Lecture 34: Cell Signaling I

Question 1

General stages of signal transduction

Answer 1

1. Primary signal (messenger) arrives at cell
2. Receptor recognizes primary signal
3. Receptor transmits signal into cell
4. Cell components (incl. 2nd messengers) pass on signal via cascades
5. Signal arrives at destination

Question 2

Types of intercellular signaling

Answer 2

Direct contact:
1. Juxtacrine
Secretion of molecules:
2. Endocrine
3. Paracrine
4. Synaptic
5. Autocrine

Question 3

Mechanisms of juxtacrine signaling

Answer 3

1. Signal protein binds receptor
2. Gap junction transmembrane channels

Question 4

Endocrine signaling

Answer 4

Sender releases hormone to blood for long-range target (low concentration, v. high affinity = slow on, slow off)

Question 5

Correlation between signal concentration and receptor affinity

Answer 5

Receptor affinity must be appropriate to detect presented signal concentration i.e. low concentration requires high affinity in order to ensure signal is transmitted. Affinity also determines how long it takes for signal to dissociate

Question 6

Paracrine signaling

Answer 6

Nearby cell releases molecules which diffuse to targets; local mediators e.g. growth factors and cytokines. Rapid on, rapid off; low to high concentration w/ low to high affinity

Question 7

Synaptic signaling

Answer 7

Extreme form of paracrine with very short diffusion distance; v. high concentration w/ v. low receptor affinity

Question 8

Autocrine signaling

Answer 8

Sender / target cell are the same; low to high concentration of slow diffusing signals w/ intermediate receptor affinity. Very rapid response.

Question 9

Primary (1st) messenger classes

Answer 9

EC signals
1. Neurotransmitters
2. Hormones
3. Growth factors
4. Cytokines

Question 10

Neurotransmitters

Answer 10

Very small, (mostly) hydrophilic molecules. Involved in very short range synaptic signaling; excitatory or inhibitory depending on receptor

Question 11

Hormones

Answer 11

Diverse molecules involved in endocrine signaling (long-range via blood). Released by specialized cells to elicit specific response. Can be hydrophilic (cell surface receptors) or hydrophobic (IC receptors, req. carrier proteins)

Question 12

Are there molecules involved in multiple different primary signaling roles?

Answer 12

Yes e.g. vasopressin (NT or hormone)

Question 13

Growth factors

Answer 13

Large group of proteins that regulate cell growth/differentiation; involved in local signaling e.g. paracrine/autocrine

Question 14

Cytokines

Answer 14

Large group of proteins that in general coordinate the immune response. Can be paracrine or autocrine.

Question 15

Odontoblast tooth layer primary messengers

Answer 15

Question 16

Ligand

Answer 16

(Usually small) molecule that binds to a protein

Question 17

Agonist

Answer 17

Ligand that activates normal response

Question 18

Antagonist

Answer 18

Ligand that induces no response, thus blocking the normal response

Question 19

Effector

Answer 19

Intracellular protein that responds to an activated receptor, generating 2nd messengers

Question 20

Coupling

Answer 20

Intracellular protein that transmits signal between active receptor and effector

Question 21

Adaptor

Answer 21

Intracellular protein without enzymatic activity but mediates protein-protein interactions i.e. certain protein domains

Question 22

Types of molecular switches

Answer 22

1. Protein phosphorylation
2. G protein cycle

Question 23

Protein phosphorylation

Answer 23

Activation/inactivation by phosphorylation/dephosphorylation mediated by a kinase/phosphatase

Question 24

G protein cycle

Answer 24

• Off state: G protein bound to GDP
• On state: G protein bound to GTP
• GDP to GTP exchange via active receptor (trimer) or GEFs (monomer)
• Hydrolysis by intrinsic GTPase activity (trimer) or GTPase + GAPs (monomer)

Question 25

GEF

Answer 25

Guanine nucleotide exchange factor. Mediates exchange of GDP for GTP, activating G protein

Question 26

GAP

Answer 26

GTPase activating protein. Catalyzes hydrolysis of GTP back to GDP on a G protein, switching it off.

Question 27

Requirements for signal receptors

Answer 27

1. Specificity
2. Appropriate binding affinity (inverse Kd)
3. Transmission of signal to cascade

Question 28

Receptor classes

Answer 28

1. Intracellular (cytoplasm, nucleus; hydrophobic signals)
2. Cell-surface (majority; GPCR, enzyme-linked, cytokine)

Question 29

General features of GPCRs

Answer 29

• Ligand activates heterotrimeric G protein which conveys signal to cascade
• Synaptic, endocrine, autocrine, paracrine
• Termination by Ser/Thr phosphorylation on C-terminus tail

Question 30

GPCR struct

Answer 30

• Outside N-terminal tail, inside C-terminal tail
• Transmembrane α-helices (5, 6, 7 = heterotrimeric G protein subunits)
• N-terminal glycosylation, C-terminal phosphorylation mods
  Large ligands bind EC loops, small ligands go to binding pocket

Question 31

Heterotrimeric G protein (GPCRs)

Answer 31

Coupling protein that diffuses under plasma membrane
- α subunit: GDP/GTP binding site + GTPase activity; hydrophilic + membrane anchor. Interacts w/ effectors
- βγ dimer complex: some effector interaction, hydrophobic + membrane anchor

Question 32

GPCR action steps

Answer 32

1. Ligand binds
2. Conform. change exposes recognition site and G protein binds
3. GDP/GTP exchange → G protein dissociation
4. α subunit binds membrane enzyme → 2nd messenger production
5. α GTPase hydrolysis; enzyme released
6. GPCR reassembly with new βγ complex

Question 33

GPCR signal amplification

Answer 33

Active G protein dissociation means a new trimer can replace it while the original ligand is still bound, therefore amplifying the signal up to 100X