molec cell test 4

1. What proteins are required for microtubule assembly?

1. γ-TuRC (γ-tubulin ring complex) 

• Function:

  • Acts as the template/nucleator for MT formation.

  • Anchors the minus end at the MTOC.

  • Determines where microtubules start growing.

2. α/β-tubulin dimers

• Function:

  • The building blocks that polymerize to form the MT protofilaments.

  • β-tubulin hydrolyzes GTP → GDP after assembly, regulating stability.

3. MAP2 & Tau

• Function:

  • Bind the sides of microtubules.

  • Help stabilize MTs.

4. +TIPs (Plus-End Tracking Proteins)

• Function:

  • Bind the growing + end of MTs.

  • Stabilize active growth and prevent catastrophe.

  • Help position MTs inside the cell.

What molecular phenotype would result if a cell did not express gamma-tubulin?

• No MT nucleation at the MTOC

• MTs cannot anchor at centrosome

• Disorganized MT network

• Mitotic spindle fails → cell can’t divide

3. To assemble a stable web of actin filaments (crisscrossed meshwork), which combination of MFAPs would be involved?

ARP2/3 complex

• Nucleates branched actin (binds side of an existing filament and creates a new branch)

• Produces the tree-like, crisscrossed mesh needed for webs

Filamin

• Crosslinks actin branches at angles

• Stabilizes the network into a strong, flexible meshwork

You want to inhibit or slow the growth of actin filaments. What proteins would help accomplish this goal? What proteins would prevent this goal?

Proteins that inhibit/slow actin growth:

• Thymosin 

• Capping protein –

• Cofilin – 

Proteins that promote actin growth:

• Profilin – loads ATP onto actin (speeds polymerization)

• Formin – nucleates and elongates straight filaments

5. Which actin filament associated proteins and microtubule associated proteins have analogous functions?

• Capping protein ↔ +TIPs (stabilize + ends)

• Cofilin ↔ Catastrophe factors (depolymerize)

• Gelsolin ↔ Katanin/Spastin (sever)

• ARP2/3 ↔ γ-TuRC (nucleate)

6. What would the molecular phenotype be if a cell was unable to express gelsolin 

Actin filaments can’t be severed

• Actin becomes too long / stable

• Poor actin remodeling, causing defects in cell movement and cytokinesis

You are examining a cell in which gelsolin has a mutation that has prevented its capping function. What molecular phenotype would you likely observe?

• Severed + ends regrow immediately

• Causes excess actin polymerization

• Actin network becomes disorganized

8. If a cell wanted to transport cellular cargo from a location near the nucleus outward to a destination at the periphery of the cell, which (i) cytoskeleton filament and (ii) molecular motor would be the best choices?

• Filament: Microtubules

• Motor: Kinesin

9. The polymerization of a cytoskeletal protein is being inhibited because its monomeric form is being held in an ADP-bound state. Which protein could cause this molecular phenotype?

Thymosin

• Holds actin monomers in a locked, ADP-bound state → prevents polymerization

10. You are examining a cell that cannot divide properly. You determine that actin is not polymerizing.  What specific (cellular) phenotype could allow you to draw this conclusion? (Hint: consider what would happen to dividing cells if actin could not polymerize.)

• No contractile ring forms

• Cytokinesis fails, leaving cells multinucleated

11. You are examining a cell that cannot divide properly. You know right away that the problem is that intermediate filaments are not depolymerizing.  What specific phenotype allowed you to draw this conclusion?

Nuclear envelope stays intact (lamins didn’t break down /

nuclear membrane did not break down)

12. What proteins are normally required for assembly and function of the actin-based structure required for cell division?

• Formin or ARP2/3 – nucleate new actin

• Profilin – speeds polymerization

• Capping protein – stabilizes filaments

• Actinin / Fimbrin / Filamin – bundle & organize actin

• Myosin – contracts the ring

• Cofilin & Gelsolin – disassemble actin after division

13. Make sure you are familiar enough with microtubules and actin filaments that you could answer general multiple choice questions, like:

• Which of the following is NOT true of actin filament assembly or disassembly?

• Which statement is true of microtubules?

MICROTUBULES

• 25 nm hollow tubes, 13 protofilaments

• α/β-tubulin dimers, uses GTP

• Nucleated by γ-TuRC at MTOC

• Regulators: MAP2, Tau, +TIPs, Stathmin, Katanin, Spastin, Catastrophe factors

• Dynamics: dynamic instability

• Functions: transport, mitotic spindle

---

⭐ ACTIN

• 7 nm, two-stranded helix

• Uses ATP

• + end more dynamic

• Nucleation: Formin (straight), ARP2/3 (branched)

• Regulators: Profilin, Thymosin, Capping protein, Cofilin, Gelsolin

• Bundling: Actinin, Fimbrin, Filamin

• Dynamics: treadmilling

• Functions: movement, contractile ring

---

14. One end of an actin filament is more dynamic than the other due to______________.

Structural polarity

15. How would cell division be affected if the pool of stathmin in a cell was all unphosphorylated?

• Stathmin binds free tubulin → blocks MT polymerization

• Mitotic spindle cannot form

• Cell cannot divide (mitosis fails)

16. Describe the cytoskeletal proteins and associated proteins required for treadmilling in the lamellipodia of a migrating cell.

• ARP2/3 → nucleates branched actin at the front

• Profilin → loads ATP onto actin → promotes plus-end growth

• Cofilin → depolymerizes old ADP-actin at the back

17. A defective cell is able to create long, unbranched actin filaments, but no actin filament webs. This cell is likely missing the nucleating protein called _____________________or it could also be missing the positive allosteric regulator of the nucleating protein called__________________ One cellular process that this defective cell would not be able to perform is ___________________________.

 

ARP2/3

ARP2/3 activating factor

amellipodia formation / cell migration

 

18. How would cell division be impacted if lamins could not be phosphorylated?

Nuclear envelope wouldn’t break down →

• Mitosis fails (chromosomes can’t separate)

19. Cancer is a disease that involves the overproliferation of cells. Chemotherapy drugs often act by inhibiting cell proliferation. What cytoskeletal processes could be targeted by chemotherapy drugs to interfere with cell proliferation? (List all answers that apply.)

• MT polymerization (spindle can’t form)

• MT depolymerization (spindle collapses)

• Contractile ring (actin + myosin)

• Lamin disassembly (nuclear envelope)

20. You are characterizing a new signal transduction pathway. To definitively determine if it is synaptic, endocrine, paracrine or juxtacrine, what are the most important considerations?

• Distance signal travels

• Delivery method (blood, local diffusion, direct contact, synapse)

21. If Oxytocin was able to bind receptors and initiate signaling in the very same cell that produced it, this type of signaling would be referred to as_____________________.

Autocrine signaling

22. You are a cell biologist who specializes in studying signal transduction. Your labmate mentions that upon adding one type of ligand to their cells, they noticed two different cellular responses were ultimately triggered—gene transcription and activation of a metabolic enzyme. What general pathway mechanism is occurring in your labmate’s cells?

Divergent signaling (divergent cross-talk)

• One ligand activates multiple intracellular pathways, producing different responses.

23. You are a cell biologist who specializes in studying signal transduction. After adding two different ligands to your cells, each of which bound to a distinct receptor, you notice that only one cellular response was triggered. What general pathway mechanism is occurring in your cells?

convergent signaling (convergent cross-talk)

24. Next, you notice that the cellular response triggered by the pathway eventually stops without you adding any other factors to your cells. What general pathway mechanism is occurring in your cells that enables the pathway to shut itself off?

Negative feedback

• A downstream component inhibits an earlier step, shutting the pathway off

25. An individual has an excess of Leptin, but is notfeeling satiated (full). What is a possible explanation?

Leptin resistance

• Target cells aren’t responding to leptin

26. Leptin is a secreted signaling molecule that travels through the (i)_______________ to reach target cells in the hypothalamus in the brain; therefore, it participates in (ii) _______________ signaling. Choose the most correct answer.

(i) Bloodstream
(ii) Endocrine signaling

27. You just started dating someone and are interested in figuring out if they are biochemically inclined to be monogamous. You somehow managed to get blood and tissue samples from this person. If you are hoping that this person is biochemically inclined to monogamy, what should you be looking for?

Higher oxytocin receptor expression

28. Nuclear receptors all contain a _______________ domain that allows them to function as

• DNA-binding domain

• transcription factors

29. You want to create a drug to help people suffering from a loss-of-function mutation in a GPCR. The mutation prevents GPCR pathway activation. Which of the following would be an effective way for your drug to activate the GPCR signaling pathway downstream of the GPCR?

• Activate the G-protein directly (force GDP→GTP)

• OR activate downstream enzymes (adenylyl cyclase or PLC)
  
  extra: (dont memorize)

• A drug that acts like a GEF → forces the Gα subunit to exchange GDP for GTP

• A drug that inhibits GAPs → keeps Gα in the active GTP-bound form

• A drug that directly activates adenylyl cyclase (cAMP pathway)

• A drug that directly activates PLC (IP₃/DAG pathway)

30. Why is oxytocin regarded as both a neurotransmitter and a hormone?

• It is released in the brain → acts as a neurotransmitter

• It is also secreted into the bloodstream → acts as a hormone

31. Which of the following accurately describes trimeric G protein activation?

• Ligand binds the GPCR

• GPCR acts as a GEF → Gα releases GDP and binds GTP

• Gα–GTP separates from Gβγ

• Both active parts (Gα–GTP and Gβγ) signal to downstream targets

32. Which of the following is NOT true regarding the Nitric Oxide (NO) receptor?

❌ “The NO receptor is a transmembrane receptor”
❌ “NO cannot cross the plasma membrane”
❌ “NO receptor is a GPCR”
❌ “NO receptor works through cAMP instead of cGMP”

33. An individual has blood vessels that do not dilate properly and are too constricted. [Based on what you learned in Lect 16/17], what could be the molecular cause of this condition?

• Impaired NO signaling

  • Could be due to:

    • Low NO production, or

    • Nonfunctional NO receptor (guanylyl cyclase) → cannot make cGMP

• Without NO → smooth muscle can’t relax → vessels stay constricted

34. In the PLC pathway, what would occur if IP₃ were unable to bind to Ca²⁺ channels?

• Ca²⁺ would not be released from the ER

• PKC would NOT be fully activated (needs DAG and Ca²⁺)

• Downstream signaling requiring Ca²⁺ would fail

35. How is PKA activated in the cAMP pathway?

cAMP binds regulatory subunits → catalytic subunits released → PKA active

36. What is the role of cAMP in the cAMP pathway?

• Second messenger that activates PKA by binding its regulatory subunits.

37. What happens to CREB when the cyclic AMP pathway is activated?

• PKA phosphorylates CREB,

• Phosphorylated CREB activates gene transcription.

38. Lithium, a drug prescribed for a number of neuropsychiatric disorders, inhibits a Wnt pathway signaling protein called GSK3. (a) When GSK3 is inhibited, how is the Wnt pathway affected? (b) If Lithium effectively treats these conditions, how is Wnt signaling involved in these neuropsychiatric disorders? (ie., What could be causing the neuropsychiatric disorders in these individuals?)

(a) GSK3 inhibited → β-catenin not degraded → Wnt signaling increases.

(b) Disorders likely caused by underactive Wnt signaling (too much β-catenin degradation)

39. What could inhibit (i.e., prevent) the target mechanism of the Wnt signaling pathway in a responding cell? (For example, activating GSK3, preventing Wnt from binding its receptor, etc...)

 

• Activate GSK3 (degrades β-catenin)

• Prevent Wnt–Frizzled binding

• Block Disheveled

• Block β-catenin entry or transcription

40. In response to high levels of ligand, GPCRs activate kinases that phosphorylate the intracellular domain of GPCRs to create a high affinity-binding site for Arrestin. What is this process an example of?

Receptor desensitization

41. The GPCR receptor involved in the cAMP pathway has mutations that prevent it from being phosphorylated. What else could cause the same phenotypic outcome for this pathway?

• Arrestin is missing or nonfunctional

• Kinases that phosphorylate the GPCR are missing

• Receptor is mutated so Arrestin can’t bind even if phosphorylated

42. You are studying Ghrelin signaling in a mouse model. Ghrelin was released after an extended period of not eating. Hunger was apparently triggered because the mouse ate. After the mouse stopped eating and was no longer hungry, you noticed that it still had very high levels of extracellular Ghrelin present in the hypothalamus. (a) If the receptor it binds to was still on the surfaceof the responding cells, but it was not triggering downstream signaling, what molecular process is occurring? (b) If the receptor it binds to is no longer present on the surface of the responding cell, what molecular processes could be occurring?

A: Desensitization (phosphorylation + Arrestin)
B: Sequestration or Down-regulation

43. Chronic users of opiates often increase the dose of opiates they consume over time. At the same time, they experience reduced feelings of euphoria. What is the molecular basis for the diminished response? What are the three variations of how this happens?

Because the opioid GPCRs become desensitized.

Three ways this happens:

1. Inactivation – receptor is phosphorylated → uncouples from G-protein

2. Sequestration – receptor is internalized into endosomes

3. Down-regulation – receptor is sent to lysosomes and destroyed

44. You want to inhibit the cAMP pathway downstream of the G protein. Describe three steps that you could block, and describe the specific mechanism of the pathway that would be missing (why/how this would stop the pathway).

45. You want to inhibit the PLC pathway downstream of the G protein. Describe three steps that you could block, and describe the specific mechanism of the pathway that would be missing (why/how this would stop the pathway).

• Block adenylyl cyclase → no cAMP made → PKA stays inactive

• Block PKA activation → catalytic subunits never released → no phosphorylation

• Block CREB phosphorylation → CREB stays inactive → no gene transcription

46. A mitogen is present and has bound to an RTK receptor, but the cell cycle is not moving past the G1/Start Checkpoint. What are possible explanations?

• Ras/MAPK not activated → no Myc

• No G1 cyclin/G1-Cdk activity

• Rb not phosphorylated → E2F stays off

• Bad conditions (no anchorage, low nutrients, high density)

47. Which of the following is required for a cell to pass the G1/Start Checkpoint? (Assume the cell is not cancerous.)

• Active G1-Cdk

• Rb phosphorylated → E2F released

• E2F activates S-phase genes (incl. S-cyclin)

• Good conditions

48. What could prevent entry into S phase?

• G1-Cdk inactive

• Rb not phosphorylated → E2F blocked

• S-phase genes not activated

• Bad conditions

49. If Ras had a mutation that prevented it from activating the MAP kinase cascade, what would the consequence be?

• No MAPK activation

• No Myc expression

• No G1 cyclin → G1-Cdk stays inactive

• Rb not phosphorylated → E2F blocked

• Cell cannot enter S phase

50. A cell passed the G1/Start checkpoint and is trying to progress through the cell cycle, but it cannot initiate M-phase because the chromosomes are not condensing and the nuclear envelope is not disassembling. What is the most likely problem?

M-Cdk is not active.

51. You are examining a cell that has a frameshift mutation in APC/C that prevents it from functioning. What would happen to this cell during the cell cycle?

• Cell is stuck in metaphase

• Sister chromatids can’t separate (securin not degraded)

• No anaphase starts

• M-cyclin isn’t degraded, so the cell can’t exit mitosis