Lecture 23: Cellular Signaling, using chemotaxis as a model system Reading: Alberts Ch 16, Pollard Chapter 24, and Phillips Ch 19.4 Cellular signaling is primarily chemical Cells can detect both chemical and physical signals. Physical signals are generally converted to chemical signals at the level of the receptor. 1
Animal cells depend on multiple extracellular signals Generic signaling pathway Long term Short term 2
Two distinct ways how extracellular signaling molecules interact with cell s molecules Two distinct ways how extracellular signaling molecules interact with cell s molecules 3
Frequently signals act via cell-surface receptors to change the behavior of the target cell 4
Intracellular signaling proteins can relay, amplify, integrate and distribute the incoming signal Many intracellular signaling proteins act as molecular switches 5
Extracellular signals can act fast or slowly Example: E. coli chemotaxis Bacterial cells sense their chemical environments and direct their motion towards attractants and away from repellents. One of the best-studied sensory signal transduction systems in biology Model for 2-component signaling pathways. 6
Run and tumble allows bacteria to climb a gradient in a signaling molecule (motor CCW) (motor CW) move in a straight line when in a higher concentration of signaling molecules (put on the gas) change direction when in a lower concentration of signaling molecules (try another direction) This will allow bacteria to overall move towards higher concentrations E. coli chemotaxis signaling pathway Annu. Rev. Biophys. 2013.42:337-359. Downloaded from www.annualreviews.org Access provided by Syracuse University Library on 11/28/18. For personal use only. A 2 CH3 +CH 3 B ATP P T 2 T 2 W Receptor Y W P Motor Tu. Ann Rev. Biophysics 2013 P Z P i R Ligand Wikipedia X Receptor dimer (T 2 ) CheX Figure 1 Illustration of the Escherichia coli chemotaxis signaling pathway. The label X refers to A, B, R, W, Y, and Z, which represent the key cytosolic chemotaxis proteins CheA, CheB, CheR, CheW, CheY, and CheZ. The phosphate group is represented by the purple circle labeled P. www.annualreviews.org Quantitative Modeling of Bacterial Chemotaxis 339 7
Components of pathway: membrane-bound receptors that interact with molecules in the environment CheW and CheA, proteins that bind to intracellular domain ofthereceptor and changetheir activity when theligand is bound messenger CheY that is phosphorylated by CheA and then interacts with the motor CheZ: removes phosphate from CheY CheR and CheB: methylate and demethylate the receptor, allowing it to adapt to vastly different overall concentrations of the signaling molecules Outline of the signal transduction pathway: If there is an attractant, the ligand is bound to receptor this causes the receptor + CheW+CheA to be (more likely) in an OFF state, where CheY is not phosphorylated In the absence of phosphorylated Che Y, the motor moves CCW, creating run behavior This makes the bacteria move in a straight line. 8
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Measuring CheY-P directly with FRET 10
Adaptation: cells can sense gradient even with HUGE changes in overall concentration. MWC model for Chemotaxis pathway On = CheY-P; Off = CheY 11
Two-component signaling pathway 12
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Annual Reviews There are three main classes of cell surface receptors 16
Signaling Mechanisms- Membrane Receptors, Transduction and Second Messengers 17
Classes of Receptors Pollard Figure 24-1 G-Protein Coupled Receptors 18
Activated GPCR activates G proteins by GDP -> GTP exchange G-protein α subunit switches itself off by hydrolyzing its bound GTP to GDP 19
Some GPCRs directly couple to ion channels Enzymes activated by G proteins increase the concentrations of small intracellular signaling molecules, such as camp 20
Example: Adrenaline induced glycogen breakdown in skeletal muscle cells Rise in intracellular camp can activate gene transcription 21
Phospholipase C activated signaling pathways Lipids as second messengers 22
More stages of GPCR signaling Pollard Figure 24-3 Receptor tyrosine kinases (RTKs) and example for enzyme coupled receptors 23
There is much variety in the RTK family Pollard Figure 24-4 RTKs activate Ras 24
Ras activates a MAP-kinase signaling module Activation of the EGR Receptor Tyrosine Kinase Pollard Figure 24-5 25
Both GPCRs and RTKs activate multiple intracellular signaling pathways Intracellular signal integration 26