MCB 252 Exam 3 Review: Cytoskeleton, Motors, and Molecular Cell Biology

Comprehensive practice flashcards covering actin dynamics, myosin motors, intermediate filaments, cytoskeletal regulation, and experimental techniques from MCB 252.

Does the addition of the actin-depolymerizing drug latrunculin to cells halt phagocytosis?

Yes, latrunculin stops phagocytosis because dynamic actin polymerization is required to extend the plasma membrane around phagocytosed particles.

As discussed in lecture, the contractile bundles (sarcomeres) found in muscle cells most

likely appeared in evolution before the contractile bundles found in stress fibers.

False

Intermediate filaments are considered equilibrium polymers, as opposed to actin

filaments which are best described as steady state filaments

True

As described in lecture, because actin filaments are helical, any motor protein that

moves along those filaments must follow a helical path

False

As described in lecture, a polyclonal antibody contains many different antibodies, each

of which recognizes a distinct epitope on the surface of an antigen.

True

To which end of an actin monomer does the regulatory protein profilin bind?

The plus (barbed) end.

Exon shuffling occurs at the level of DNA in the genome; whereas, splicing occurs at the

level of RNA

True

As described in lecture, Homo sapiens migrated out of Africa before Neanderthals

migrated out of Africa

False

In lecture, we discussed the modular construction of some genes and the evolution of

genes (and proteins) by the addition of domains. As described in lecture, this process

occurs in prokaryotes and eukaryotes

False

As described in lecture, all cytoskeletal polymers are multi-stranded.

True

Which property of eukaryotic gene regulatory regions, as discussed in lecture, allowed

for the evolution complex regulatory regions?

That enhancers can act over long distances

Which of the following statements is/are TRUE, and which is/are FALSE?

i = The majority of different colors of fluorescent proteins used by cell biologist are all

naturally occurring proteins that were isolated from different types of aquatic

organisms.

ii = As described in lecture, when purifying a protein of interest one needs an assay that is

specific for that particular protein.

i is FALSE, and ii is TRUE.

Congratulations! You finally managed to build a ray gun that allows you to shrink yourself

down to the size of a molecule. Like any normal person, your first idea is to stand on top of a

formin protein. That formin protein that you are standing on is immobilized on a solid

surface. Actin monomers then start to arrive at the formin upon which you are standing.

Looking down the filament, what do you see?

The minus end of the actin filament is moving away from you.

You have now elected to stand on an Arp2/3 complex. The Arp2/3 complex just nucleated a

new actin filament, and that daughter filament has polymerized to add several thousand actin

subunits. Now you see that a myosin II molecule (similar to the myosin that drives muscle

contraction) has landed on the middle of the daughter filament produced by the Arp2/3

molecule upon which you are standing. What do you see?

No worries. The myosin is moving away from you.

Which of the following does not require one or more motor proteins for movement?

Movement of Listeria in the cytoplasm of eukaryotic cells

As discussed in lecture, there are approximately how many different types of cells in

humans?

210

In order to determine the structure of a protein by X-ray crystallography you need

which of the following?

D. Both A and B

Which of the following is correct with regard to the two statements below?

i = As described in lecture, the concentration of actin monomers in cells is approximately

100x the critical concentration.

ii = As described in lecture, the plus end of a typical actin filament has a higher critical

concentration than the minus end.

i is TRUE, and ii is FALSE.

Which step in the ATPase cycle of myosin II results in rotation of the head into the

“cocked” position?

Hydrolysis of ATP to ADP + Pi

Which of the two statements below is/are TRUE, and which is/are FALSE?

i = Dictyostelium MIA myosin is more closely related to Acanthamoeba HMW myosin

than it is to Human brush border MI myosin.

ii = Chicken MV myosin is more closely related to Budding yeast Myo4 than it is to

Acanthamoeba MII.

i is FALSE, and ii is TRUE

The image below was discussed in lecture. What type of cell is shown in this image?

A red blood cell

Technique:

i = column chromatography

ii = X-ray crystallography

iii = production of a GFP fusion protein in a living cell

Goal:

X = determine the structure of a protein

Y = determine the sub-cellular localization of a protein

Z = purify a protein

i = Z, ii = X, iii = Y

You are carrying out in vitro chemical reactions using an enzyme that produces a

deadly cyanide gas when you add the substrate for that enzyme. You have produced a

monoclonal antibody and a polyclonal antibody against your cyanide-producing

enzyme. Normally when you work with your enzyme you wear protective gear to

prevent you from inhaling the deadly gas that is produced. However, today you

accidentally add the substrate to the enzyme before you put on your protective gear

and rapidly find yourself in a forming cloud of cyanide gas. You start to become weak

and realize you need to stop the reaction in order to save your life. Luckily, you have

taken MCB 252 and realize that your two tubes of antibodies could potentially help you

out. Unfortunately, you only have time to add one of the tubes of antibodies before you

are overcome by the gas. Which tube of antibodies would be best to add?

The tube containing the polyclonal antibody.

In Xenopus laevis, proper tissue development requires which of the following?

E. All of the above (A, B, and C)

In this actin polymerization time course curve, what molecular processes account for phases A and B, and what occurs at phase C if total actin is doubled?

Phase A is the nucleation (lag) phase where monomers form trimers; phase B is the elongation phase with rapid monomer addition; at phase C (steady state), free actin concentration remains unchanged at the critical concentration ($C_c$).

If formins associate with actin plus ends at the leading edge of a lamellipodium, which biophysical model of membrane protrusion is favored?

The model in which actin filament elongation exerts pushing force directly against an anchored transmembrane protein.

What family of crosslinking proteins is denoted by 'X' linking actin filaments to transmembrane receptors?

ERM family members (Ezrin, Radixin, and Moesin).

What is a muscle fiber?

A single, multinucleated muscle cell formed by the developmental fusion of myoblasts.

In this in vitro gliding assay where immobilized myosin heads propel actin filaments in the direction shown, which end is labeled 'X'?

The plus (barbed) end, because myosin heads walk toward the plus end, thereby propelling the minus end forward and leaving the plus end at the trailing rear.

What conclusion was established by observing motility of ActA-coated beads in cell extracts?

Actin polymerization nucleated by ActA is sufficient by itself to produce mechanical force and propel a membrane or bead without motor proteins.

Which actin-binding protein packages actin filaments into tightly spaced, parallel bundles of identical polarity?

Fimbrin.

In this skeletal muscle ultrastructure diagram, what is structure Y, and which labeled structure stores calcium ($\text{Ca}^{2+}$)?

Structure Y is a transverse tubule (T-tubule), and structure X (the sarcoplasmic reticulum) stores intracellular $\text{Ca}^{2+}$.

What are the hallmarks of a biological system existing at steady state?

At least one parameter remains constant over time while others can fluctuate, and maintenance of the state requires a continuous consumption/flux of energy.

From which primary embryonic germ layer does the neural tube originate in Xenopus?

Ectoderm.

How does injecting phalloidin affect Listeria comet tails in infected host cells?

Comet tails become significantly longer because phalloidin stabilizes F-actin and blocks filament depolymerization while new assembly proceeds.

Match each Rho family small GTPase (Cdc42, Rac, Rho) with the specific actin arrangement it promotes.

Cdc42 induces filopodia; Rac promotes lamellipodia; Rho stimulates the assembly of contractile stress fibers.

What common properties characterize highly conserved proteins across evolution?

They are typically essential for viability, engage in macromolecular interactions with multiple binding partners, and often perform multiple cellular roles.

Define and contrast transfection, stable transformation, and transgenics.

Transfection transiently introduces foreign DNA into cultured cells; stable transformation involves genomic integration of foreign DNA in cultured cell lines; transgenics integrates DNA into the germline so that it is stably inherited throughout an entire organism.

Which structural feature of the myosin molecule governs the step size it takes along actin filaments?

The length of the lever arm (neck domain).

Match titin, nebulin, and troponin with their structural or regulatory functions in the sarcomere.

Titin acts as a molecular spring; nebulin acts as a molecular ruler specifying actin thin filament length; troponin binds $\text{Ca}^{2+}$ to displace tropomyosin and permit myosin-actin binding.

Name three cellular assemblies or protein complexes that incorporate actin or actin-related proteins (ARPs).

Erythrocyte cytoskeletal junctional complexes, SWI/SNF-type chromatin remodeling complexes, and the Arp2/3 nucleation complex.

In modular multidomain proteins, how are the N- and C-termini of individual folded domains typically oriented?

They point in opposite directions, allowing successive domains to be linked end-to-end without steric clash.

What are the specific biochemical activities of thymosin $\beta_4$, profilin, and cofilin on actin?

Thymosin $\beta_4$ sequesters G-actin monomers; profilin catalyzes ADP-to-ATP nucleotide exchange on monomers; cofilin severs and promotes disassembly of ADP-actin filaments.

Mutations giving rise to the keratin family of intermediate filaments facilitated which major evolutionary transition?

The transition of animals from marine environments to terrestrial life by providing mechanical resilience and protection against desiccation.

What type of mutant allele was engineered to uncover the function of keratin 14 in transgenic mouse models?

A dominant negative allele.

What was the ancestral evolutionary function of the earliest intermediate filament proteins?

Providing structural support and organization to the nuclear envelope (as nuclear lamins).

Are adult stem cells pluripotent or multipotent, and what is the outcome when a stem cell divides?

Adult stem cells are multipotent (not pluripotent), and asymmetric division yields one self-renewed stem cell and one committed progenitor cell.

What is the principal experimental advantage of GFP fusion proteins over immunofluorescence microscopy?

GFP tagging enables live-cell imaging of dynamic protein localization and behavior in intact living cells and whole organisms without chemical fixation.

What is the primary mechanical role of the intermediate filament cytoskeleton?

Providing high tensile strength and mechanical integrity to cells, tissues, and the nuclear envelope (they do not support motor protein transport).

Which modern human phenotypes and protein classes are notably influenced by introgressed Neanderthal alleles?

Skin pigmentation pathways and keratin intermediate filaments.

Given a $100\,\mu\text{M}$ actin solution with $99\%$ F-actin in AMPPNP and $90\%$ F-actin in ADP, what are the critical concentrations ($C_c$) of the plus and minus ends?

The plus-end $C_c$ is $1\,\mu\text{M}$ ($1\%$ of $100\,\mu\text{M}$ remaining unpolymerized with ATP analog), and the minus-end $C_c$ is $10\,\mu\text{M}$ ($10\%$ of $100\,\mu\text{M}$ unpolymerized with ADP).

In this optical trap setup, if a laser beam strikes a bead and is refracted toward the South-West as shown, in which direction is the bead pulled?

North (the refraction deflects photons southward, generating an equal and opposite reactionary momentum force that pulls the bead northward toward the center of the trap).

In this myosin V stepping cycle where inorganic phosphate is released just before step W, in what conformational state is the leading head in panels W and X?

The leading head is in the post-powerstroke conformation in both W and X.