intracellular signaling

rate of cell signaling

depends on how the cell processes the signal

altered protein function is fast

• cel movement, secretion, metabolisim

altered protein synthesis is slow

• cell growth, cell division, differentiation

on v off state of molecular switches - phosphorylation

30-50% of human proteins get phosphorylated

phosphate group added to OH of AA side chains

• Ser, Thr, Tyr

serine/threonine kinases are most common class across eukaryotes

tyrosine kinases are common in multicellular animals

on v off state of molecular switches - GTP binding

GTP binding proteins with GTPase activity

GTPase activating proteins (GAPs) trigger hydrolyzation of GTP

guanine nucleotide exchange factors (GEFs) promote GDP release so a new GTP can be bound

cell surface receptor proteins

3 major classes

• ion channel coupled receptors (neurotransmitters)

• G protein coupled receptors

• enzyme coupled receptors

most extracellular signal molecules bind cell-surface receptors but do not enter the cell

cell-surface receptors communicate ligand binding into intracellular signals (signal transduction)

GPCR - structure

largest family

core to sense of sight, smell, taste

signal molecules (ligands) are varied and the same signaling molecule can activate multiple GPCRs

single 7 pass transmembrane protein with ligand binding site in core

use G-proteins to relay signal into cell interior

GPCR signaling

G protein: heterotrimeric GTP binding protein. couples receptor to effect protein (typically enzymes or ion channels). alpha, beta, and gamma subunits

G proteins are specific to their receptors

1. inactive GDP bound alpha subunit

2. ligand binds GPCR

3. conformation changes act as GEF to alpha subunit

4. alpha releases GDP and binds GPT. conformational change of alpha

5. dissociate and release beta and gamma dimer

6. alpha-GTP and beta-gamma relay signal to effectors

7. GPCR stays active as long as ligand bound, can activate many G proteins

8. when alpha hydrolyzes GTP = inactivated. some regulatory proteins act as GAPs to promote shut off

downstream signaling -cAMP

synthesized from ATP by Adencylyl cyclase

rapidly and continually destroyed by cAMP phosphodiesterases.

short lived signal

effector proteins of an activated g-alpha can activate or inhibit adenylyl cyclase

cAMP and PKA

cAMP activates Protein Kinase A (PKA)

• serine/threonine kinase

• target proteins vary by cell type, so cAMP leads to different responses in different cells

inactive PKA: tetramer of 2 regulatory subunits + 2 catalytic subunits. anchor proteins bind regulatory subunits and localize complex to organelle, cytoskeleton, etc

active PKA: cAMP binding to regulatory subunits changes conformation, releasing catalytic subunits. catalytic subunits phosphorylate targets. some pathways lead to gene expression via CREB, CBP and CRE

secondary messengers

relay signal, generated in large amounts for signal amplification, spread throughout the cell, transmitting signal

cAMP, Ca, lipids like diacylglycerol

Rapidly rebalance to resting state (cAMP phosphodiesterases, Ca pumps)

cell ready to respond again (rapid response requires a rapid reset)

enzymes coupled receptors

single pass transmembrane proteins that bind extracellular ligands

cytosolic domain has enzyme activity or associates with enzyme

Receptor Tyrosine Kinases

many subfamilies

extracellular portion binds ligand, intracellular portion is a kinase the phosphorylates tyrosines

RTKs act as dimers when ligands binds (often monomers w/o ligand)

transautophospohrylate

• fully activates kinase domains

• phosporylation creates new binding sites

examples of RTKs

PDGF (platelet derived growth factor): dimeric ligand, brings 2 RTKs together

Insulin receptor: RTK already a dimer, ligand binding changes confromation and brings kinase domains into proximity

EGF (epidermal growth factor): 2 ligands bind to RTKs and induce dimerization, only 1 kinase activates, not transautophosphorylation

Ras

family of GTPases relay signaling for many RTKs

membrane bound via lipidation, RTK activates a GEF that activates Ras

ras signals via MAP kinases

• 3 component signaling cascade (Raf, Mek, Erk)

humans have 3: H-Ras, K-Ras, N-Ras

signaling pathways in plants

both plants and animals use kinases, GTPases and Ca

but plants don’t use cAMP, GPCRs and RTKs

plants use receptor serine/threonin kinases, particularly leucine rich reapeat receptor kinases

differsent set of growth factors: ethylene, auxin , cytokinins, gibberellins, abscisic acid, brassinosteriods