E. coli chemotaxis

E. coli cells swim toward amino acids (serine and aspartic acid), sugars (maltose, ribose, galactose, glucose), dipeptides, pyrimidines and electron acceptors (oxygen, nitrate, fumarate).  The cells swim away from potentially noxious chemicals, such as alcohols and fatty acids.  These gradient-tracking behaviors rely on exquisitely sensitive chemoreceptors, known as methyl-accepting chemotaxis proteins (MCPs) that can detect concentration differences as small as 0.1%.  Changes in receptor ligand occupancy modulate the activity of a cytoplasmic, receptor-associated kinase (CheA) to control the flux of signaling phosphoryl groups to two effector proteins: CheY~P controls the direction of rotation of the cell's flagellar motors; CheB~P modulates the sensitive range of the receptors through a feedback sensory adaptation circuit.  The interplay of rapid (~100 msec) motor responses and slow (3-4 second) sensory adaptation enables the cells to "remember" chemical conditions in their recent past and to determine their direction of travel in chemical gradients. (more information)

Our studies of E. coli chemotaxis address - in molecular detail - three mechanistic questions common to many signal transduction systems in biology.

Transmembrane signaling - How do chemoreceptor molecules communicate sensory information between their stimulus-sensing (input) domains and their signaling (output) domains?

Attractant ligands induce an asymmetric conformational change in receptor molecules that involves a small (~2 Å) displacement of one membrane-spanning segment toward the cytoplasm.  This "piston" motion in turn favors a large, symmetric conformational change in the kinase-controlling portion of the receptor.  A 50-residue HAMP domain lies between the input and output regions and negotiates their conformational interactions.

HAMP domains play central roles in many types of bacterial signaling proteins and are key to understanding the transmembrane signaling mechanism in chemoreceptors.  Accordingly, we are conducting an extensive genetic analysis of the HAMP domain in the serine receptor (Tsr) to: (i) establish its in vivo structure in the kinase-ON and kinase-OFF signaling states; (ii) determine how ligand-binding signals modulate transitions between HAMP signaling conformations; (iii) determine how HAMP signal states influence the structure and activity of the kinase-controlling output domain of the receptor.

Kinase control - How do receptors modulate the autokinase activity of their associated CheA molecules? 

The CheA histidine autokinase functions as a homodimer with five discrete domains.  Autophosphorylation involves interaction of the phosphorylation site (substrate) domain in one subunit with the ATP-binding (catalytic) domain in the other subunit.  Chemoreceptors, in conjunction with the small CheW adapter protein, most likely control CheA activity in allosteric fashion, by modulating interactions between the substrate and catalytic domains.  The structure and function of the ternary receptor/CheW/CheA signaling complex are poorly understood.  To better understand the signaling complex, we are conducting genetic, biochemical, and structural studies of Tsr, CheW, and CheA to: (i) identify the critical binding determinants in each protein for ternary complex assembly; (ii) test various models of CheA activtion and control; (iii) develop mini-CheA and single-chain, trimeric CheW proteins to facilitate structural studies of the ternary complex.

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Signal amplification - What is the source of the prodigious signal gain in chemoreceptor signaling complexes? 

When a single chemoreceptor molecule changes signaling state, it modulates approximately three dozen kinase molecules, nearly all of which are associated with other receptor molecules.  Different receptors appear to act in highly cooperative fashion through physical connections that link them into a large "antenna array".  Our studies have shown that the fundamental unit of receptor signaling (a "team") is based on trimers of receptor dimers.  Different types of receptors can form mixed trimers that cooperatively control their associated CheA molecule(s).  In the receptor network, different trimer teams probably communicate through shared CheW/CheA connections.  These recent findings have raised intriguing questions that are currently under active study in our group.  For example: (i) What is the structure of a receptor team? (ii) How are different receptor teams networked in the array?  How is stimulus information relayed between receptor teams?  (iv) What factors influence the effective size of a receptor array?  (v) How is the array assembled and why is it located at the pole of the cell?  (vi) How does the receptor array integrate multiple or conflicting stimulus inputs? (more information)

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