Cytoskeleton

Question 1

What is the cytoskeleton?

Answer 1

• Network of filaments in the cytosol
• All filaments bind a target; link organelles
to each other and to the PM
• Composed of 3 main types of filaments:
• Microfilaments—whole-cell movement,
muscle contraction, cell shape, cytokinesis
• Intermediate filaments—strength
• Microtubules—movement of cilia and
flagella, intracellular trafficking, mitotic
spindle formation

Question 2

Actin is a central component of microfilaments

Answer 2

• Actin present in all cells but
different isoforms predominate in
various cell types
• G-actin monomers polymerise →
F-actin microfilament

Question 3

Functions of Actin

Answer 3

Forms structural and motility
systems:
• Helps maintain cell shape
• cf. spectrin
• Provides structural support for
plasma membranes (eg.
dystrophin), including
protrusions (eg. microvilli)

Question 4

Functions of Actin

Answer 4

Restricts diffusion of organelles
• At the cell cortex, actin network excludes
organelles
• Provides anchor for cell adhesion
molecules
• Contraction (cf. myosin)
• Muscle cells
• During telophase of mitosis, actomyosin
contractile ring forms

Question 5

Cells migrate along extracellular matrix via actin

Answer 5

Necessary during embryogenesis, movement of axons in response to growth factors, movement of white blood cells toward site of infection, phagocytosis • Leading edge temporarily attaches to extracellular matrix

Question 6

What are Thin Filaments?

Answer 6

Name given to actin, especially in muscle cells • Composed mainly of F -actin • Tropomyosin • Troponin complex (skeletal, cardiac cells) • TnI • TnC • TnT

Question 7

Myosin

Answer 7

Thick filament • Actin-binding, motor protein • Binds ATP • Found in muscle, non-muscle cells. • Different isoforms exist

Question 8

Intermediate Filaments

Answer 8

Diameter of microtubules > IF > microfilaments • Impart mechanical strength to resist mechanical forces placed on cell • Eg. Fluid sheer stress in blood vessels; stress in contracting skeletal muscles • Subunits are heterogeneous • No ATP nor GTP required for polymerization • No polarity • Major IFs: keratins, vimentins (incl. desmin), neurofilaments, lamins, nestin

Question 9

Epithelial IFs form strong attachment sites at cell surface

Answer 9

• Keratins are IFs found in cytosol of epithelial cells • Interconnect desmosomes in neighbouring cells to help stabilize epithelial sheets • Desmosome: adhesive junction that helps keep adjacent cells joined together • Associated with strong adhesion between epithelial cells and underlying ECM

Question 10

Neurofilaments

Answer 10

Neurofilaments in nerve cell axon serve as structural support to resist breakage • Extend along length of axon

Microtubules (green), intermediate filaments (purple) and actin filaments (red).

Question 11

Vimentin, Desmin

Answer 11

Vimentin family of proteins includes desmin and vimentin • Vimentin found in mesoderm-derived cells. Widely distributed in the body • Desmin expressed in all types of muscle cells • Important for stabilizing the contractile apparatus

Question 12

Lamins

Answer 12

Lamins found exclusively in all nuclei: lamins A, B, C are part of nuclear lamina • Line inner nuclear surface, protecting chromatin from damage due to mechanical stress • Disassemble at start of mitosis • Re-form lamina at end of mitosis

Question 13

Nestin

Answer 13

Expressed widely: neural progenitor cells, many other cell types • Associated with cell proliferation

Question 14

Microtubules

Answer 14

Cytosolic, hollow, polar cylinders made of αand β-tubulin heterodimers

• Functions include: motility (cilia and flagella), chromosomal movements during cell division (mitosis, meiosis), transport of intracellular vesicles
• Can rapidly assemble and disassemble
• Can re-direct traffic in a cell by disassembling m’tubules in one region and assembling in another

Question 15

Assembly of Microtubules

Answer 15

• α- and β-tubulin subunits each have bound GTP during assembly • Add to plus end • Rapidly assemble, disassemble • For mitosis, m’tubules disassemble, reassemble to form mitotic spindle, then disassemble for chromosomal separation

Question 16

Assembly of Microtubules

Answer 16

• Centrosome composed of 2 centrioles plus pericentriolar material • Centrosome complex caps minus end during elongation of m’tubule

Question 17

Dynamic instability—Growth and breakdown of microtubules

Answer 17

Assembly then catastrophic breakdown of m’tubules = dynamic instability • Minus end capped at centrosome complex • Tubulin subunits, each with bound GTP, add to plus end • β-tubulin GTP hydrolysed to GDP. (αtubulin always maintains its GTP) • Presence of GTP at + end encourages faster assembly • For elongation, GTP-tubulin addition must occur faster than nucleotide hydrolysis

Question 18

Dynamic instability—Growth and shrinkage of microtubules

Answer 18

• Disassembly occurs from plus end • Capping plus end (microtubule capping proteins) prevents disassembly

Question 19

Vesicle and Organelle Transport

Answer 19

Movement of intracellular vesicles, secretory vesicles, organelles, etc. • Transport requires energy (ATP) • M’tubule motor proteins (ATPases) needed • Kinesin, kinesin-related proteins— anterograde transport • Cytoplasmic dynein—retrograde transport

Question 20

Movement of Cilia and Flagella —Axoneme

Answer 20

Cilia found in respiratory tract lining (clear mucus), lining of oviduct (transport eggs to uterus) • Flagellum allows sperm to swim • Axoneme: 9+2 arrangement of m’tubules extending from basal body, anchored in plasma membrane • Axonemal dynein (an ATPase) needed for motility • Axoneme composed of m’tubules and other proteins to allow ‘whipping’ of cilia and wave -like motion of flagella • These m’tubules are stable; resist breakdown

Question 21

Dynamics in Mitosis

Answer 21

• In interphase, • Centrosome is replicated • M’tubules disassemble • New centrosomes form poles of spindle • M’tubules orient spindle for cell division • M’tubules growing from each centrosome attach chromatids to spindle poles • Transports chromosomes to poles during anaphase

Question 22

A 39-year-old man presents with progressive weakness in the muscles of his lower limbs. Investigations reveal a mutation in the gene coding for desmin. Which of the following would be most directly affected in these muscle cells of the patient?

A) Breakdown of nuclear membrane B) Maintenance of structural support C) Movement of acetylcholine-containing vesicles D) Movement of cilia E) Movement of entire cell

Question 23

Which of the following is most effective in promoting polymerization of the cytoskeletal filament important for the movement of a white blood cell?

A) Cross-linking of actin with myosin B) Interaction of keratin with GTP C) Interaction of tubulin with GTP D) Phosphorylation of ADP bound to actin E) Phosphorylation of lamins

Question 24

A 15-year-old male is brought to the physician because of shortness of breath during a sporting activity. He was diagnosed with cystic fibrosis at 3 months of age. Which of the following describes the most likely route or mechanism by which normal copies of the affected protein would be transported to their final destination?

A) Along polymers of tubulin B) Along vimentin filaments C) Binding to importin at nuclear pores D) Through activity of flagella E) Through activity of cilia F) Via axonemal dynein