Two Component system.pptx

Two component system

1. BY SHARIQ AISHA DEPARTMENT OF BIORESOURCES UNIVERSITY OF KASHMIR [email protected]

2. Two component system   Two- component signaling system is the most common form of signaling pathway that responds to extracellular events in bacteria and plants. Protein phosphorylation is the key mechanism for regulating signal transduction pathways in both prokaryotes and eukaryotes . TWO COMPONENT SIGNALING MECHANISMS IN BACTERIAL CHEMOTAXIS   In pioneering studies of chemotaxis in bacteria, Julius Adler showed that Escherichia coli responds to nutrients in its environment, including sugars and amino acids, by swimming toward them, propelled by one or a few flagella. A family of membrane proteins have binding domains on the outside of the plasma membrane to which specific attractants/ repellents bind. Ligand binding causes an intrinsic kinase activity of the receptor to phosphorylate a His residue in its cytosolic domain.

3. This first component of the two-component system, the receptor histidine kinase, then catalyzes transfer of the phosphoryl group from the His residue to an Asp residue on a second, soluble protein, the response regulator.

4. MOLECULAR MECHANISM OF TWO COMPONENT PHOSPHORYLATION RELAY SYSTEM   Genetic studies indicate that four cytoplasmic proteins - CheA , CheW , CheY , and CheZ - are involved in the intracellular signaling pathway that couples the chemotactic receptors to the flagellar motor.   CheY acts at the effector end of the pathway to control the direction of flagellar rotation. When activated, it binds to the motor, causing it to rotate clockwise and thereby inducing tumbling; mutants that lack this protein swim constantly without tumbling.   CheA is a histidine protein kinase. When bound to both an activated chemotactic receptor and CheW , it phosphorylates itself on a histidine residue and almost immediately transfers the phosphate to an aspartic acid residue on CheY ’  

5. The phosphorylation of CheY activates the protein so that it binds to the flagellar motor and causes clockwise rotation and tumbling.   CheZ rapidly inactivates phosphorylated CheY by stimulating its dephosphorylation .

6. The binding of a repellent to a chemotactic receptor increases the activity of the receptor, which in turn increases the activity of CheA and thereby the phosphorylation of CheY , which causes tumbling.   These phosphorylations occur rapidly: the time required for the tumbling response after adding a repellent is about 200 milliseconds.   The binding of an attractant has the opposite effect. It decreases the activity of the receptor, which decreases the activity of CheA , so that CheY remains dephosphorylated, the motor continues to rotate counterclockwise, and the bacterium swims smoothly.   In the absence of any environmental stimulus, the direction of rotation of the disc reverses every few seconds, producing a characteristic pattern of movement in which smooth swimming in a straight line is interrupted by abrupt, random changes in direction caused by tumbling.

7. CHEMOTAXIS RECEPTORS   There are four types of plasma membrane chemotaxis receptors, each concerned with the response to a small group of chemicals. Type 1 and 2 receptors mediate responses to serine and aspartate, respectively, by directly binding these amino acids and transducing the binding event into an intracellular signal. Type 3 and 4 receptors mediate responses to sugars and dipeptides, respectively, in a slightly less direct fashion.

8. Plant Two Component system

9. CYTOKININ TWO-COMPONENT SIGNALING(IN PLANTS)   Cytokinins are perceived and transduced by a two- component signaling system . Typically these systems are composed of two functional elements: a sensor histidine kinase, to which a signal binds, and a downstream response regulator, whose activity is regulated via phosphorylation by the sensor histidine kinase . TYPES OF TWO-COMPONENT SIGNALING SYSTEM SIMPLE TWO-COMPONENT SIGNALING SYSTEM In simple two-component systems, the input domain is the site where the signal is sensed. This regulates the activity of the histidine kinase domain, which when activated autophosphorylates on a conserved His residue. The phosphate is then transferred to an Asp residue that resides within the receiver domain of a response regulator. Phosphorylation of this Asp regulates the activity of the output domain of the response regulator, which in many cases is a transcription factor

10. PHOSPHORELAY TWO-COMPONENT SIGNALING SYSTEM In the phosphorelay -type two-component signaling system, an extra set of phosphotransfers is mediated by a histidine phosphotransfer protein ( Hpt ), called AHP in Arabidopsis. The Arabidopsis response regulators are called ARRs. H = histidine , D = aspartate.    

11. MODEL FOR CYTOKININ SIGNAL TRANSDUCTION VIA MULTISTEP PHOSPHORELAY SYSTEM

12. STEPS Cytokinin binds to CRE1 ( cytokinin response receptor-1), which is likely to occur as a dimer.Cytokinin binds to an extracellular portion of CRE1 called the CHASE domain.Two other hybrid sensor kinases (AHK2 and AHK3) containing a CHASE domain are also likely to act as cytokinin receptors in Arabidopsis. 2 . Cytokinin binding to these receptors activates their histidine kinase activity. The phosphate is transferred to an asparate residue (D) on the fused receiver domains . 3. The phosphate is then transferred to a conserved histidine present in an AHP protein (Arabidopsis Histidine Phosphotransfer protein ).

13. 4 . Phosphorylation causes the AHP protein to move into the nucleus, where it transfers the phosphate to an asparate residue located within the receiver domain of a type-B ARR. 5 . The phosphorylation of the type-B ARR activates the output domain to induce transcription of genes encoding type-A ARRs.   6 . The type-A ARRs are likely also to be phosphorylated by the AHP proteins.   7 . The phosphorylated type-ARRs interact with various effectors to mediate the changes in cell function appropriate to cytokinin (indicated in the model as " cytokinin responses").

14. REFRENCES : Lehninger Principles of Biochemistry(David and Michael M. Cox ) Molecular Biology of The Cell (Bruce Alberts ) Plant Physiology (Lincoln Taiz and Eduardo Zeiger ) Introduction to Plant Physiology ( Willaim G. Hopkins and Norman P. A. Huner )

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