Cell signalling 2: GPCR signalling Flashcards
3 main classes of cell surface signalling receptor
ion-channel coupled receptors
GPCR
Enzyme coupled receptors
Ion channel coupled receptors
transmitter gated ion channels
Change conformation allowing ions to enter and depolarise the membrane (3 sorts of conformational change)
GPCRs
Receptors couple to G proteins - leads to raised levels of second messenger
No intrinsic enzymatic activity
Enzyme coupled receptors
Do have intrinsic enzymatic activity
Signals activate enzyme activity integral to the receptor
Changes in protein conformation allow
switching between active and inactive states- reversible
GPCRs
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
GPCR structure
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
alpha subunits are
GTPase enzymes
Hydrolyse GTP into GDP
How does GPCR signalling work
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)
How can rapid responses to signals occur
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)
Interaction of active alpha/GTP subunit
associates with specific target enzymes to activate their activity
Target enzyme remains active while bound to alpha/GTP subunit and will propagate the signal
What does interaction with its target enzyme do
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
What do the enzymes do
When activated by alpha/GTP subunits they raise levels of small second messenger molecules, diffuse to act on intracellular signalling proteins
GsG proteins
raise level of cAMP
GqG proteins
raise levels of Ca2+ ions
alpha Gs
stimulates adenlyl cyclase
catalyses cyclization of adenosine triphosphate (ATP) into cyclic adenosine monophosphate (cAMP)
High levels of cAMP activates
A-kinase
Structure of A-kinase
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
How does cAMP activate A-kinase
One cAMP binds to regulatory subunits, the catalytic ones are released and become active kinase molecules .
What can A-kinase do
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
alpha Gq
stimulates phosholipase C
How does stimulation of phospholipase C work and what are the effects
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
Calmodulin
2 binding sites for Ca at each end
Calmodulin activation
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
The Gs and Gq signalling pathways lead to
Changes in altered cytoplasmic protein function and altered gene expression
