Organization and role of microtubules and microfilaments
a) Microtubules
- Compared to the other cytoskeletal fibers, the microtubule is rather large (15 to 35 nm diameter).
- Microtubules are composed of a globular protein, tubulin.
- The tubulin subunit is a heterodimer of alpha- and beta-tubulin .
- The microtubule itself is made up of 13 “proto-filaments”, which are each composed of alternating alpha and beta subunits.
- These protofilaments are cylindrically arranged to form a hollow tube.
- It is the arrangement of proto-filaments that makes up the microtubule.
- Microtubules are polar molecules i.e. they have a fast growing “plus end” and a slow growing “minus end”.
- These strands are in a constant state of flux, termed “dynamic instability” (i.e. they continuously grow and fall apart).
- Free GTP (guanosine triphosphate) binds to the beta-tubulin, which alters its protein structure and enables it to join to the growing end of the strand.
- Then, there is a delayed GTP hydrolysis reaction to yield GDP (guanosine diphosphate).
- Thus, at the terminus of the filament a GTP cap is present (i.e. tubulin subunits are bound to GTP), while further down the strand, tubulin subunits are bound to GDP.
- The presence of GDP creates a weak bond between tubulin subunits, and therefore the filament is more likely to de-polymerize.
- In other words, if all the GTP is hydrolyzed in the filament, the filament is susceptible to falling apart (i.e. under a microscope, the microtubules are observed to shrink or even disappear).
- There are various proteins that are able to stabilize the microtubule, preventing de-polymerization these are known as “cap-proteins” or MAPs (Microtubule-Associated Proteins).
- There are also various other chemicals that can either stabilize or destabilize microtubules
- Another group of cytoskeletal proteins are the intermediate filaments (IFs).
- IFs have an intermediate size between microtubules and actin filaments (7 to 10 nm diameter).
- There are, however, two general types of IFs:
- (1) cytoplasmic IFs, and
- (2) nuclear lamina.
- Cytoplasmic IFs are for mechanical stress and cell-to-cell junctions.
- Nuclear lamina create a meshwork beneath the inner nuclear membrane.
- One important difference to note between IF proteins and proteins for microtubules or for microfilaments, is that most IF protein subunits are filamentous, rather than globular (i.e. tubulin and actin subunits are globular).
b) Microfilaments
- Actin filaments (or “microfilaments” these terms are used interchangeably) are the smallest of the cytoskeletal fibers (3 to 6 nm diameter).
- Microfilaments are flexible doubled-stranded fibers composed of polymers of the protein actin (contrast this structure to microtubules, which are hollow tubes composed of 13 protofilaments).
- Actin is present in all eukaryotes, and microfilaments are typically found in the cell cortex (i.e. just beneath the cell membrane).
- The actin subunit is globular and has a molecular mass of 43 kDa.
- As with microtubules, actin filaments are also dynamic and polar molecules (i.e. they have a fast growing “plus end” and a slow growing “minus end”).
- Free actin binds ATP (adenosine-triphosphate, as opposed to GTP in tubulin subunits) which enables polymerization, while ATP hydrolysis to ADP favours depolymerization.
- Unlike microtubules which undergo dynamic instability, actin filaments may use a different dynamic process.
Actin Binding Proteins.
- The actin-based cytoskeleton functions for bearing of tension and for compression resistance (i.e. it is like a shock suspension system, which gives mechanical strength and maintains structural integrity of a cell).
- In fact, there are different microfilament arrangements for a variety of functional purposes.
- There are, however, two general classes of microfilament arrangement:
- (1) bundles (parallel and contractile), and
- (2) gel-like networks.
- Different proteins (i.e. actin-binding proteins) mediate the different arrangements.
- Parallel bundles are structures where microfilaments are oriented with the same polarity (i.e. plus ends are all “pointed” in the same direction) and which are closely spaced.
MCQs on Microtubules and Microfilaments
- What is the typical diameter range of microtubules?
A) 3–6 nm
B) 7–10 nm
C) 15–35 nm
D) 50–70 nm
Answer: C) 15–35 nm - Microtubules are made up of which protein subunit?
A) Actin
B) Tubulin
C) Keratin
D) Myosin
Answer: B) Tubulin - Each microtubule is composed of how many protofilaments?
A) 5
B) 9
C) 13
D) 20
Answer: C) 13 - What type of dimer forms the basic building block of microtubules?
A) Gamma and beta
B) Alpha and gamma
C) Alpha and beta
D) Beta and delta
Answer: C) Alpha and beta - The process of continuous growth and disassembly of microtubules is termed:
A) Dynamic equilibrium
B) Dynamic instability
C) Molecular fluctuation
D) Treadmilling
Answer: B) Dynamic instability - Which molecule binds to beta-tubulin to promote polymerization?
A) ATP
B) ADP
C) GTP
D) GDP
Answer: C) GTP - What happens when GTP in a microtubule is hydrolyzed to GDP?
A) The microtubule elongates
B) The filament becomes more stable
C) Depolymerization is favored
D) MAPs are released
Answer: C) Depolymerization is favored - Which proteins stabilize microtubules by capping them?
A) Actin-binding proteins
B) MAPs (Microtubule Associated Proteins)
C) Dyneins
D) Keratin
Answer: B) MAPs - Intermediate filaments are mainly involved in:
A) Transport of organelles
B) ATP synthesis
C) Mechanical strength and support
D) DNA replication
Answer: C) Mechanical strength and support - The nuclear lamina is a type of:
A) Microtubule
B) Actin filament
C) Cytoplasmic IF
D) Nuclear IF
Answer: D) Nuclear IF
- What is the diameter range of actin filaments?
A) 3–6 nm
B) 7–10 nm
C) 13–16 nm
D) 20–25 nm
Answer: A) 3–6 nm - Actin filaments are made of:
A) Tubulin dimers
B) Globular actin monomers
C) Keratin fibers
D) Lamin units
Answer: B) Globular actin monomers - Which nucleotide is used by actin to polymerize?
A) GTP
B) GDP
C) ATP
D) ADP
Answer: C) ATP - Which structure is typically found just beneath the plasma membrane and composed of actin filaments?
A) Nucleolus
B) Cell cortex
C) Centriole
D) Ribosome
Answer: B) Cell cortex - Microfilaments are involved in which type of cellular function?
A) Spindle formation
B) Compression resistance and tension bearing
C) DNA replication
D) Lipid synthesis
Answer: B) Compression resistance and tension bearing - Which term best describes actin filaments with plus and minus ends?
A) Non-polar
B) Asymmetric
C) Bipolar
D) Polar
Answer: D) Polar - What happens to actin filaments when ATP is hydrolyzed to ADP?
A) Polymerization is promoted
B) Depolymerization is promoted
C) Treadmilling stops
D) Actin-binding proteins detach
Answer: B) Depolymerization is promoted - What are the two general arrangements of actin filaments?
A) Helical and radial
B) Bundles and networks
C) Loops and spirals
D) Rigid and flexible
Answer: B) Bundles and networks - Which type of actin bundle has all filaments oriented in the same direction?
A) Contractile bundle
B) Antiparallel bundle
C) Parallel bundle
D) Circular bundle
Answer: C) Parallel bundle - Which proteins are responsible for different arrangements of actin filaments?
A) MAPs
B) Dyneins
C) Actin-binding proteins
D) Kinesins
Answer: C) Actin-binding proteins
- Which of the following best describes the structure of microtubules?
A) Single strand of actin
B) Hollow tube of 13 protofilaments
C) Triple helix of actin
D) Sheet of beta-pleated proteins
Answer: B) Hollow tube of 13 protofilaments - Which cytoskeletal element shows treadmilling behavior in some cases?
A) Intermediate filaments
B) Actin filaments
C) Collagen
D) Ribosomes
Answer: B) Actin filaments - What does a ‘GTP cap’ on a microtubule indicate?
A) Microtubule is shrinking
B) Microtubule is stable and growing
C) Microtubule is completely depolymerized
D) Protein synthesis is active
Answer: B) Microtubule is stable and growing - Which of the following favors the disassembly of microtubules?
A) Presence of GTP-bound tubulin
B) MAP binding
C) Hydrolysis of GTP to GDP
D) Presence of ATP
Answer: C) Hydrolysis of GTP to GDP - The term ‘dynamic instability’ is mostly associated with:
A) Actin filaments
B) Intermediate filaments
C) Microtubules
D) Plasma membrane
Answer: C) Microtubules - Which cytoskeletal component is considered the most stable and least dynamic?
A) Microtubules
B) Microfilaments
C) Intermediate filaments
D) Centrioles
Answer: C) Intermediate filaments - Which of the following statements is true about actin?
A) Actin binds GTP for polymerization
B) Actin forms hollow filaments
C) Actin polymerizes in the presence of ATP
D) Actin is not found in animal cells
Answer: C) Actin polymerizes in the presence of ATP - Which protein assists in cross-linking actin filaments into gel-like networks?
A) Tubulin
B) Spectrin
C) Myosin
D) ATPase
Answer: B) Spectrin - Contractile actin bundles are typically found in:
A) Flagella
B) Muscle cells
C) Spindle fibers
D) Cilia
Answer: B) Muscle cells - Which cytoskeletal element is most involved in chromosome separation during mitosis?
A) Intermediate filaments
B) Actin filaments
C) Microtubules
D) Myosin fibers
Answer: C) Microtubules - Which statement about actin filaments is FALSE?
A) They are polar structures
B) They are composed of tubulin
C) They use ATP for polymerization
D) They form bundles and networks
Answer: B) They are composed of tubulin - Depolymerization of actin is favored when actin is bound to:
A) ATP
B) ADP
C) GTP
D) GMP
Answer: B) ADP - Which process is microtubule-dependent?
A) Formation of nuclear lamina
B) Cytokinesis
C) Cilia movement
D) Transcription
Answer: C) Cilia movement - Which one of the following is NOT a feature of intermediate filaments?
A) Composed of globular subunits
B) Provide mechanical strength
C) Form nuclear lamina
D) Do not show polarity
Answer: A) Composed of globular subunits - Which of the following is a key difference between microtubules and microfilaments?
A) Microfilaments are hollow
B) Microtubules are more flexible
C) Microtubules are composed of actin
D) Microfilaments are composed of actin, not tubulin
Answer: D) Microfilaments are composed of actin, not tubulin - What is the molecular weight of globular actin monomer?
A) 20 kDa
B) 32 kDa
C) 43 kDa
D) 56 kDa
Answer: C) 43 kDa - Which cytoskeletal component is involved in vesicle transport inside cells?
A) Actin
B) Microtubules
C) Intermediate filaments
D) Myosin
Answer: B) Microtubules - Which motor protein moves along actin filaments?
A) Kinesin
B) Dynein
C) Myosin
D) Tubulin
Answer: C) Myosin - Microtubule-organizing centers (MTOCs) are found at:
A) Cell membrane
B) Endoplasmic reticulum
C) Centrosome
D) Golgi body
Answer: C) Centrosome - Which of the following is a function of actin-binding proteins?
A) Catalyze ATP synthesis
B) Organize actin filaments into functional structures
C) Produce tubulin
D) Hydrolyze DNA
Answer: B) Organize actin filaments into functional structures
- Which of the following drugs binds tubulin and prevents microtubule polymerization?
A) Taxol
B) Colchicine
C) Phalloidin
D) Cytochalasin D
Answer: B) Colchicine - Which of these microtubule-associated proteins (MAPs) is primarily involved in neuronal stability?
A) MAP1
B) MAP2
C) Tau
D) Tubulin
Answer: C) Tau - Microfilaments are predominantly located in which part of the cell?
A) Nucleoplasm
B) Centrosome
C) Cell cortex
D) Mitochondria
Answer: C) Cell cortex - Which of the following stabilizes actin filaments by binding along their sides?
A) Thymosin
B) Profilin
C) Tropomyosin
D) Gelsolin
Answer: C) Tropomyosin - Which of the following structures is formed primarily by actin filaments?
A) Mitotic spindle
B) Flagella
C) Filopodia
D) Centrioles
Answer: C) Filopodia - Which motor protein moves toward the plus end of microtubules?
A) Dynein
B) Myosin
C) Actin
D) Kinesin
Answer: D) Kinesin - Which organelle is responsible for nucleating microtubule formation in animal cells?
A) Nucleolus
B) Centrosome
C) Ribosome
D) Golgi body
Answer: B) Centrosome - Which statement correctly describes intermediate filaments?
A) Composed of actin
B) Involved in intracellular transport
C) Provide mechanical strength
D) Exhibit dynamic instability
Answer: C) Provide mechanical strength - What is the function of gelsolin in actin dynamics?
A) Polymerizes actin
B) Binds ATP to actin
C) Caps and severs actin filaments
D) Cross-links microtubules
Answer: C) Caps and severs actin filaments - Which cytoskeletal system is essential for cilia and flagella motion?
A) Actin filaments
B) Intermediate filaments
C) Microtubules
D) Keratin filaments
Answer: C) Microtubules - Which cytoskeletal element participates in phagocytosis?
A) Actin filaments
B) Intermediate filaments
C) Microtubules
D) Keratin filaments
Answer: A) Actin filaments - Which of these proteins promotes actin filament polymerization by facilitating ATP-actin addition?
A) Thymosin
B) Formin
C) Tropomyosin
D) Gelsolin
Answer: B) Formin - In cell division, the spindle apparatus is primarily made of:
A) Actin filaments
B) Intermediate filaments
C) Microtubules
D) DNA
Answer: C) Microtubules - Which actin-binding protein prevents polymerization by sequestering actin monomers?
A) Profilin
B) Gelsolin
C) Tropomyosin
D) Thymosin
Answer: D) Thymosin - Actin treadmilling involves:
A) Only addition at the plus end
B) Polymerization at both ends
C) Addition at plus end and loss at minus end
D) Addition at minus end and loss at plus end
Answer: C) Addition at plus end and loss at minus end - Which type of cytoskeletal filament has no inherent polarity?
A) Actin filaments
B) Microtubules
C) Intermediate filaments
D) All of the above
Answer: C) Intermediate filaments - Microtubule dynamics are essential for which of the following?
A) Nuclear envelope formation
B) Mitochondrial DNA replication
C) Chromosome segregation
D) Ribosome synthesis
Answer: C) Chromosome segregation - Which compound stabilizes microtubules rather than causing their disassembly?
A) Colchicine
B) Vinblastine
C) Taxol
D) Nocodazole
Answer: C) Taxol - The basal body of cilia is structurally similar to:
A) Ribosome
B) Centrosome
C) Golgi body
D) Actin filament
Answer: B) Centrosome - Which of the following structures do not involve microtubules?
A) Cilia
B) Mitotic spindle
C) Nuclear lamina
D) Flagella
Answer: C) Nuclear lamina