Cell Biology: Microtubules and Cytoskeleton

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Questions and Answers

Which protein is primarily responsible for microtubule nucleation?

Kinesin

MAP2

Dynein

γTURC (correct)

Which of the following statements about motor proteins is true?

They do not play a role in cell division.

They are powered by chemical energy from ATP or GTP. (correct)

They stabilize microtubules.

They only transport organelles along microfilaments.

What role do microtubules play during cell division?

They guide the movement of cytokinesis.

They form the mitotic spindle. (correct)

They maintain the cell's 3D shape.

They form the cell membrane.

How do cilia and flagella achieve movement?

Through a repeated back-and-forth or wave-like motion. Signup and view all the answers

What is one of the roles of actin microfilaments (MF) mentioned?

Guiding cytoplasmic currents for molecule movement. Signup and view all the answers

What is the role of ARP2/3 in cell migration?

It assists in the assembly of actin microfilaments. Signup and view all the answers

Which myosin family protein is primarily responsible for muscle contraction?

Myosin II Signup and view all the answers

What occurs when the balance between polymerization and depolymerization in microtubules is disturbed?

Disaster phenomenon leading to instability. Signup and view all the answers

Which G-protein is responsible for the formation of lamellipods that aid in cell migration?

Rac Signup and view all the answers

What is the primary function of Tropomyosin in relation to actin microfilaments?

To protect against depolymerization. Signup and view all the answers

What type of tubulins combine to form protofilaments in microtubule assembly?

Alpha and beta tubulins. Signup and view all the answers

Which of the following proteins prevents actin microfilament polymerization?

CapZ Signup and view all the answers

What is the role of Ran in cellular mechanisms?

Molecule transport into and out of the nucleus. Signup and view all the answers

What is the primary composition of the cytoplasm?

70% water and macromolecules Signup and view all the answers

What is NOT a function of the cytoskeleton?

Energy production Signup and view all the answers

Which protein structure forms actin monomers?

G-actin Signup and view all the answers

What initiates actin polymerization?

Increased G/ATP concentration Signup and view all the answers

What type of cytoskeletal protein is primarily involved in maintaining cell shape?

Actin microfilaments Signup and view all the answers

What process describes the movement of white blood cells outside of blood vessels?

Diapedesis Signup and view all the answers

Which of the following is considered an inert inclusion in the cytoplasm?

Lipid droplets Signup and view all the answers

Which cytoskeletal component is characterized by its larger diameter?

Microtubules Signup and view all the answers

Actin polymerization to form F-actin requires which of the following?

ARP2/3 complex involvement Signup and view all the answers

Which role is NOT associated with actin microfilaments?

Chromosome disassembly Signup and view all the answers

What is the primary role of intermediate filaments in cells?

Maintain cell shape and structure Signup and view all the answers

Which of the following is true about the assembly of intermediate filaments?

They consist of stabilized tetramers that form a rope-like structure Signup and view all the answers

Which of the following statements correctly describes the function of colchicine?

It binds to free tubulin and prevents its polymerization. Signup and view all the answers

Which intermediate filament type is specifically found in muscle cells?

Desmin Signup and view all the answers

What can intermediate filaments be used to determine in a clinical setting?

Type of cancer through immunohistochemistry Signup and view all the answers

What condition is caused by genetic abnormalities affecting the links in intermediate filaments?

Epidermolysis bullosa Signup and view all the answers

What is one of the main functions of intermediate filaments related to mechanical stress?

They act as a scaffold linking desmosomes. Signup and view all the answers

Which drug is known for preventing mitotic spindle depolymerization?

Taxol Signup and view all the answers

Study Notes

Chapter 3: Cytoskeleton

The cytoskeleton is a network of filamentous proteins within the cytoplasm
It provides mechanical support, maintains cell shape, and enables cell motility
It facilitates chromosome migration during mitosis, organelle movement, and biochemical reactions
The cytoskeleton includes actin microfilaments (AF), intermediate filaments (IF), and microtubules (MT)

Cytoplasm Composition

Cytoplasm (cytosol, hyaloplasm) is a jelly-like fluid with
Water (70%)
Small molecules (low MW): ions, gases, glucose, amino acids, fatty acids, urea, glycerol
Large molecules (high MW): proteins, lipids, carbohydrates, nucleic acids (RNA)
Inert inclusions (non-living)
Proteins (proteasome)
Lipids (droplets)
Carbohydrates (rosettes, little clouds)

Cytoskeletal Components

Actin Microfilaments (AF): 5-8 nm diameter
Intermediate Filaments (IF) : 8-10 nm diameter
Microtubules (MT): 20-30 nm diameter

Actin Microfilaments (AMF) Structure and Nucleation

Monomer: G-actin (globular protein) with 1 ATP groove
Polymer: Several G-actins bind together to form F-actin. ATP grooves on same side. Involves ARP2/3 (a complex) forming 2 twisted chains.

Actin Polymerization/Depolymerization

ATP hydrolysis drives polymerization and depolymerization
Polymerization occurs at the plus end, while depolymerization occurs at the minus end
Factors like ARP2/3, Profilin, and CapZ regulate polymerization
The polymerization/depolymerization rate depends on G-actin/ATP concentration
The plus end is referred to as a barbed end and the minus end is a pointed end

AMF Roles

Maintain cell shape using cellular cortex (actin layer)
Enable cytoplasmic currents (movement of molecules and organelles)
Support structures like microvilli (intestinal and kidney cells)
Facilitate vesicle exchanges (endocytosis/exocytosis)
Essential during cell division: forms a contractile ring
Enables diapedesis (movement of white blood cells out of vessels)
Enables cell migration by the polymerization and depolymerization of AMFs

Microtubules (MT)

Organize the positions of organelles
Direct intracellular transport
Composed of tubulin dimers (α-tubulin and β-tubulin) arranged in a hollow tube structure with 13 protofilaments per circumference.

MT Structure and Assembly

Monomer: α-tubulin and β-tubulin
Polymerization: Tubulin dimers bind to each other using GTP, forming protofilaments. 13 protofilaments combine to form a microtubule
The γ-TuRC complex is crucial for MT nucleation (initiating assembly). It's considered a strong nucleator.
MTs are stabilized by accessory proteins. MAP2 is a common example.

MT Polymerization

Depends on equilibrium between free and bound tubulins. Free GTP form leads to polymerization; otherwise depolymerization.
Imbalance in the GTP-bound to GDP-bound tubulin ratio can cause instability and collapse. This is called a disaster phenomenon.

MT Associated Proteins

γ-TuRC: MT nucleation
MAP2: MT stability
Kinetochore: Attaches chromosomes to mitotic spindle during mitosis
Motor proteins (Kinesin and Dynein): convert chemical energy into mechanical energy for transport of cargo along microtubules. Kinesin moves towards the + end, Dynein towards the minus end.

Intermediate Filaments (IF)

Stability: Never polymerize or depolymerize
Cytoskeleton Role: Maintain cell shape
Nuclear Lamina: Form a nuclear lamina in the nucleus (3 types of lamins: A, B, and C). Reinforces the nuclear envelope. They are crucial against the pressure of the nucleus.
Diagnostics: Used in immunohistochemistry (IHC) to determine the cancer type
Families:
- Keratins (epithelial cells) - crucial in diagnosing carcinomas
- Desmin (muscle cells, myoblastomas)
- Vimentin (connective tissue, sarcomas)
- Neurofilaments (neurons, neuroblastomas)
- GFAP (glial cells, gliomas)

IF Structure and Assembly

Composed of filamentous monomers with two heads
Monomers form dimers, followed by tetramers, then a rope-like structure. The specific tetramer arrangements within the rope will depend on the type of intermediate filament.

IF Functions

Morphogenesis: Forms a scaffold for maintaining cell shape.
Muscles: In muscle cells, IFs orient actin and myosin to maintain organization, and thus contraction.
Resistance: IFs link desmosomes and hemidesmosomes to each other for cell resistance. Disruptions in this can cause diseases like epidermolysis bullosa.

Cilia and Flagella

Microtubules control their beating
Cilia move fluids (e.g., mucus in the throat)
Flagella propel cells (e.g., sperm)

Axonal Transport

Kinesin and Dynein move cargo along microtubules. Kinesin moves towards the + end and Dynein moves towards the - end.
Cargo can be transported in either direction.

Centrosomes and Centrioles

Centrosomes have a pair of centrioles at its core
Centrioles are made of microtubules arranged in a specific configuration that differ in their longitudinal and cross sections.

Pharmacological Effects on MTs

Certain drugs (e.g., colchicine, taxol, vincristin, podophyllotoxin) interfere with MTs, impacting polymerization, depolymerization, and overall microtubule function. This is a major target in chemotherapy and cancer treatment.

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Description

Test your knowledge on microtubules and cytoskeletal proteins with this quiz. It covers key concepts such as protein roles in cell division, movement mechanisms of cilia and flagella, and the function of major proteins like myosin and tropomyosin. Assess your understanding of cell migration and the cytoplasmic composition.