Cell signalling 2: GPCR signalling

Question 1

3 main classes of cell surface signalling receptor

Answer 1

ion-channel coupled receptors
GPCR
Enzyme coupled receptors

Question 2

Ion channel coupled receptors

Answer 2

transmitter gated ion channels

Change conformation allowing ions to enter and depolarise the membrane (3 sorts of conformational change)

Question 3

GPCRs

Answer 3

Receptors couple to G proteins - leads to raised levels of second messenger
No intrinsic enzymatic activity

Question 4

Enzyme coupled receptors

Answer 4

Do have intrinsic enzymatic activity

Signals activate enzyme activity integral to the receptor

Question 5

Changes in protein conformation allow

Answer 5

switching between active and inactive states- reversible

Question 6

GPCRs

Answer 6

7 TM domains- 7 regions of hydrophobic AAs
Found in all eukaryotes- conserved evolutionarily over long time period
Many functions and receive wide range of signals eg odorants, hormones and nts

Question 7

GPCR structure

Answer 7

Inactive G protein tethered to inner leaflet of plasma membrane and alpha subunit associated with a molecule of GDP
G protein made up of alpha, beta and gamma subunit
Alpha and beta subunits have lipid side chains to associate with the membrane

Question 8

alpha subunits are

Answer 8

GTPase enzymes

Hydrolyse GTP into GDP

Question 9

How does GPCR signalling work

Answer 9

Signal binding changes the receptor conformation and allows binding of the G protein the receptor
The receptor-G protein interaction alters the G protein conformation and promotes release of GDP, which is replaced by GTP
Causes a further conform change in the alpha subunit, which is activated and dissociates both from the B/gamma complex and from the receptor (keeps GTP)

Activated alpha with GTP interacts with another target protein (membrane associated)

Question 10

How can rapid responses to signals occur

Answer 10

Rapid responses to signals occur in some cells through GPCR signalling in which the activate beta-gamma subunit interacts with and opens ion channels on the cell surface (depolarisation)

Question 11

Interaction of active alpha/GTP subunit

Answer 11

associates with specific target enzymes to activate their activity
Target enzyme remains active while bound to alpha/GTP subunit and will propagate the signal

Question 12

What does interaction with its target enzyme do

Answer 12

Activates GTPase activity of alpha/GTP subunit. After a short delay the alpha subunit hydrolyses GTP to GDP, , which restores the inactive alpha subunit conformation, causing its release from target enzyme
Restores resting state unless signal still present

Question 13

What do the enzymes do

Answer 13

When activated by alpha/GTP subunits they raise levels of small second messenger molecules, diffuse to act on intracellular signalling proteins

Question 14

GsG proteins

Answer 14

raise level of cAMP

Question 15

GqG proteins

Answer 15

raise levels of Ca2+ ions

Question 16

alpha Gs

Answer 16

stimulates adenlyl cyclase

catalyses cyclization of adenosine triphosphate (ATP) into cyclic adenosine monophosphate (cAMP)

Question 17

High levels of cAMP activates

Answer 17

A-kinase

Question 18

Structure of A-kinase

Answer 18

A-kinase is a 4 protein complex in its resting state. There are 2 regulatory subunits and 2 inactive catalytic subunits which are proteins that act as a kinase

Question 19

How does cAMP activate A-kinase

Answer 19

One cAMP binds to regulatory subunits, the catalytic ones are released and become active kinase molecules .

Question 20

What can A-kinase do

Answer 20

They phosphorylate cytoplasmic enzymes to change their activity

And phosphorylates TFs that can alter gene expression- can activate them

Activate cytoplasmic proteins eg to release stored energy

Question 21

alpha Gq

Answer 21

stimulates phosholipase C

Question 22

How does stimulation of phospholipase C work and what are the effects

Answer 22

Activated alpha subunit changes conformation
Produces IP3- main target is CA channels, interaction opens channels (on ER)
In cytoplasm the CA conc is low- so pumps on the cell surface pump it out and pumps on ER shuttle it in
Ca rises rapidly when Ca channels open- binds to protein kinase C , which associates with diacylglycerol and becomes activated

Question 23

Calmodulin

Answer 23

2 binding sites for Ca at each end

Question 24

Calmodulin activation

Answer 24

conform change when Ca binds
When it encounters target proteins it can wrap around them- activate them and change their conformation and activity
Mainly the Ca/calmodulin modulated kinases are the proteins calmodulin interacts with

Question 25

The Gs and Gq signalling pathways lead to

Answer 25

Changes in altered cytoplasmic protein function and altered gene expression