim smoothly in a run

• Methylation of the MCPs increases during this period cuz CheB-P isn’t present to demethylate cuz there’s no CheA-P

• Fully methylated MCPs no longer respond to the attractant (i.e constant level) “become noseblind” so attractant is released so CheW helps CheA-P → CheY-P (tumbling cells) and CheB-P (demethylation) to reset receptors

What happens when there’s a high level of repellents?

• Rate of autophosphorylated CheA is high, which leads to CheY-P (tumbling cells) and CheB-P (demthylation)

• Fully methylated MCPs respond best to an increasing gradients of repellents and send a signal for cell tumbling to move off in a random direction while MCPs are slowly demthylated

What does Chemotaxis achieve?

• It achieves the ability to monitor small changes in concentrations (gradients) of both attractants and repellents over time