The Cytoskeleton: Structure and Function Inside the Cell

Introduction to the Cytoskeleton

The cytoskeleton is a dynamic, complex network of protein filaments that provides structural support, facilitates cell movement, and organizes the contents of eukaryotic cells. It is composed of three main types of protein polymers: actin filaments (microfilaments), intermediate filaments, and microtubules. Each type has distinct structures and functions, but all contribute to the cell's shape, internal organization, and ability to move.

• Actin filaments: Thin, flexible fibers involved in cell movement and shape.

• Intermediate filaments: Rope-like fibers providing mechanical strength.

• Microtubules: Hollow tubes that serve as tracks for organelle movement and are essential for cell division.

Overview Table: Cytoskeletal Filaments

The following table summarizes the main features of the three cytoskeletal filament types:

Actin Filaments (Microfilaments)

Structure of Actin Filaments

Actin filaments are the thinnest cytoskeletal polymers, composed of two intertwined strands of actin subunits. They exhibit structural polarity, with a plus (+) end that grows faster than the minus (−) end. Actin filaments can form bundles or cross-linked networks, contributing to the cell's mechanical properties.

• Diameter: ~7 nm

• Subunit: Globular actin (G-actin)

• Polarity: Plus and minus ends

Functions of Actin Filaments

Actin filaments are essential for maintaining cell shape, enabling cell movement, and facilitating various cellular processes. They interact with the motor protein myosin, which uses ATP to generate force and movement.

• Maintain cell shape by forming a supportive cortex beneath the plasma membrane

• Enable cell crawling and muscle contraction through actin-myosin interactions

• Drive cytokinesis (division of the cytoplasm) in animal cells

• Facilitate cytoplasmic streaming in plant cells

Actin-Myosin Interactions

Many cellular movements are based on the interaction between actin filaments and myosin motor proteins. Myosin "heads" bind to actin and, through ATP hydrolysis, move along the filament, causing sliding and contraction.

• Muscle contraction: Myosin slides actin filaments past each other

• Cytokinesis: Actin-myosin ring contracts to divide the cell

• Cytoplasmic streaming: Actin-myosin interactions move cytoplasm in plant cells

Intermediate Filaments

Structure of Intermediate Filaments

Intermediate filaments are composed of various proteins (such as keratins and lamins) that form coiled-coil dimers, which assemble into tough, rope-like fibers. Unlike actin filaments and microtubules, intermediate filaments lack structural polarity.

• Diameter: 8–12 nm

• Subunits: Diverse (keratin, nuclear lamins, etc.)

• No structural polarity

• Highly durable and resistant to stretching

Functions of Intermediate Filaments

Intermediate filaments provide mechanical strength and structural support to cells and tissues. They are especially important in cells exposed to mechanical stress, such as skin and muscle cells.

• Support the nuclear envelope (nuclear lamina)

• Anchor organelles in place

• Form the structural basis of hair, nails, and claws (keratin)

Example: Mutations in nuclear lamins can cause progeria, a genetic disease characterized by premature aging due to a weakened nuclear envelope.

Microtubules

Structure of Microtubules

Microtubules are the largest cytoskeletal elements, consisting of hollow tubes made from α- and β-tubulin heterodimers. They have structural polarity, with a plus (+) end that grows faster than the minus (−) end. Microtubules are dynamic, constantly assembling and disassembling.

• Diameter: 25 nm

• Subunit: α- and β-tubulin dimers

• Polarity: Plus and minus ends

• Dynamic instability: rapid growth and shrinkage

Microtubule Organization: The Centrosome

Microtubules originate from the microtubule organizing center (MTOC), which anchors the minus end and allows the plus end to grow outward. In animal cells, the MTOC is called the centrosome, which contains a pair of centrioles.

• Centrosome: Main MTOC in animal cells

• Centrioles: Cylindrical structures within the centrosome

• Microtubules radiate outward from the centrosome

Functions of Microtubules

Microtubules play critical roles in maintaining cell shape, facilitating intracellular transport, and enabling cell division and locomotion.

• Provide structural support and resist compression

• Serve as tracks for motor proteins (kinesin and dynein) to move organelles and vesicles

• Form the mitotic spindle for chromosome separation during cell division

• Compose the core of cilia and flagella for cell movement

Dynamic Instability of Microtubules

Microtubules exhibit dynamic instability, characterized by rapid phases of growth and shrinkage. This property is essential for their cellular functions, especially during mitosis.

• Growth occurs by addition of GTP-bound tubulin at the plus end

• Catastrophe: sudden shrinkage when GTP cap is lost

• Allows rapid reorganization of the cytoskeleton

Microtubules in Cell Division and Transport

During cell division, microtubules form the mitotic spindle, which separates chromosomes. Motor proteins such as kinesin and dynein move vesicles and organelles along microtubule tracks, using ATP for energy.

• Kinesin moves toward the plus end; dynein moves toward the minus end

• Essential for vesicle transport, organelle positioning, and chromosome movement

Cilia and Flagella: Microtubule-Based Motility Structures

Eukaryotic cilia and flagella are motile structures composed of microtubules arranged in a "9 + 2" pattern, known as the axoneme. Dynein motor proteins cause bending by sliding microtubule doublets against each other, resulting in movement.

• Axoneme: 9 doublet microtubules surrounding 2 central microtubules

• Dynein "walks" along adjacent doublets, causing bending

• Prokaryotic flagella are structurally different (composed of flagellin, not microtubules)

Summary Table: Cytoskeletal Elements

The following table summarizes the key properties of the three cytoskeletal filament types: