Cell CommunicationCell Communication

Signal-transduction Signal-transduction pathwaypathway

Cell signaling is the way that cells Cell signaling is the way that cells have to respond to external stimulihave to respond to external stimuli• Increase in temperatureIncrease in temperature• Fight of flight responseFight of flight response• Increased aerobic activityIncreased aerobic activity

In each case, In each case, signal transduction signal transduction pathways pathways are a key component in are a key component in cell communicationcell communication

Signal-transduction Signal-transduction pathwaypathway

SignalSignal on a cell’s surface is converted into a specific on a cell’s surface is converted into a specific cellular cellular responseresponse (binds to a receptor) (binds to a receptor)

TransducedTransduced = changed (protein changes shape) = changed (protein changes shape) Cell junctions connect cell to cellCell junctions connect cell to cell

• Animal – gap junctionsAnimal – gap junctions• Plant – plasmodesmataPlant – plasmodesmata

Direct Contact communicationDirect Contact communication• Cell –cell recognition, important in development and immune systemCell –cell recognition, important in development and immune system

Local signaling (short distance): Paracrine (Local regulators - growth factors)Synaptic (neurotransmitters – electrical to chemical signal)

Long distance: hormones

HormonesHormones

Endocrine SystemEndocrine System Travel through Travel through

the circulatory the circulatory systemsystem

Plant hormones = Plant hormones = growth regulators growth regulators

Signal transductionSignal transduction

Stages of cell signalingStages of cell signaling 3 steps:3 steps:

1.1. Reception: target cell detection Reception: target cell detection

2.2. Transduction: single-step or series of Transduction: single-step or series of changeschanges

3.3. Response: triggering of a specific cellular Response: triggering of a specific cellular responseresponse

Cell signalingCell signaling

Step 1: ReceptionStep 1: Reception

Only Only Target CellsTarget Cells have have the receptor to bind the the receptor to bind the signal moleculesignal molecule

Signal molecule is a Signal molecule is a ligandligand – only binds to – only binds to specific moleculesspecific molecules

Binding Usually changes Binding Usually changes shape of receptorshape of receptor• Activates receptorActivates receptor• Causes aggregation Causes aggregation

(clumping)(clumping)

Step 2: TransductionStep 2: Transduction

Usually multistepUsually multistep Amplifies Amplifies

responseresponse Mostly involves Mostly involves

proteinsproteins Relay molecules Relay molecules

are activated or are activated or deactivated by deactivated by phosphorylationphosphorylation

Protein phosphorylationProtein phosphorylation

Protein activity regulation (ON/OFF switch)Protein activity regulation (ON/OFF switch) Adding phosphate from ATP to a protein Adding phosphate from ATP to a protein

activates proteinsactivates proteins Enzyme: protein Enzyme: protein kinaseskinases (1% of all our (1% of all our

genes) genes) Reversal enzyme: protein phosphatasesReversal enzyme: protein phosphatases

• Removes the phosphate groupRemoves the phosphate group• Phosphorylation (activation) is only temporaryPhosphorylation (activation) is only temporary• Do not want the protein to be continually “ON”Do not want the protein to be continually “ON”

Second messengersSecond messengers

Non-protein signaling Non-protein signaling pathway pathway

Ex: cyclic AMP (cAMP)Ex: cyclic AMP (cAMP) Ex: Glycogen breakdown Ex: Glycogen breakdown

with epinephrinewith epinephrine• Epinephrine activates Epinephrine activates

receptor in membranereceptor in membrane Enzyme: adenylyl cyclaseEnzyme: adenylyl cyclase G-protein-linked receptor G-protein-linked receptor

in membrane (guanosine in membrane (guanosine di- or tri- phosphate)di- or tri- phosphate)

Cellular responses to Cellular responses to signalssignals

Cytoplasmic Cytoplasmic activity regulationactivity regulation

Cell metabolism Cell metabolism regulationregulation

Nuclear Nuclear transcription transcription regulationregulation

Plant and Animal Plant and Animal HormonesHormones

G-Protein-Linked G-Protein-Linked ReceptorsReceptors

One example of secondary messenger One example of secondary messenger is the G-protein-linked receptorsis the G-protein-linked receptors

We will discuss the steps as an exampleWe will discuss the steps as an example• Step 1: There is a “loosely” bound Step 1: There is a “loosely” bound

protein bound to the cytosol side of protein bound to the cytosol side of the plasma membrane (G-protein)the plasma membrane (G-protein)– Inactive if bound with GDPInactive if bound with GDP– Active if bound with GTP (like ATP, Active if bound with GTP (like ATP,

but with guanine)but with guanine)

G-Protein-Linked G-Protein-Linked ReceptorsReceptors

Step 2: Another Step 2: Another protein acts as protein acts as the receptor the receptor proteinprotein• When a chemical When a chemical

signal binds to the signal binds to the receptor protein, receptor protein, changes shape changes shape and allows the G-and allows the G-protein to bindprotein to bind

G-Protein-Linked G-Protein-Linked ReceptorsReceptors

Step 3: When the G-Step 3: When the G-protein binds to the protein binds to the receptor protein, the receptor protein, the G-protein changes G-protein changes shapeshape

Step 4: The new Step 4: The new conformation of the conformation of the G-protein causes GTP G-protein causes GTP to displace GDP (G-to displace GDP (G-protein is now protein is now active)active)

G-Protein-Linked G-Protein-Linked ReceptorsReceptors

Step 5: Activate G-Step 5: Activate G-protein now moves protein now moves free of the receptor free of the receptor proteinprotein

Step 6: The G-Step 6: The G-protein moves along protein moves along the plasma the plasma membrane until it membrane until it binds to another binds to another membrane protein membrane protein (enzyme)(enzyme)

G-Protein-Linked G-Protein-Linked ReceptorsReceptors

Step 7: The G-Step 7: The G-protein binds to the protein binds to the membrane-bound membrane-bound enzyme and changes enzyme and changes its shape (activates its shape (activates the enzyme)the enzyme)

Step 8: Allows the Step 8: Allows the enzyme to perform enzyme to perform its function (ie. its function (ie. Convert ATP to Convert ATP to cAMP)cAMP)

G-Protein-Linked G-Protein-Linked ReceptorsReceptors

Note: The G-protein is Note: The G-protein is also an enzymealso an enzyme• It acts as a GTPase It acts as a GTPase

enzyme (catalyzes the enzyme (catalyzes the breakdown of GTP to breakdown of GTP to GDP)GDP)

• This takes a little bit of This takes a little bit of time so the G-protein time so the G-protein can be active for a can be active for a limited timelimited time

• This allows the G-This allows the G-protein to “regenerate”protein to “regenerate”