Cytoskeleton Study Notes: Microtubules, Actin Filaments, and Intermediate Filaments

Overview

The cytoskeleton comprises three main types of cytoskeletal proteins that form distinct filament networks within the cell:
- Microtubules (MTs)
- Actin microfilaments (F-actin)
- Intermediate filaments (IFs)
Each filament type forms a separate meshwork with specific functions essential for cell shape, movement, intracellular transport, division, and mechanical integrity.
Key visual cues from the slides:
- Blue = actin microfilaments
- Green = microtubules
- Red = intermediate filaments

Filament Diameters and Basic Structure

Microtubules
- Structure: Column of tubulin dimers (α-tubulin and β-tubulin) that polymerize into hollow tubes
- Diameter: $25\,\text{nm}$
- Composition: Tubulin dimer built from alpha and beta tubulin; forms dynamic hollow tubes
- Basic features: Highly dynamic polymer that grows and shrinks via addition/removal of dimers
Actin microfilaments
- Structure: Actin monomers assemble into long, dynamic fibers; actin monomers can dimerize and coil
- Diameter: $7\,\text{nm}$
- Composition: Actin subunit; forms a flexible polymer
- Basic features: Rapidly assembling and disassembling in the cell
Intermediate filaments
- Structure: Fibrous subunits (e.g., keratins) coil together to form cables
- Diameter: $8-12\,\text{nm}$
- Composition: Various keratin-family proteins; typically form nonpolar, coil-coiled filaments

Actin Cytoskeleton (Actin Microfilaments)

Composition and assembly
- Made of actin protein monomers that form dynamic, long fibers
- Actin monomer is the basic unit; filaments form by polymerization and turnover
- Actin turnover involves ATP/ADP exchange and ATP-actin incorporation
- Actin filaments are the smallest of the three filament types
Dynamic behavior
- Actin filaments are dynamically assembling and disassembling in living cells
- ATP-bound actin adds to filaments; after incorporation, ATP is hydrolyzed to ADP-actin, regulating turnover
- Dynamic actin networks continually remodel to support cellular processes
Organization in the cell
- The actin array spans across the cell and tends to accumulate at the edges, forming a cortical network
- In some images the actin cytoskeleton is colored blue to highlight its distribution
Key functions ( Actin cytoskeleton )
1) Mechanical support: provides resistance to mechanical stress and helps maintain cell shape
- Actin stress fibers span parts or all of a cell, providing tensile strength
  2) Cell migration: essential for crawling cells; actin microfilaments project into the leading edge of migrating cells
- Example: Actin dynamics at the leading edge drive protrusions during migration; video references show migrating cytotoxic T lymphocytes
  3) Adhesion and junctions: cytoplasmic anchor for extracellular matrix (ECM) or cell–cell adhesions such as tight junctions and adherens junctions
- Tight junctions are part of the cell–cell adhesion system connected to the actin cytoskeleton
  4) Intracellular cargo transport: serves as tracks for myosin motor proteins to move organelles and vesicles
- Example pathways include actin cables used for vacuole segregation and organelle positioning (e.g., Myo2 motor proteins, Vac8, Vac17, Inp2, etc.)
  5) Muscle contraction (in muscle cells): forms thin filaments essential for contraction
- Note: This aspect is often covered in anatomy; not a primary focus for development but demonstrates functional versatility
  6) Neutrophil locomotion: actin filaments dynamically push out the cell membrane at the leading edge to propel the cell toward a chemoattractant (e.g., bacteria)
Notable terms connected to actin function
- Leading edge: region at the front of migrating cells rich in actin polymerization
- Stress fibers: contractile actin bundles that provide tensile strength
- Myosin motors: enzymes that convert chemical energy from ATP into mechanical work along actin filaments

Intermediate Filaments (IFs)

Organization
- Form an array surrounding the nucleus; networks are consistently spaced around the nucleus across the cell
- Keratin is a representative IF protein in many cell types
- IFs create a stabilizing scaffold that maintains nuclear shape and perinuclear integrity
Dynamics
- IF cytoskeletons are relatively static compared with MTs and actin; they do not undergo rapid assembly/disassembly
- They retain their structural integrity regardless of changes in movement or other cytoskeletal dynamics
Primary functions
- Provide tensile strength: resist pulling forces to prevent cell rupture or deformation
- Provide compressive resistance: help cells maintain shape under mechanical stress
- Overall, IFs contribute to mechanical resilience and integrity of the cell

Microtubules (MTs)

Structure and assembly
- MTs are built from tubulin dimers (α-tubulin and β-tubulin) that polymerize into hollow tubes
- Tubulin dimers form heterodimers; MTs are highly dynamic, constantly assembling and disassembling
- MTs grow from centrosomes, also called the microtubule organizing center (MTOC)
- MTs radiate outward from the MTOC like spokes of a wheel
Centrosome and MTOC
- The centrosome acts as the primary MTOC in many animal cells, organizing MT nucleation and anchoring
- MTs emanate from the centrosome to reach various cellular regions
MT-associated functions (major roles) 1) Structural support: maintain overall cell architecture 2) External motility structures: form the internal structure of cilia and flagella; MTs are essential for swimming cells
- Cilia and flagella contain axonemes with MT-based architecture; include components like IFT motors (kinesin/dynein) and dynein motors
  3) Intracellular transport platforms: tracks for vesicles, organelles, and molecules; powered by motor proteins such as kinesin and dynein
  4) Chromosome separation during mitosis and meiosis: MTs form the mitotic spindle to pull chromosomes apart
- Key spindle components include kinetochore microtubules, interpolar microtubules, astral microtubules, the spindle midzone, and the central spindle
- Spindle organization features include the mitotic spindle, contractile ring, centrosome, and midbody during cytokinesis
MT-based processes and structures (detailed references)
- Mitotic spindle assembly and function: chromosomes attach to kinetochore MTs; spindle poles organize MTs; pole-to-kinetochore forces align chromosomes
- Spindle microtubule classes: kinetochore, interpolar, astral, central spindle, and midbody structures
- The MT cytoskeleton interfaces with actin for coordinated cell division and intracellular transport
Imaging and visualization
- In some cellular images, DNA is stained with DAPI (blue); MTs appear green and IFs appear red depending on staining strategy
Additional notes on MT-based structures
- The ciliary axoneme is organized around MTs and uses motor proteins for motility; ciliary pocket and transition zone are key regions in cilia base
- The centrosome/MTOC and MT dynamics are central to understanding cell polarity and division

Comparative Overview (Key Distinctions)

Size and composition
- MTs: $25\,\text{nm}$ diameter; tubulin heterodimers (α/β) polymerize to form hollow tubes
- Actin filaments: $7\,\text{nm}$ diameter; actin monomers polymerize into flexible filaments
- IFs: $8-12\,\text{nm}$ diameter; keratin-like fibrous subunits coil to form cables
Dynamics
- MTs: dynamic instability (rapid growth/shrinkage); nucleation at MTOC; highly dynamic
- Actin: dynamic turnover with ATP-actin; rapid remodeling, especially at leading edge; supports movement and shape changes
- IFs: relatively static; provide stable structural support
Primary cellular roles
- MTs: structural framework, cilia/flagella architecture, intracellular transport, chromosome segregation during division
- Actin: mechanical support, migration, cell–cell and cell–ECM adhesion, intracellular transport, and muscle contraction components
- IFs: mechanical resilience, tensile/compressive support, nuclear and perinuclear stabilization
Interaction with motors
- MTs interact with kinesin and dynein for long-range transport
- Actin filaments interact with myosin motors for short-range transport and muscle contraction
Spatial organization
- MTs often radiate from the MTOC (centrosome) outward
- Actin networks are enriched near the plasma membrane and at the leading edge
- IF networks frame the perinuclear region and extend through the cytoplasm to provide structural support

Key Terms and Definitions (Glossary)

Tubulin dimer: The basic building block of microtubules, composed of α-tubulin and β-tubulin
α/β tubulin: The two subunits forming the tubulin dimer; possess GTP-binding sites important for polymerization dynamics
Microtubule organizing center (MTOC): The cellular site where microtubules nucleate and organize; centriole-containing centrosome in many animal cells
Centrosome: Primary MTOC that organizes MT nucleation and radial growth
Kinesin: Plus-end–directed motor protein that moves cargos along microtubules toward the cell periphery
Dynein: Minus-end–directed motor protein that moves cargos toward the cell center
Kinetochore: Protein complex at the centromere of chromosomes that attaches to MTs during mitosis/meiosis
Axoneme: Core MT-based structure of cilia/flagella
IFT (intraflagellar transport): Motor-driven movement along cilia/flagella essential for assembly and function
Actin monomer: The basic unit of actin; ATP-bound when polymerizing
Myosin: Family of motor proteins that move along actin filaments, converting ATP to mechanical work
Stress fiber: Contractile actin bundle contributing to cell stiffness and tension
Tight junctions/adherens junctions: Cell–cell adhesions linked to the actin cytoskeleton
Neutrophil: An example cell type illustrating actin-driven membrane protrusion during migration

Practical Connections and Implications

Cellular mechanics: The balance between MTs, actin, and IFs determines how a cell resists deformation and adapts to mechanical stress.
Development and disease relevance: Proper assembly and regulation of the cytoskeleton are crucial for cell division, migration, and tissue organization; defects can contribute to developmental disorders and disease states.
Real-world examples from the slides illustrate organelle positioning (actin-mediated transport), cell migration (leading edge dynamics), and chromosome segregation during division (mitotic spindle construction).
Visualization cues: Color-coding (blue actin, green MTs, red IFs) helps in understanding the spatial organization of the cytoskeleton in cells.

Quick Reference: Key Numerical Details

Microtubules diameter: $25\,\text{nm}$
Actin filament diameter: $7\,\text{nm}$
Intermediate filament diameter: $8-12\,\text{nm}$

Summary of Core Concepts

The cytoskeleton is composed of three filament systems—MTs, actin filaments, and IFs—each with distinct structures, dynamics, and functions.
Microtubules are dynamic, MTs organize around the MTOC, and they function in structural support, cilia/flagella architecture, intracellular transport, and chromosome separation during division.
Actin filaments form dynamic networks that drive cell shape changes, migration, adhesion, cargo transport with myosin motors, and contribute to muscle contraction; actin turnover is tightly regulated by ATP/ADP cycling.
Intermediate filaments are relatively static, forming a robust scaffold that provides tensile and compressive strength, especially around the nucleus, contributing to overall cellular integrity.
The three networks work together to enable complex cellular behaviors observed during development, immune responses, and tissue organization.