Learn: BCM 475 Chapter 14 - Signal Transduction

Three pathways that I care about

beta-adrenergic receptor, insulin receptor, EGF receptor

What does the beta-adrenergic receptor do?

mobilizes stored energy to be turned into usable energy

What does the insulin receptor do?

increase glucose uptake by turning glucose into glycogen(stored energy)

What does the EGF receptor do?

expresses growth-promoting genes

Signal Transduction

process in which environmental change, hormone, or chemical signal is transmitted through cell, resulting in change in cellular processes

Basic signal transduction cascade steps

1. release of primary message as response to physiological circumstance

2. reception of primary message by receptor

3. relay of message by generation of second messenger

4. activation of effector molecules by second messenger

5. termination

common secondary messengers

cAMP, cGMP, Ca2+, IP3, DAG

7TM receptor

type of receptor with 7 transmembrane helices with many different types and functions

ligand binding outside induces detectable change inside of the cell

How does a 7TM receptor work?

binding of substrate outside of cell on receptor causes structural change that can be detected on the inside of the cell

G protein

aka guanine nucleotide-binding proteins, family of proteins that act as molecular switches inside of cells

-transmit signals from stimuli outside of a cell to its interior

What regulates G protein activity?

ability to bind to GTP and ability to hydrolyze GTP to GDP

G proteins on vs off

G proteins are on when they are bound to GTP

G proteins are off when they are bound to GDP

GTPase

larger group of enzymes which G proteins belong to

What occurs when epi binds to receptor?

alpha subunit dissociates from beta+gamma dimer and exchanges GDP for GTP

this alphas-subunit bound to GTP transmits signal to other cellular components

another name for 7TM receptors

GPCR: g protein coupled receptors

always associated with G proteins

What does the activated Gas activate in the epi pathway?

stimulates adenylate cyclase

What does adenylate cyclase do?

converts ATP into cAMP

what does activation of adenylate cyclase do in this cascade?

produces the secondary messenger cAMP

What does cAMP activate in the epi pathway?

PKA by binding and freeing its catalytic subunits, allowing for further phosphorylation of many targets

How do G proteins reset themselves?

1. resets via GTPase activity which cleaves bound GTP into GDP, then reassociates with beta+gamma dimer

2. epi receptor is reversible, when concentration of epi falls, receptor will no longer be active

3. signal cascade activates beta-adrenergic receptor kinase, which phosphorylates and inactivates receptor

4. beta-arrestin protein binds to phosphorylated receptor, inhibiting ability to activate G-protein

Phosphoinositide Cascade

activated by some GPCR

-involved Gaq protein in trimeric G protein complex

purpose is to activate PKC

Phosphoinositide cascade mechanism

1. Gaq activates phospholipase C, cleaves phosphatidylinositol bisphosphate into IP3 and DAG

2. IP3 binds to IP3 gated channel in ER, allowing influx of Ca2+ ions into cytoplasm

3. DAG and Ca2+ activate Protein Kinase C, SER/THR kinase

Insulin signalling cascade receptor type

receptor tyrosine kinase

Insulin receptor structure

Dimer of two alpha(out) and two beta(in) subunits

Where does kinase activity reside in insulin receptor?

the two beta subunits on the inside of the cell, only upon crossphosphorylation of the two beta subunits

How does the receptor tyrosine kinase of the insulin receptor activate?

when insulin binds to receptor on alpha subunits, beta subunits are cross phosphorylated and activated

-now will phosphorylate targets

What does the activated beta subunits of the insulin receptor phosphorylate to transmit its signal?

beta subunits phosphorylate insulin-receptor substrates(IRSs), which start signal transduction pathway

What is the final purpose of insulin receptor pathway?

increased glucose uptake, converting glucose into glycogen, removing sugar from blood stream

How is the insulin pathway terminated?

three types of phosphatase enzymes

1. protein tyrosine phosphatase - removes {P} from tyrosine residues on receptor and IRS proteins

2. Iipid phosphatases - hydrolyzes PIP3 to PIP2

3. Protein Serine Phosphatases - remove {P} from activated protein kinases

How are the phosphatases that terminate signaling recruited?

they are activated or recruited by response to insulin, so that initial binding also has a means of termination of signal

EGFR

epidermal growth factor receptor

EGFR structure

4 domains

-EGF binding domain

-transmembrane domain

-kinase domain

-tyrosine rich C-terminal domain

exists as monomer, dimerizes when each monomer binds EGF

EGFR dimerization mechanism

binding of EGF free dimerization arm which allows two receptors to interact

What occurs after dimerization of EGFR?

C-terminal domain of one receptor is put into active site of parter, allowing cross-phosphorylation

kinase is in active conformatoin without phosphorylation, meaning it responds quickly to phosphorylation, saves time basically

What is Ras?

small G protein

What happens when insulin receptor is activated?

phosphotyrosines of tyrosine rich c-terminal domains act as docking site for other domains, specifically Grb2

What docks to activated EGFR?

Grb2

What does Grb2 bind to after docking?

Sos

What does Sos do after being bound by Grb2?

binds to and activates Ras by facilitating release of GDP and allowing formation of Ras-GTP complex

What does Ras do in the EFGR cascade?

initiates kinase cascade

What is the mechanism of Ras intiating protein kinase cascade?

1. Ras binds and activates Raf(kinase)

2. Raf, activates MEK

3. MEK activate ERK

4. ERK phosphorylate many things to signal growth factors

Termination methods for EGFR cascade

1. EGF signalling serine/threonine phosphatases and tyrosine phosphatases

2. Ras inherent GTPase activity hydrolyzes GTP into GDP, turns off

3. GTPase activity of Ras can be enhanced by GTPase-activating proteins

Cholera

infection caused bymodification of Gas protein which stabilizes GTP-Gas complex, continuously activating PKA

-result in excessive loss of NaCl and water

Whooping Cough

toxin modifies Gai, which prevents binding of Gi, locking it in off state