cell physiology exam 2

What makes a phospholipid amphipathic?

It contains a hydrophilic head group and hydrophobic fatty-acid tails.

What happens to membrane fluidity when saturation increases?

Fluidity decreases because saturated tails pack tightly together.

What happens to membrane fluidity when unsaturation increases?

Fluidity increases because double bonds create kinks that reduce packing.

How does increasing tail length affect membrane fluidity?

Longer tails increase van der Waals interactions and decrease fluidity.

What is the role of cholesterol at high temperatures?

Stabilizes the membrane and prevents excessive fluidity.

What is the role of cholesterol at low temperatures?

Prevents phospholipids from packing too tightly and maintains fluidity.

What is membrane fluidity?

The ability of lipids and proteins to move laterally within the membrane.

How can phospholipid head groups affect signaling?

Different head groups have different charges and structures that recruit different proteins.

What is a phosphoinositide (PI)?

A membrane phospholipid whose head group can be phosphorylated and used in signaling.

What does PI3K do?

Converts PIP2 into PIP3.

What does PIP3 do?

Recruits signaling proteins and GEFs to the membrane.

What does phospholipase C (PLC) do?

Cleaves PIP2 to release a diffusible signaling molecule.

Why is asymmetric phospholipid distribution important?

Different lipids on different membrane leaflets create distinct signaling environments.

What kind of ligand typically activates a nuclear receptor?

Small hydrophobic molecules such as steroid hormones.

Describe the nuclear receptor pathway.

Ligand diffuses through membrane → binds receptor → conformational change → NLS exposed → nucleus entry → transcription factor activity.

Example of a nuclear receptor from class?

Androgen receptor (AR).

What does the ligand-binding domain (LBD) do?

Binds testosterone or another ligand.

What does the DNA-binding domain (DBD) do?

Binds specific DNA sequences.

What does the N-terminal domain (NTD) do?

Interacts with transcriptional machinery.

Mutation in AR LBD causes what?

Receptor cannot sense testosterone.

Mutation in AR DBD causes what?

Receptor cannot regulate target genes.

Mutation in AR NTD causes what?

Poor transcriptional activation.

What does SREBP sense?

Cellular cholesterol levels.

Why is SREBP membrane-bound instead of cytoplasmic?

It directly senses cholesterol within membranes.

What activates the SREBP pathway?

Low cholesterol.

Describe the SREBP pathway.

Low cholesterol → ER to Golgi transport → cleavage → transcription factor release → nucleus entry → cholesterol synthesis genes activated.

What type of feedback loop is SREBP?

Negative feedback.

What is the ultimate output of SREBP signaling?

Increased cholesterol synthesis.

What does RTK stand for?

Receptor Tyrosine Kinase.

How are RTKs activated?

Ligand-induced dimerization followed by autophosphorylation.

What enzyme activity do RTKs have?

Tyrosine kinase activity.

What is autophosphorylation?

RTKs phosphorylate their own tyrosine residues.

What is the purpose of RTK phosphorylation?

Creates docking sites/scaffolds for signaling proteins.

Why use phosphorylation cascades?

Signal amplification and rapid information transfer.

What is Ras?

A monomeric GTPase involved in RTK signaling.

How is Ras activated?

A GEF exchanges GDP for GTP.

If an RTK cannot recruit a GEF, what happens to Ras?

Ras remains inactive.

Would loss of GEF recruitment stop PIP signaling?

No, PIP signaling uses different docking sites.

What is the key requirement for RTK activation?

Receptor clustering/dimerization.

What did the EGFR/MβCD experiments suggest?

EGFR can become activated without ligand if clustering occurs.

What does pEGFR indicate?

Activated EGFR.

What does pERK indicate?

Active downstream signaling.

What does GPCR stand for?

G-protein coupled receptor.

What enzymatic activity does an activated GPCR perform?

Acts as a GEF for Gα.

Describe GPCR activation.

Ligand binds GPCR → GDP released → GTP binds Gα → downstream signaling.

What does Gα-GTP represent?

Active Gα.

How is GPCR signaling normally terminated?

GTP hydrolysis on Gα.

What does GRK do?

Phosphorylates activated GPCRs.

What does β-arrestin do?

Promotes receptor desensitization and endocytosis.

What happens if Gα cannot hydrolyze GTP?

Signaling remains active longer.

What happens if β-arrestin cannot bind?

Endocytosis decreases and signaling lasts longer.

What is signaling bias?

Different ligands stabilize different receptor conformations and favor different downstream pathways.

What is a balanced opioid drug?

Activates both G-protein and β-arrestin branches.

What is a biased opioid drug?

Preferentially activates G-protein signaling.

Why are GPCRs more diverse than RTKs?

Different G-protein combinations, receptor promiscuity, pleiotropy, and biased signaling.

What is G-actin?

Globular actin monomer.

What is F-actin?

Filamentous actin polymer.

What end of actin grows most rapidly?

Plus (barbed) end.

What end loses monomers most frequently?

Minus (pointed) end.

What is nucleation?

Formation of the initial stable actin nucleus.

Why is nucleation rate-limiting?

Stable 3–4 monomer nuclei are difficult to form.

What is actin treadmilling?

Addition at the plus end and loss at the minus end.

What happens if ATP-G-actin concentration drops?

Filaments shrink.

What happens if ATP hydrolysis is blocked?

Filaments become more stable and tend to grow.

What happens if actin nuclei are stabilized?

Faster actin polymerization.

How does actin push a membrane without a motor?

Monomer addition at the plus end physically pushes the plasma membrane forward.

What are lamellipodia?

Broad sheet-like protrusions of branched actin.

What are filopodia?

Thin finger-like protrusions of bundled actin.

What are stress fibers?

Contractile actin bundles associated with myosin II.

Key protein for lamellipodia?

Arp2/3.

Key proteins for filopodia?

Formin and Fascin.

Key protein for stress fibers?

Myosin II.

Which Rho-family member activates lamellipodia?

Rac.

Which Rho-family member activates filopodia?

Cdc42.

Which Rho-family member activates stress fibers?

RhoA.

Do lamellipodia push or pull?

Push.

Do filopodia push or pull?

Push/sense.

Do stress fibers push or pull?

Pull.

What does a GEF do?

Activates a GTPase by exchanging GDP for GTP.

What does a GAP do?

Inactivates a GTPase by stimulating GTP hydrolysis.

What does a GDI do?

Locks a GTPase in the GDP-bound inactive state.

Rac pathway?

Rac-GTP → Arp2/3 → Lamellipodia.

Cdc42 pathway?

Cdc42-GTP → Formin/Fascin → Filopodia.

RhoA pathway?

RhoA-GTP → ROCK → Myosin phosphorylation → Stress fibers.

What is ROCK?

Rho-associated kinase.

What does ROCK do?

Phosphorylates myosin light chain.

What activates myosin II?

Myosin light chain phosphorylation.

If RhoGEF is always active, what happens?

Increased RhoA-GTP, increased contraction, increased migration.

If RhoGAP is nonfunctional, what happens?

Increased RhoA-GTP signaling.

If GDI is overexpressed, what happens?

Reduced RhoA signaling.

If myosin light chain cannot be phosphorylated, what happens?

Reduced contraction and migration.

What are integrins?

α/β heterodimer transmembrane adhesion receptors.

Do integrins have enzymatic activity?

No.

Why are integrins important for migration?

They connect the cell to the ECM.

What is the migration cycle?

Reach → Anchor → Pull.

What is inside-out activation?

Internal signaling activates integrins.

What is outside-in activation?

ECM binding activates integrins and signaling.

What protein directly opens integrins?

Talin.

What protein binds activated talin and actin?

Vinculin.

What is a focal adhesion?

A cluster of activated integrins linked to the cytoskeleton.