Lecture 2: Cellular Communication and Signal Transduction Part 1

True or False: Gap junctions allow for electrical and metabolic coupling of neighboring cells.

true

What was gap junctions filled with?

water

What do gap junctions do?

facilitate the passage of inorganic, small molecules

What is the permeability of gap junctions regulated by?

• intracellular calcium concentration

• cAMP

• pH

Where does Ca2+-dependent interactions of cadherins take place?

adhering junctions

What does cadherin clustering lead to?

intracellular catenin clustering

Adhering junctions are sites of attachment for what?

the actin cytoskeleton

What roles do adhering junctions play in organ development and remodeling?

• disruption of adhering junctions leads to B-catenin disassociating from cadherin

• increase in free B-catenin and translocation to the nucleus

• change in gene transcription

What type of transmembrane proteins do tight junctions have?

• claudin

• occludin

What do tight junctions do?

create a diffusion barrier

What type of protein is zonula occludin 1?

an intracellular protein

What role do intracellular proteins play in tight junctions?

stabilize the complex and have intracellular signaling roles through links to serine/threonine kinases

Describe membrane-associated ligands.

interaction of receptor in the plasma membrane with the ligand that is a membrane protein on an adjacent cell membrane

What can membrane-associated ligands do?

provide spatial cues in migtating cells

Describe ephrin.

• membrane-associated ligand

• interacts with Eph receptor on a different cell

What are integrins?

cell-surface receptors that interact with the extracellular matrix (ECM)

True or False: When integrin binds to its receptor, a conformational change occurs.

true

What does an accumulation of signaling molecules in focal adhesion on the cytosolic side include?

small GTP-binding proteins and intracellular kinases

What does an accumulation of signaling molecules in focal adhesion on the cytosolic side play a role in?

• cell proliferation

• migration

• differentiation

What are some chemical messengers?

• odorants

• metabolites

• ions

• hormones

• growth factors

• neurotransmitters

True or False: Lipid-soluble molecules → diffuse across plasma membrane → cytosolic or nuclear receptors.

true

True or False: Non-lipid soluble molecules → interact with cell-surface receptors → opening of membrane channels or intracellular 2nd messengers systems.

true

Describe the signaling events initiated by membrane receptors.

1. recognition of signal by receptor and conformational change

2. transduction of extracellular message to intracellular signal

3. transmission to appropriate effector(s)

4. modulation of the effector(s)

5. response of cell to initial stimulus

6. termination of response

Describe ligand-gated ion channels.

• hybrid ligand receptors that are also ion channels (ionotropic)

• integral membrane proteins

• binding of signaling molecule to specific site(s) controls opening and closing of ion channels

Describe GPCRs.

• metabotropic receptors

• largest family of receptors (more than 1000 members known or predicted)

• single polypeptide chain with 7 membrane-spanning a-helical segments

• extracellular N-terminus

• large hydrophilic cytoplasmic loop between helices 5 and 6

• hydrophilic domain on cytoplasmic C-terminus

For GPCRs, where do ligands bind?

at the site formed membrane-spanning segments

Where is the major site of G protein activation?

between helices 5 and 6

Describe GTP-binding proteins.

• heterotrimeric G proteins

• examples: alpha, beta, and gamma

Is the GTP-bound or GDP-bound G-protein active?

• GTP-bound is active

• GDP-bound is inactive

Describe the heterotrimeric subunit diversity.

• 16 alpha subunits

• 5 beta subunits

• 11 gamma subunits

Describe the alpha subunit?

• binds, hydrolyzes GTP

• interacts with downstream effect proteins like adenylyl cyclase

Describe the beta-gamma complex.

interacts with different effector molecules than alpha subunits

True or False: Both the alpha and gamma subunits are involved in anchoring the complex to the membrane.

true

True or False: G-protein subunits have distinct tissue distributions and interact with different receptors/effectors.

true

Describe G-protein activation.

1. in the resting state, the receptor associates with the inactive G protein heterotrimer

2. upon ligand binding, the receptor G-protein complex undergoes a conformational change that promotes the exchange of GDP for GTP

3. G-protein dissociates from the receptor

4. alpha-GTP and beta-gamma subunits dissociate

5. both alpha-GTP and beta-gamma can now interact with their appropriate effectors (E1 and E2)

6. alpha catalyzed hydrolysis of GTP to GDP inactivates alpha and promotes reassembly of the trimer

7. alpha-catalyzed hydrolysis of GTP to GDP inactivates alpha and promotes reassembly of the trimer

8. cycle repeats

9. 
   

True or False: The alpha-beta-gamma complex is stimulated by GDP-GTP exchange and dissociates from the receptor.

true

True or False: Members of the RGS family of G-protein regulators stimulate GTP hydrolysis with some but not all alpha subunits.

true

What are the downstream effetors of alpha subunits?

• adenylyl cyclase

• phospholipases

The alpha subunit is an enzyme that does what?

the hydrolysis of GTP to GDP + inorganic phosphate

True or False: The beta-gamma subunit can activate ion channels or other effectors.

true

What happens after the inactive alpha-GDP complex dissociates from the downstream effector?

the complex re-associates with the beta-gamma subunit

What stabilizes the alpha-GDP interaction and slows the rate of GDP-GTP exchange?

beta-gamma

What does adenylyl cyclase do?

converts ATP to cAMP

True or False: Adenylyl cyclase is either activated or inhibited by G-protein signaling, depending on the association with GTP-bound form of: G-alpha(s) → stimulatory and G-alpha(i) → inhibitory.

true

Describe the transducin process.

1. alpha-t subunit

2. activates cGMP phosphodiesterase (PDE)

3. catalyzes the breakdown of cGMP to GMP

List the retinal phototransduction process.

1. light stimulation

2. decrease in intracellular cGMP levels

3. closure of cGMP-dependent channels

What do phospholipases do?

catabolize phospholipids

What are some families of phospholipases?

• A2

• C

• D

Which subunits activate phospholipase C?

G-alpha(q)

What are the two processes that PIP2 undergoes?

• PIP2 → PLC → DAG → stimulates protein kinase C (PKC)

• PIP2 → PLC → IP3 → Ca2+ release from ER stores

What are some examples of downstream beta-gamma complex effectors?

• ion channels

• modulation of adenylyl cyclase and PLC

• stimulation of phospholipase A2

True or False: The effects of beta-gamma can be independent of, synergize with, or antagonize actions of the alpha subunit.

true