Cell Bio Unit 3- lecture 2

g protein coupled receptors

7 transmembrane alpha helix domains with extracellular binding pocket

largest family of receptors

contribute to our basic senses and physiology

approximately 700 FDA approved drugs directly target GPCSs

g proteins

what GCPR promote signaling intracellularly through

are lipid-modified to associate with the plasma membrane, exists as a timer when inactive (a, b, y subunits)

stimulating a GPCR promote complex formation between the GPCR and this

GPCR acts like a GEF for the a subunit, causing it to dissociae from the by subunits

regulator of G protein signaling

proteins act like CAPs to inactivate a subunit and re-from inactive G-protein complex

aka RGS

GPCR kinases

active GPCRs will be phosphorylated by this

arrestin

recruited by phosphorylated GPCR which prevents subsequent activation of G-protein

cAMP

a second messenger downstream of many GPCRs

made from ATP by adenylyl cyclase

some GPCRs promot adenlylyl cyclase activity to turn ATP into this

its a small molecule that can spread rapdily throughout the cell

protein kinase A

generally in an inactive state, with catalytic subunits held by regulatory subunits (4 total)

regulatory subunits require binding of 2 cAMP molecules to release active catalytic subunits- ensure specifcity and threshold of activation

its a kinase that phosphorylates many targets and cause them to become active/inactive

CREB

cyclic AMP response element-binding protein is a transcription factor phosphorylated by PKA

when active, binds CREB-binding protein (CBP) to create a transcription factor complex

this + CBP act as trancriptional activator of many genes that control cell metabolism, invasion, and survival

phospholipase c-b

an enzyme at the plasma membrane that acts on PIP2

cleaves PIP2 to make two products that can do their own signaling: DAG and IP3

IP3

leaves the membrane and binds to ligand-gated channels in the ER to release Ca2+

CICR results in positive feedback and large increases in Ca2+ conentration

DAG

stays at the membrane and interacts with PKC

Ca2+ is required for this to activate PKC which has many targets

by subunits

seprate from active a subunit and activate PLCb

PLCb cleaves PIP2, producing DAG and IP3

IP3 diffuses into the cytosol and open Ca2+ channels to release Ca2+ into cytosol

DAG + Ca2+ activates PKC which can phosphorylate and regylate many different targets

calcium

binding can activate or inactivate many proteins

can be rapidly released, but also rapidly put back into storage

can even more into neighboring cells

rhodopsin

a GPCR found in rod cells in the eye

in the dark, rod cells have Na+/Ca2+ channels open leading to constant depolarization and release of inhibitory neurotransmitter

when light hits the rod cells, this becomes active and closes the membrane channels, hyperpolarizing the cell and stopping the release of inhibitory neurotransmitter

acts through cGMP

b-adrenergic receptors

a GPCR expressed in the heart and activated by ligands including adrenaline, leading to cAMP » PKA » contraction

in heart failure, a GRK is upregulated which reduces the activity of this GPCR and leads to reduced signaling and contractility

signaling is also reduced in the aging heart, however the mechanism is still unknown

receptor tyrosine kinases

have intrinsic enzyme activity in cytosolic domain

over 60 RTKs in humans with differential expression in different cell types

RTK extracellular domains bind to specific ligands (growth factors, hormones)

cytosolic domains phosphorylate effector proteins

trans-autophosphorylation

allows RTKs to activate each other at tyrosines

activated RTKs can interact with and active intracellular signaling proteins

SH2 domain

domain on some signaling proteins that interact with specific phosphorylated tyrosines on the RTK

SH3 domains

domain that allows proteins to bind other signaling proteins

proteins may have combinations of this and SH2 domains like a barcode to determine which RTKs and partners to interact with

Ras GTPases

molecular switches like other GTPases

are anchored to the membrane and can respond to RTK signaling

active RTKs facilitate interactions between Ras GEFs and Ras GTPases to activate Ras and cause downstream signaling

Grb2

binds active RTK using SH2 domain and recruits GEF sos using SH3 domain

this brings Sos close to its target and leads to activation of Ras GTPase

MAP kinase

activated by Ras GTPase

MAPKKK (raf) » MAPKK (mek) » MAPK (erk)

generally, negative feedback limits the duration of the signal

leads to changes in protein activity or changes in gene expression

cells can organize cascades into specific pathways using scaffolds that hold certain ones of these in sequences

PI3K

can be activated by RTK and phosphorylate PIPs

can turn PIP3 into PIP3, which can scaffold signaling proteins containing PH domains at the membrane

one of these signaling proteins, Atk, can dissociate from the plasma membrane and promote cell survival

mTOR complex 1

growth factors activate RTKs, which act through both PI3K/Akt and Ras/MAPK signaling to promote activation of this

can also be activated by high levels of AA or lysosome, which indicate high nutrient levels

both pathways promote cell growth

RTK

this that prmotes cell survival can become overactive in cancer, overriding other signals that may indicate cell death

many drugs have been developed to reduce activity of these themselves or the intracellular signaling proteins

these can be highly effective, but can also cause side effects since these signaling mol have other roles