Difference between revisions of "Part:BBa K1639003"

Revision as of 08:03, 21 September 2015

TlpB

Helicobacter Pylori TlpB protein coding region. This part gives a negative chemotactic response to H+ ions. This makes bacteria capable of moving from acidic pH to more neutral pH levels.

Usage and Biology

Chemotaxis, movement toward or away from chemicals, is a universal attribute of motile cells and organisms. E. coli cells swim toward amino acids (serine and aspartic acid), sugars (maltose, ribose, galactose, glucose), dipeptides, pyrimidines and electron acceptors (oxygen, nitrate, fumarate).

E. coli's optimal foraging strategy

In isotropic chemical environments, E. coli swims in a random walk pattern produced by alternating episodes of counter-clockwise (CCW) and clockwise (CW) flagellar rotation (Fig. 3, left panel). In an attractant or repellent gradient, the cells monitor chemoeffector concentration changes as they move about and use that information to modulate the probability of the next tumbling event (Fig. 3, right panel. These locomotor responses extend runs that take the cells in favorable directions (toward attractants and away from repellents), resulting in net movement toward preferred environments. Brownian motion and spontaneous tumbling episodes frequently knock the cells off course, so they must constantly assess their direction of travel with respect to the chemical gradient.

Figure 1: Random and biased walks. Left: A random walk in isotropic environments. When the cell's motors rotate CCW, the flagellar filaments form a trailing bundle that pushes the cell forward. When one or more of the flagellar motors reverses to CW rotation, that filament undergoes a shape change (owing to the torque reversal) that disrupts the bundle. Until all motors once again turn in the CCW direction, the filaments act independently to push and pull the cell in a chaotic tumbling motion. Tumbling episodes enable the cell to try new, randomly-determined swimming directions. Right A biased walk In a chemoeffector gradient. Sensory information suppresses tumbling whenever the cell happens to head in a favorable direction. The cells cannot head directly up-gradient because they are frequently knocked off course by Brownian motion.

H. pylori is a Gram-negative bacterium that resides in thestomachs of over half the world’s population. Its gastric habitatcontains a marked pH gradient from the highly acidic lumen,which can reach pH 2, to the more neutral environment adjacentto the epithelial lining, which is typically pH 7. Based on genome sequence analysis, the H. pylori chemotaxismachinery resembles that of the well-studied model, E. coli,with a few notable variations including the absence of themethylation enzymes involved in receptor adaptation (Sweeneyand Guillemin, 2011).

The chemotaxis signaling pathway of E.coli

E. coli senses chemoeffector gradients in temporal fashion by comparing current concentrations to those encountered over the past few seconds of travel. E. coli has four transmembrane chemoreceptors, known as methyl-accepting chemotaxis proteins (MCPs), that have periplasmic ligand binding sites and conserved cytoplasmic signaling domains (Fig. 4). MCPs record the cell's recent chemical past in the form of reversible methylation of specific glutamic acid residues in the cytoplasmic signaling domain (open and filled circles in Fig. 4). Whenever current ligand occupancy state fails to coincide with the methylation record, the MCP initiates a motor control response and a feedback circuit that updates the methylation record to achieve sensory adaptation and cessation of the motor response. A fifth MCP-like protein, Aer, mediates aerotactic responses by monitoring redox changes in the electron transport chain. Aer undergoes sensory adaptation through a poorly-understood, methylation-independent mechanism. The five MCP-family receptors in E. coli utilize a common set of cytoplasmic signaling proteins to control flagellar rotation and sensory adaptation (Fig. 4). CheW and CheA generate receptor signals; CheY and CheZ control motor responses; CheR and CheB regulate MCP methylation state.

Figure 2: Signaling components and circuit logic. E. coli receptors employ a common set of cytoplasmic signaling proteins: CheW and CheA interact with receptor molecules to form stable ternary complexes that generate stimulus signals; CheY transmits those signals to the flagellar motors, CheZ controls their lifetime; CheR (methyltransferase) and CheB (methylesterase) regulate MCP methylation state. Abbreviations: OM (outer membrane); PG (peptidoglycan layer of the cell wall); CM (cytoplasmic membrane)

As in many biological signaling systems, the signaling currency in the E. coli chemotaxis pathway is reversible protein phosphorylation (Fig. 5). However, the principal signaling chemistry is a bit different in prokaryotes and eukaryotes. CheA is a kinase that uses ATP to autophosphorylate at a specific histidine residue. Phospho-CheA molecules then serve as donors for autokinase reactions that transfer phosphoryl groups to specific aspartate residues in CheY and CheB. Phospho-CheY enhances CW rotation of the flagellar motors; phospho-CheB has high MCP methylesterase activity. The active forms of these response regulators are short-lived because they quickly lose their phosphoryl group through spontaneous self-hydrolysis. CheZ further enhances the dephosphorylation rate of phospho-CheY to ensure rapid locomotor responses to changes in the supply of signaling phosphoryl groups to CheY.

CheW couples the autophosphorylation activity of CheA molecules to chemoreceptor control. Receptors, CheW, and CheA form stable ternary signaling complexes that modulate the influx of phosphoryl groups to the CheY and CheB proteins in response to chemoeffector stimuli.

Figure 3: Phosphorelay signaling. The flagellar motors of E. coli spin CCW by default; the signaling pathway modulates the level of phospho-CheY, the signal for CW rotation. Reactions and components that augment CW rotation are depicted in green; those that augment CCW rotation are depicted in red.

Chemoreceptor signaling states in E.coli

The signaling activities of chemoreceptors are described by a two-state model (Fig. 6). Receptor complexes in the CW signaling state activate CheA, producing high levels of phospho-CheY. Receptors in the CCW signaling state deactivate CheA, resulting in low levels of phospho-CheY. Thus, the behavior of the flagellar motors reflects the relative proportion of receptor signaling complexes in the kinase-on and kinase-off conformations. Both chemoeffector binding or release and methylation or demethylation can shift receptor signaling complexes from one state to the other. For example, attractant ligands drive receptors toward the kinase-off state; subsequent addition of methyl groups shifts receptors toward the kinase-on state, reestablishing the steady-state (adapted) balance between the two states and restoring random walk movements.

TlpB in Helicobacter Pylori:

H. pylori is a Gram-negative bacterium that resides in thestomachs of over half the world’s population. Its gastric habitatcontains a marked pH gradient from the highly acidic lumen,which can reach pH 2, to the more neutral environment adjacentto the epithelial lining, which is typically pH 7. Based on genome sequence analysis, the H. pylori chemotaxismachinery resembles that of the well-studied model, E. coli,with a few notable variations including the absence of the methylation enzymes involved in receptor adaptation (Sweeneyand Guillemin, 2011).

Sequence and Features