Cytoskeletal Systems: Structure, Function, and Dynamics

Cytoskeletal Systems

Modularity of the Cytoskeleton

The cytoskeleton is a modular system composed of a small number of elements that can be arranged in various ways to support diverse cellular structures and functions. This modularity allows cells to reuse components for different purposes, similar to how a human skeleton uses a few building materials for many structures.

• Three major cytoskeletal elements: Microtubules, Microfilaments (Actin Filaments), and Intermediate Filaments

• Accessory proteins: Regulate structure, create functional diversity, and assist in organization and movement

• Septins: Sometimes considered a 'fourth cytoskeleton,' involved in cell division and contractile ring formation

Prokaryotic cytoskeleton: Bacteria possess cytoskeletal elements analogous to eukaryotes, such as MreB (actin-like), FtsZ (tubulin-like), and Crescentin (IF-like).

Dynamic nature: Cytoskeletal elements are constantly assembled and disassembled, enabling cell movement, shape changes, division, and intracellular transport.

Comparison of Cytoskeletal Elements

Feature	Microtubules	Microfilaments	Intermediate Filaments
Size	25 nm	7 nm	8–12 nm
Protein	Tubulin	Actin	Various fibrous proteins
Polarity	Yes	Yes	No
Energy	GTP	ATP	None
Main Role	Transport, division	Movement, contraction	Strength, stability
Dynamics	Very dynamic	Dynamic	Stable

Microtubules

Structure and Properties

Microtubules are the largest cytoskeletal elements, with a diameter of about 25 nm. They are hollow tubes composed of 13 protofilaments made from α- and β-tubulin heterodimers. Microtubules exhibit polarity, with a plus (+) end and a minus (−) end, and require GTP for assembly.

• Types: Cytosolic (dynamic, loosely organized) and Axonemal (highly organized, stable)

• Functions: Intracellular transport, chromosome movement during mitosis, cell motility (cilia, flagella), cell shape and organization

• Polarity: Plus end grows/shrinks faster; minus end often anchored at the centrosome

• Isoforms and modifications: Multiple α- and β-tubulin isoforms; chemical modifications (e.g., acetylation) affect stability

Microtubule Assembly and Dynamics

Microtubules form by reversible polymerization of tubulin dimers, involving nucleation (formation of oligomers) and elongation (addition of dimers at ends). The assembly process is characterized by phases: lag (nucleation), elongation (rapid growth), and plateau (balance of growth and shrinkage).

• Critical concentration: Tubulin concentration where assembly equals disassembly

• Treadmilling: Addition at plus end and loss at minus end; overall length remains constant but subunits move through the filament

Dynamic instability: Microtubules rapidly switch between growth and shrinkage, regulated by GTP hydrolysis on β-tubulin. A GTP cap at the plus end stabilizes the microtubule; loss of the cap leads to rapid shrinkage (catastrophe), while regaining it allows growth (rescue).

• Key terms: Catastrophe (growth to shrinkage), Rescue (shrinkage to growth)

Microtubule-Organizing Centers (MTOCs)

MTOCs nucleate and anchor microtubules, establishing polarity within the cell. The centrosome is the main MTOC in animal cells, containing two centrioles and pericentriolar material. γ-tubulin ring complexes (γ-TuRCs) nucleate microtubules and anchor minus ends.

• Centrioles: Made of 9 triplet microtubules; important for basal body formation and MT nucleation

• Other MTOCs: Golgi apparatus, noncentrosomal sites in specialized cells

• Cell-specific polarity: Axons (plus ends outward), dendrites (mixed), cilia (minus ends at basal body), red blood cells (mixed)

Microtubule-Binding Proteins

• Microtubule-Associated Proteins (MAPs): Stabilize, bundle, and space microtubules; Tau (tight bundles in axons, linked to Alzheimer disease), MAP2 (looser bundles in dendrites)

• +TIP Proteins: Stabilize growing plus ends; EB1 binds GTP-tubulin at plus ends

• Destabilizing/Severing Proteins: Stathmin/Op18 (prevents polymerization), Catastrophins (promote depolymerization), Katanin (severs microtubules)

Drugs Affecting Microtubules

• Colchicine/Colcemid: Bind β-tubulin, inhibit assembly, promote disassembly

• Nocodazole: Inhibits polymerization, reversible

• Vinblastine/Vincristine: Sequester tubulin dimers, prevent formation

• Paclitaxel (Taxol): Stabilizes microtubules, prevents disassembly

These antimitotic drugs interfere with the mitotic spindle and are used as anticancer agents.

Microtubule Structure Variants

• Singlet: Standard cytosolic microtubule (13 protofilaments)

• Doublet: Found in cilia and flagella (A tubule + incomplete B tubule)

• Triplet: Found in basal bodies and centrioles (A tubule + incomplete B and C tubules)

Doublets and triplets are stabilized by microtubule inner proteins (MIPs).

Microfilaments (Actin Filaments)

Structure and Properties

Microfilaments are the thinnest cytoskeletal elements (~7 nm), composed of actin protein. The monomer is G-actin (globular), which polymerizes into F-actin (filamentous), forming two intertwined chains. Microfilaments are polar, with a fast-growing plus end and a slower minus end.

• ATP binding: Actin binds ATP; after incorporation, ATP is hydrolyzed to ADP, making ADP-actin less stable

• ATP cap: Stabilizes the filament; loss leads to instability

Assembly and Dynamics

• Assembly steps: Nucleation (formation of small clusters), Elongation (rapid growth)

• Critical concentration: Different at plus and minus ends, leading to treadmilling

• Treadmilling: Addition at plus end, loss at minus end; filament length remains constant but subunits move

Functions of Microfilaments

• Cell cortex: Actin network under plasma membrane maintains cell shape and stiffness

• Cell movement: Drives cell crawling, lamellipodia (sheet-like protrusions), filopodia (spikes)

• Muscle contraction: Actin interacts with myosin for contraction via sliding mechanism

• Cytoplasmic streaming: Movement of cytoplasm, especially in plant cells

• Cytokinesis: Forms contractile ring to pinch cell into two

Actin-Binding Proteins

• Profilin: Promotes actin assembly

• Cofilin: Disassembles actin

• Arp2/3 complex: Creates branched filaments

• Fimbrin/α-actinin: Bundling proteins

• Myosin: Motor protein that moves along actin using ATP, driving muscle contraction and vesicle movement

Intermediate Filaments

Structure and Properties

Intermediate filaments (IFs) provide mechanical strength and stability. They are rope-like, twisted cables made of fibrous proteins (not globular), with a diameter of 8–12 nm. IFs lack polarity and do not require ATP or GTP for assembly, making them more stable and less dynamic than microtubules or actin filaments.

• No motor proteins: No directionality, so motor proteins are not involved

• No treadmilling: Assembly does not involve treadmilling

Types of Intermediate Filaments

• Keratin: Epithelial cells (skin, hair)

• Vimentin: Connective tissue

• Desmin: Muscle cells

• Neurofilaments: Neurons

• Lamins: Nuclear envelope

Functions of Intermediate Filaments

• Mechanical strength: Resist stretching and stress, prevent cell tearing

• Structural support: Maintain cell integrity

• Nuclear lamina: Lamins support the nuclear envelope

• Cell junctions: Anchor at desmosomes, connect cells together

Assembly of Intermediate Filaments

• Monomers assemble into dimers, then tetramers, and finally into rope-like filaments

Clinical Relevance

• Defects in IFs: Lead to fragile cells; e.g., keratin defects cause skin blistering diseases

Summary Table: Cytoskeletal Elements

Feature	Microtubules	Microfilaments	Intermediate Filaments
Size	25 nm	7 nm	8–12 nm
Protein	Tubulin	Actin	Fibrous proteins
Polarity	Yes	Yes	No
Energy	GTP	ATP	None
Function	Transport, mitosis	Movement, contraction	Strength
Dynamics	Very dynamic	Dynamic	Stable

Techniques Used to Study the Cytoskeleton

• Fluorescence microscopy: Proteins are labeled to visualize their location

• Live-cell imaging: Observes cytoskeletal dynamics in real time

• Electron microscopy: Provides high-resolution images of individual filaments

Key Equations and Concepts

• Critical concentration for polymerization:

$C_c = \frac{k_{off}}{k_{on}}$

• Treadmilling condition:

$C_{plus} < [Tubulin] < C_{minus}$

• Dynamic instability: GTP hydrolysis on β-tubulin leads to instability

$\text{MT stability} \propto \text{GTP cap presence}$

Additional info:

• Septins are increasingly recognized as a distinct cytoskeletal system, especially in yeast and animal cells.

• MAPs and Tau proteins are clinically significant in neurodegenerative diseases.

• Actin-myosin interactions are fundamental to muscle contraction and many forms of cell motility.