GPCR 2 - Gq + IP3 Flashcards
examples of G1-activating signals + responses
Liver
- vasopressin
= glycogen breakdown
Smooth muscle
- thrombin
= platelet aggregation
Gq proteins activate inositol phospholipid pathway
GPCR activated
->activates alpha subunit
- > activates phospholipase C-beta
- > cleaves PIP2
- > IP3 + DAG
- > initiate downstream events
IP3 role
increases cytoplasmic [Ca2+] from ER
- > activates calmodulin
- > activates CAM kinases
DAG role
activates protein kinase C
-> actives IkB/NF-k-B pathway
OR
-> MAP-kinase pathway
Gq + Ras signalling pathways
cross-talk between the 2
Raf->Mek->Erk->
nucelar proteins
C-kinase activates Raf by phosphorylation
protein kinase C inactivates inhibitor of NFk-B
Ik-B bound to NF-kB
PKC phosphorylates IkB
- > Ik-B degraded through proteasome
- > releases Nf-kB
- > enters nucleus and laters transcription
IP3 increases cystolic [Ca2+] + activates PKC
- activated Gq activates phospholipase C-beta
- converts PIP2 to IP3 + DAG
- IP3 binds to gated Ca2+ channels on ER
- Ca2+ release
- Ca2+ and DAG bind to and activate protein kinase C
treating cells with TPA + Ionomycin
- induces..?
proliferation
treating cells with TPA + Ionomycin
-mechanism
TPA mimics DAG action
- binds to PKC
Ionomycin mimics IP3-gated channels
- binds + transports Ca2+ across ER membrane
Ca2+
- role
- effects mediated by?
triggers embryo development post-fertilisation
muscle contraction
secretion in nerves + other secretory cells
mediated by Ca2+ response proteins
mechanisms that keep [Ca2+] low
Ca2+ pumps on PM re-establish Ca2+ levels in resting cells
Organellar Ca2+ pumps
Ca2+ binding proteins in cytosol reduce free [Ca2+]
Ca2+ pumps on PM re-establish Ca2+ levels in resting cells
secretory cells, nerves + muscles:
Na+/Ca2+ antiport in the PM
all cells:
Ca2+ ATPase in PM
Organellar Ca2+ pumps re-establish resting [Ca2+]
Ca2+ ATPase in ER membrane
-> directs Ca2+ into ER from cytoplasm
active Ca2+ import in mitochondria
(emergency mechanism e.g. membrane breached)
Ca2+ release due to positive feedback
- Ca2+ binds to IP3-gated channels
- Ca2+ binds to ryanodine channels alos on ER
= rapid release of Ca2+ in cytoplasm
reversal of IP3 signal involves negative feedback
enzymes that turn over IP3 are activated by Ca2+
IP3 -> lipid phosphatase removes P -> IP2 -> can't bind to IP3-gated channels = no Ca2+ released
IP3 -> lipid kinase adds P -> IP4 -> Ca2+ ATPase activity = negative feedback
feedback generates Ca2+ waves + oscillations
local signalling events open IP3-gated channels
- > Ca2+ out of ER
- > Ca2+ opens other channels
- > propagates the wave
reversed by high levels
- > channels start to close
- > resting state
hepatocyte response to increasing [vasoperessin]
Ca2+ amplitude is constant
BUT frequency increases with vasopressin concentration
intracellular Ca2+ levels oscillate due to feedback
- positive
Ca2+ induced Ca2+ release
via IP3 and ryanodine-gated channels
intracellular Ca2+ levels oscillate due to feedback
- negative
high Ca2+ stops further Ca2+ release
high Ca2+ stimulates lipid kinase -> IP4
IP4 -> promotes Ca2+ removal from cytosol
Ca2+ oscillation frequency-dependent responses
release pituitary hormones is pulsatile
-> pulses follow Ca2+ spike freq
some cells activate gene transcription in response to spike freq
calmodulin and Ca2+
has x4 Ca2+ binding sites
-> Ca2+ causes conformational change (kink)
- > can then recognise specific target proteins
e. g. Ca2+ ATPase in PM as part of -ve feedback
CaM-kinases
serine/threonine kinases
an activate CREB
- cross-talk with Gs/cAMP signalling
cell type specific response to increasing [Ca2+]
CaM-kinase specificity
Narrow:
myosin light-chain kinase
- smooth muscle contraction
phosphorylase kinase
- glycogen breakdown in liver
Broad:
CaM-KII
- multifunctional
CaM-kinase II
- molecular memory
activity continues after [Ca2+] drop
and Ca2+/calmodulin dissociates from it
due to autophosphorylation
CaM-kinase II
- hexameric complex
functions as a homo-hexameric complex
6 subunits bound
Ca2+/Calmodulin activates each subunit
CaM-KII activity based on Ca2+ oscillation freq
- low freq
- high freq
CAMK-II dephosphorylated after each Ca2+ spike
CAM-II activity ratchets up as more molecules phosphorylated
when Cam-KII activity reaches max…?
increasingly difficult for phosphates to compete
when all subunits of each CaMK-II complex are phosphorylated
-> full activity maintained at relatively low Ca2+ levels
desensitisation of GPCR
phosphorylation by PKA, PKC or GRK (=GPCR regulatory kinase)
of serine/threonine reissues on cyto loops
arrestin binds to phosphorylated GPCR
- blocks GPCR from receptor
