Cytoskeleton, Cell Motility & Junctions – Review

100 question-and-answer flashcards summarizing key concepts on cytoskeletal filaments, motor proteins, cell junctions, and related diseases.

1. What are the three main classes of cytoskeletal filaments?

Microfilaments (actin), intermediate filaments, and microtubules.

2. What is the primary function of the cytoskeleton in a cell?

To provide shape, mechanical strength, and an intracellular ‘highway’ for organelle and vesicle movement.

3. Which cytoskeletal filament is about 7 nm in diameter?

Actin microfilaments.

4. Which cytoskeletal filament is roughly 10 nm in diameter and found only in animal cells?

Intermediate filaments.

5. Which cytoskeletal filament is approximately 25 nm in diameter?

Microtubules.

6. Name the four major types of intermediate filaments found in the cytoplasm.

Vimentin, keratin, neurofilaments, and desmin (muscle-type vimentin).

7. Which intermediate-filament type is located inside the nucleus?

Nuclear lamins.

8. What is the basic building block (monomer) of intermediate filaments?

An α-helical polypeptide that forms a coiled-coil dimer.

9. How many monomers coil to form an intermediate-filament dimer?

Two monomers.

10. What structure is formed when two intermediate-filament dimers align in parallel?

A tetramer (two dimers side-by-side).

11. How are two tetramers arranged to form a stable intermediate-filament rope?

They align antiparallel and staggered, then eight such strands twist together.

12. Why are intermediate filaments considered mechanically strong?

Their rope-like, staggered tetramer structure resists stretching (high tensile strength).

13. Which cell junction uses keratin filaments to connect epithelial cells?

Desmosomes.

14. What cellular feature allows intermediate filaments to resist stretching without breaking?

The extensive lateral packing of staggered tetramers.

15. What is the main role of nuclear lamins?

They provide rigidity and shape to the nuclear envelope.

16. Mutation in lamin A causes which premature aging disorder?

Progeria syndrome.

17. Which motor proteins travel along microtubules?

Kinesins and cytoplasmic dyneins.

18. In which direction do kinesins generally move?

Toward the microtubule + (plus) end, usually toward the cell periphery.

19. In which direction do cytoplasmic dyneins generally move?

Toward the microtubule − (minus) end, usually toward the cell interior/centrosome.

20. What is the basic subunit of a microtubule?

A heterodimer of α-tubulin and β-tubulin.

21. How many protofilaments form a single microtubule?

Thirteen.

22. What molecule binds to both α- and β-tubulin?

GTP.

23. Which tubulin subunit can hydrolyze GTP?

β-tubulin.

24. Where do most microtubules originate inside animal cells?

The centrosome (microtubule-organizing center).

25. What protein ring complex nucleates microtubule growth at centrosomes?

γ-Tubulin ring complex (γ-TuRC).

26. What term describes the rapid switching between growth and shrinkage of microtubules?

Dynamic instability.

27. Name one disease in which defective Tau protein destabilizes microtubules.

Alzheimer’s disease.

28. What antiviral pathway is exploited by rabies virus to reach neurons?

Retrograde dynein-mediated transport along axonal microtubules.

29. Which anticancer drug stabilizes microtubules and blocks mitosis?

Taxol (paclitaxel).

30. What is the diameter of a typical actin filament?

Approximately 7 nm.

31. What nucleotide binds to globular actin monomers?

ATP (or ADP after hydrolysis).

32. Which end of an actin filament most rapidly adds new subunits?

• (plus) end.

33. Which proteins promote the branching of actin filaments at 70° angles?

The Arp2/3 complex.

34. What enzyme promotes unbranched actin filament elongation while preventing capping?

Formin.

35. Which protein exchanges ADP for ATP on actin monomers to speed elongation?

Profilin.

36. What actin-based protrusion forms thin, finger-like sensors at the leading edge?

Filopodia.

37. Which broader, sheet-like protrusion drives cell crawling?

Lamellipodium.

38. During cytokinesis, what structure composed of actin and myosin divides the cytoplasm?

The contractile ring.

39. Which type of myosin is responsible for muscle contraction?

Myosin II.

40. What happens to the myosin head when ATP binds?

It detaches from actin (leading to relaxation).

41. What causes myosin heads to pull actin filaments during contraction?

Release of inorganic phosphate after ATP hydrolysis triggers the power stroke.

42. What type of junction seals neighboring epithelial cells to prevent paracellular leakage?

Tight junction.

43. Name two key transmembrane proteins found in tight junction sealing strands.

Claudins and occludin.

44. Which junction connects actin filaments of one cell to actin filaments of another?

Adherens junction.

45. Cadherin-based junctions are dependent on which extracellular ion?

Calcium (Ca²⁺).

46. Which cadherin is typical of epithelial cells?

E-cadherin.

47. Which cadherin is common in nervous tissue?

N-cadherin.

48. What type of binding occurs when identical cadherins on adjacent cells interact?

Homophilic binding.

49. Differential cadherin expression drives what cellular process during embryogenesis?

Cell sorting (tissue segregation and morphogenesis).

50. Which adaptor protein links classical cadherins to actin filaments?

β-Catenin (together with α-catenin).

51. Name the protein that reinforces adherens junctions under mechanical tension.

Vinculin.

52. Which junction connects intermediate filaments between neighboring cells?

Desmosome.

53. What two non-classical cadherins form desmosomes?

Desmoglein and desmocollin.

54. Which plaque protein links desmosomal cadherins to keratin filaments?

Desmoplakin (with plakoglobin and plakophilin).

55. What junction attaches intermediate filaments to the extracellular matrix?

Hemidesmosome.

56. What type of integrin-based junction links actin to the extracellular matrix?

Focal adhesion (actin-linked cell–matrix junction).

57. How many connexin subunits make one connexon?

Six connexins.

58. Two connexons align to form what kind of intercellular channel?

A gap junction channel.

59. What size limit (in Daltons) applies to molecules passing through gap junctions?

Approximately 1,000 Da.

60. What triggers the activation of integrins from a low-affinity to high-affinity state?

Inside-out signaling via talin, kindlin, or RIAM binding to the β-integrin tail.

61. Which cytoskeletal linker binds activated β-integrin tails to actin filaments?

Talin (often followed by vinculin).

62. Which leukocyte adhesion molecule mediates rolling along the endothelium?

Selectins (E-, P-, or L-selectin).

63. Which immunoglobulin-like molecule provides firm adhesion to integrins on leukocytes?

ICAM (Intercellular Adhesion Molecule).

64. What small GTPase regulates actin dynamics during adherens junction formation?

Rac (also Rho family members).

65. What is the role of centrosomal centrioles in relation to microtubules?

They provide structural support for the centrosome and help organize microtubule nucleation.

66. Which microtubule end is typically embedded in the centrosome?

The − (minus) end.

67. What is the lumen of a microtubule?

The hollow interior space inside the 13-protofilament cylinder.

68. Which actin nucleator is functionally similar to γ-tubulin for microtubules?

The Arp2/3 complex (acts as a nucleation core).

69. What term describes the network of actin beneath the plasma membrane that supports cell shape?

The actin cortex.

70. Which myosin motor moves toward the minus end of actin filaments?

Most myosins, including myosin VI, move toward the minus end (unlike others that move toward plus).

71. What energy molecule is hydrolyzed by motor proteins to generate movement?

ATP.

72. What structural polarity do actin filaments exhibit?

They have distinct + (barbed) and − (pointed) ends.

73. What happens to actin subunits after ATP hydrolysis within the filament?

They become ADP-actin, lowering affinity and favoring dissociation at the − end.

74. Which cellular structures are supported by actin-rich microvilli?

Epithelial absorptive surfaces such as the small intestine.

75. Name the ring of actin filaments that anchors tight junction proteins.

The perijunctional actin belt.

76. What is the approximate angle at which Arp2/3 branches new filaments?

About 70 degrees.

77. Which disease involves microtubule-dependent axonal transport of viral particles?

Rabies (viral spread to the brain via dynein transport).

78. Which cytoskeletal element forms the axoneme of cilia and flagella?

Microtubules (organized in a 9 + 2 arrangement).

79. What is the microtubule arrangement in a typical centriole?

Nine triplet microtubules arranged in a cylinder.

80. Name the protein complex that generates force to separate chromosomes during mitosis.

The mitotic spindle (kinesin- and dynein-based motor complexes on microtubules).

81. What happens to microtubules when GTP on β-tubulin is hydrolyzed?

The protofilaments curve outward, causing rapid depolymerization (catastrophe).

82. What property of intermediate filaments allows them to remain intact when stretched?

Their staggered, nonpolar rope-like architecture distributes tensile stress.

83. Which type of filament has no intrinsic polarity?

Intermediate filaments.

84. Which cytoskeletal element is most important for resisting tensile forces?

Intermediate filaments (e.g., keratin in skin).

85. In muscle cells, what keeps actin filaments anchored at the Z-disc?

α-Actinin and associated proteins.

86. During cell crawling, which end of actin filaments faces the plasma membrane at the leading edge?

The + (plus/barbed) end.

87. What role does myosin-II play at the rear of a crawling cell?

It contracts actin filaments, pulling the trailing edge forward.

88. Which tight junction protein family determines selective ion permeability?

Claudins.

89. What is the function of occludin in tight junctions?

Helps regulate barrier properties and junction stability.

90. How do claudins and occludin connect to the actin cytoskeleton?

Via scaffolding proteins such as ZO-1, ZO-2, and ZO-3 containing PDZ, SH3, and GK domains.

91. Which adaptor protein complex binds to the cytoplasmic tails of tight-junction proteins?

The ZO (zonula occludens) protein family.

92. What are sealing strands?

Interconnected rows of claudins/occludin that form the tight junction barrier between adjacent cells.

93. What is the extracellular matrix ligand for hemidesmosomal integrins?

Laminin (and type IV collagen in basal lamina).

94. Name two intracellular linker proteins in hemidesmosomes.

Plectin and BP230 (bullous pemphigoid antigen 1).

95. What effect does Taxol have on microtubule dynamics?

It stabilizes microtubules, preventing depolymerization and blocking mitosis.

96. How does profilin influence actin polymerization?

It binds actin-ADP, promotes ADP–ATP exchange, and delivers ATP-actin to the + end.

97. What cytoskeletal problem contributes to Alzheimer's disease?

Hyperphosphorylated Tau detaches from microtubules, causing their disassembly.

98. What is dynamic instability?

The stochastic switching of microtubules between phases of growth and rapid shrinkage.

99. Which end of a microtubule shows faster growth when GTP-β-tubulin is present?

The + (plus) end.

100. How do selectins and integrins cooperate during leukocyte extravasation?

Selectins mediate initial rolling adhesion, then integrins bind ICAMs for firm arrest and transmigration.