VL 40 (Otto Baumann) Flashcards
Cells react to signals from the exterior
- exposure to something → cell → response
- cells also have to sense their neighbours → signaling between them (called cell-cell signaling)
Types of cell-cell signaling
1. contact-dependent
2. gap junctions
3. autocrine
4. paracrine
5. synaptic
6. endocrine
autocrine: cell secretes a hormone or chemical messenger (called the autocrine agent) that binds to autocrine receptors on that same cell
synaptic: signal producing cell (neuron); signal at synapse → target cell: postsynaptic cell (neuron/muscle cell); translation: elec. signaling → chem. signaling
paracrine: dependent on endocrine cells; hormones released in blood stream; cell produces a signal to induce changes in nearby cells
hundreds of different signaling molecules:
* peptides & proteins
* amino acids
* nucleotides
* steroids
* fatty acid derivatives
* gases
Receptors for extracellular signals
- signal detection via receptors
- intracellular receptors
–> binding of signaling molecule →translocated into nucleus
Responses are cell-type specific
- ACh→different reaction on same signaling molecule (cell-type dependent)
- reaction depends on
–> signaling in between receptor – effector
–> effector
–> receptor
Surface receptors
Surface receptor 1
GPCR:
* regulate activity of separate, in PM integrated target protein
* ligand binds receptor → receptor binds G protein → interacts with E → active E
Surface receptor 2
E-coupled receptors:
* act as E/combine with E that activate them
* ligand (dimer) binds inactive domain + catalytic
→active domain + catalytic
* ligand binds receptor → dimerization → active associated
Response strength depends on [signal]
- Sigmoidal: low c → no response
- all-or-none: low c → no response;
c-threshold → complete reaction - cell fate depends on c
- different receptors need different ligand numbers → activated
–> 1 Ligand: hyperbolic
–> More ligands → steeper dose-response curve - increasing number of effector molecules that bind target protein → activation → steeper dose-response curve
Surface receptors act via a signaling cascade
- Activated receptor→signaling molecules→activate large number of target proteins
- →signal amplification
Surface receptors often act via 2nd messengers:
Intracellular signaling proteins:
Regulation of GTPase activity of G proteins:
Temporal response patterns
- phasic: response as stimulus beginning → decrease
- tonic: reaction as long as stimulus is present; reaction same
- phasic-tonic: stimulus-on set → response peak → decrease → plateau phase as long as stimulus is
present - oscillating response, amplitude-modulated: the higher the stimulus, the higher the amplitude
- oscillating response, frequency-modulated: higher stimulus, higher oscillation frequency
positive and negative feed back loop
Example: S phosphorylates, activates E; I dephosphorylates, inactivates E
positive feedback:
* enhanced activity of activated molecule
* larger response; response stays on after stimulus
→all-or-none/sigmoidal
negative feedback:
* inhibits/reduces reaction
→oscillating (short delay: peak, little oscillation, plateau phase; long delay: oscillation)
→increasing dephosphorylation rate of E
Adaptation / desensitization
- Signaling molecule bound to receptor → receptor internalized by endocytosis → complex inactivation → receptor recycling
- Signal molecule + receptor complex internalized via endosomes → fusion with lysosomes → degradation
- Receptor inactivation via neg. feedback loop from cofactor which binds to activated receptor
- Receptor interacts with intracellular signaling proteins → neg. feedback loop on intracellular signaling protein via cofactor
- Inhibitor synthesis
G-Protein-coupled receptors (GPCRs)
- largest family of membrane-bound receptors: 1000-2000 (> 1% of human genome)
- evolutionary quite old
- detect various extracellular signals
- one signal may be different GPCRs → differential expression, reactions
- > 50% of pharmaceutica act on GPCRs and/or GPRC-dependent signaling
Structure
* 7 transmembrane α-helices (“7-pass transmembrane receptors”/”heptahelical receptors”)
* N-terminus (E1): extracellular
* C-terminus (C4): cytosolic
* Signal binding: either via transmembrane domains, E1, extracellular loops (E2/3)
* → binding, activation of heterotrimeric G-proteins via C3/4
* Often: palmitoyliaction of Cys in C-terminal region
GPCR act via heterodimeric G Protein:
* 3 SU: αβγ
* Membrane-attached via fatty acyl chains on α, γ SU
* α SU: GDP-bound; GPCR interaction; GTPase
* activated GPCR = GEF for α SU
* target protein = GAP → GTPase of α SU high
Heterodimeric G Protein at work
Effector protein: adenylyl cyclase
Protein kinase A (PKA)
- cAMP-dependent
- tetramer of 2 catalytic (Ser/Thr kinase) + regulatory SU
- each regulatory SU: 2 cAMP-binding sites
- 4 cAMP-binding → catalytic SU dissociate → active
cAMP-dependent signaling pathway:
- Epinephrine binds receptor → activates G protein → dissociates → GTP-alpha → AC → cAMP → binds PKA → dissociaties → catalytic SU act phosphorylates effector proteins
- Amplification steps:
–> Each active receptor → different G-proteins
cAMP may effect gene transcription
There are several heterotrimeric G proteins
Gβγ may also affect target proteins
Desensitization of GPCR-dependent signaling
- Rhodopsin
- Bound
PLC/IP3/Ca2+ signaling pathway:
Turning off the IP3 signal:
Ca2+ signaling:
EF-hand motif:
- Ca2+-binding motif
- HLH
- 30aa
- Ca2+ surrounded by 6 O-atoms (polypeptide), 1 O (H2O)
- dissociation constant: ~ 1 μM Ca2+
- usually present in pairs
Calmodulin:
- = calcium-binding modulator protein
- 148aa
- 17kDa
- 2 EF hand – flexible linker - 2x EF hand
- →4 Ca2+ binding sites
- cooperative Ca2+-binding → allosteric activation
- Kd (Ca2+): 0.5 – 5 μM
- all eukaryotic cells
- up to 1% of total protein (= 1-10 μM)
- cytosol, nucleus
Calmodulin at work:
- Ca2+-binding
- →conformation change (EF-hand-opening)
- →exposure: hydrophobic aa residues
- binding to target sequences within an α helix of (many) other proteins
