BIOL 1510 Chapter 12

8 shared functions of signaling

1. specificity

2. sensitivity

3. amplification

4. integration

5. divergence

6. localization

7. modulation

8. desensitization/adaptation

Specificity

Signal ligand fits the binding site of its complementary receptor while other ligands do not

• think of it like enzyme-substrate interactions

Sensitivity

How tightly ligand binds its receptor

• low vs high Km

Amplification

Activation of a receptor by a ligand triggers a growing intracellular cascade as a response

• allows for quick responses

Modularity

Allows for sharing of signals between pathways, where the same downstream protein can be activated on different conditions

• usually through phosphorylation

Desensitization/adaptation

Receptor activation triggers a feedback circuit that tones down a response

• can shut receptor off

• can take receptor off the membrane

Integration

Two similar OR opposing signals work together to produce a unified response

Divergence

Same ligand activates two or more pathways

Localized response

response focuses on the area where the message exists in space→signal is degrade before it can diffuse to distant points

Qorum sensing

A certain threshold must be hit to produce a response. Controls group/social behavior in bacteria

Examples of qorum sensing

• DNA uptake

• virulence gene expression

• bioluminescence

How do bacteria communicate?

secreted signals called autoinducers

Types of receptors

• G-protein coupled receptor

• Receptor enzyme (tyrosine kinase)

• ligand gated ion channel

• nuclear receptor

Examples of GPCRs in the human body

• beta-adrenergic receptors

• odorant and tastant receptors

• neurotransmitter receptors→muscarnic ACh and serotonin

• chemotactic receptors

Agonist

activates receptor

Antagonist

deactivates/inhibits receptor

Mechanism of action of GPCR

1. ligand binds extracellular part of 7TM protein

2. 7TM protein changes conformation, causing the heterotrimeric g proin to release GDP and bind GTP instead

3. alpha subunit dissociates from the beta-gamma subunit

4. alpha subunit activates adenylyl cyclase

5. alpha subunit hydrolyzes GTP to GDP, becoming inactive

6. adenylyl cyclase turns ATP into cAMP

7. cAMP activates other protein, such as cAMP

cAMP

a second messenger molecule that is synthesized by a cell in response to an external signal

Mechanism of action of cAMP on PKA

cAMP binds the regulatory subunit of PKA, which induces a conformational change and causes the catalytic subunits to dissociate and become active

A-Kinase Anchoring Proteins

Proteins that bind the regulatory portion of PKA to the cell membrane in order to confine a response to a specific region of a cell

What cellular component explains why many ligands can use the same secondary messengers?

AKAPs

Ways to terminate a signal

• degrade the secondary messenger (cAMP)

• downregulate the receptor

How is cAMP degraded?

degraded by phosphodiesterase

Phosphodiesterase activity

Constitutively active→whether cAMP rises is based on if cAMP product outweights cAMP degradation

beta-adrenergic receptor downregulation

• beta-adrenergic receptor kinase phosphorylates a serine reside on the the receptor’s intracellular c-terminus

• beta-arrestin binds to phosphorylated tail and triggers endocytosis of the receptor

beta-gamma subunit affinity for 7 TM protein

low affinity→has potential to activate other enzymes

GAPs and RGSs

modulators of GTPase activity of alpha subunit

GEFs

GTP-GDP exchange factors

• help alpha subunit exchange GDP for GTP

Non-cAMP dependent GPCR pathway

1. alpha subunit binds protein lipase C (PLC)

2. PLC cleaves PIP2 into IP3 and diacylglycerol (DGA)

3. Increasing IP3 levels leads to Ca2+ release, which can act a secondary messenger

4. diacylglycerol stays in the membrane and activates other enzymes

Effects of Ca2+

Important secondary messenger that binds calmodulin, which can go on to activate other proteins/kinases

Mechanism of action of receptor tyrosine kinases

1. ligand binds to each receptor subunit

2. receptor dimerizes

3. subunits cross-phosphorylate each other, leading to an activation

4. Active receptors trigger cascade of events involving kinases

Grb2

adaptor protein that binds IRS1 and Sos simultaneously to propagate the signal from RTK to kinase cascade

Sos

guanine exchange factor that helps Ras swap GDP for GTP, starting kinase cascade

What kind of receptor is the insulin receptor?

receptor tyrosine kinase

Functions of insulin

• puts more GLUT4 receptors on the membrane

• prevents inactivation of glycogen synthase

Two strategies of cross-talk between beta-adrenergic receptor and insulin receptor

1. Insulin RTK phosphorylates C-terminus tail of beta-adrenergic receptor, leading to its downregulation/sequestration away from cell membrane

2. Insulin RTK phosphorylates another unrelated GPCR, creating a nucleation point with the help of Grb2, which localizes and strengthens insulin signaling

Purpose of insulin receptor beta-adrenergic receptor cross-talk

The two trigger opposing processes:

• epinephrine tells the cell to break down glycogen for energy

• insulin tells the cell to make glycogen for energy storage

Two-component systems

signal transduction pathway in bacteria involving a histidine kinase transmembrane protein and a cytosolic response regulator

Mechanism of action of two component system

1. signal binds to extracellular part of histidine kinase

2. Intracellular His residue phosphorylated

3. Pi group transferred to aspartate residue on response regulator

4. response regulator can go on to alter gene expression

Can multiple histidine kinases be responsible or the same response regultor?

Yes

Where are the phosphate groups coming from in a kinase cascade?

Pi groups are being TRANSFERRED protein to protein. Not getting new ones from ATP

How is BARK recruited?

By beta-gamma subunit

What effect does increasing the concentration of beta gamma subunits have on GPCR mediated signaling?

Dampens. This is because beta gamma subunit binds to the GDP form of alpha, preventing alpha from exchanging GDP for GTP another time

Can cAMP cross the plasma membrane?

No! Charged

Similarity between Gs of GPCR and Ras

Both bind GTP/GDP. Both have intrinsic GTPase activity. Both activate a downstream protein. Both are activated by GTP.

What are heterotrimeric G proteins?

membrane associated GTPases that bind to GPCRs for activation

What are second messengers?

A small intracellular molecule that relays information from the receptor to other parts of the cell