Cell Biology Actin Structures Quiz

Cytoskeleton and Cellular Motility

• The cytoskeleton is a network of protein filaments that extends throughout the cytoplasm of all eukaryotic cells.
• It provides a structural framework, acting as a scaffold that determines cell shape, positions organelles, and organizes the cytoplasm.
• The cytoskeleton is also responsible for cell movements.
• The cytoskeleton is composed of three main types of protein filaments: actin filaments, intermediate filaments, and microtubules.
• Accessory proteins link these filaments together and to subcellular organelles and the plasma membrane.

Actin Filaments

• Actin filaments (microfilaments) are abundant beneath the plasma membrane, forming a network.
• This network provides mechanical support, determines cell shape, and enables cell surface movement, migration, engulfment, and division.
• Actin filaments are thin, flexible fibers, approximately 7 nm in diameter and up to several micrometers in length.
• Actin monomers polymerize to form filaments (F-actin).
• Actin monomers are rotated by 166° in the filament, creating a double-stranded helix structure.
• Actin filaments have polarity; barbed (plus) and pointed (minus) ends are distinguishable.

Assembly and Disassembly of Actin Filaments

• Individual actin molecules (G-actin) are globular proteins of 375 amino acids (approximately 43 kd).
• Actin monomers polymerize to form filaments (F-actin).
• Polymerization begins with the formation of dimers and trimers, followed by adding monomers to both ends.

Organization of Actin Filaments

• Actin filaments are organized into bundles or networks.
• Bundles are cross-linked into closely packed parallel arrays by actin-bundling proteins.
• These proteins (e.g., fimbrin) are small and rigid, forcing filaments to align closely.
• Networks are cross-linked in orthogonal arrays, forming three-dimensional meshworks with semisolid properties.
• Cross-linking proteins are large and flexible, allowing cross-linking of perpendicular filaments.

Actin Bundles and Networks

• Actin bundles and networks are observed via electron microscopy.
• Actin bundles support cell surface projections (e.g., filopodia).
• Actin networks are present beneath the cell membrane giving mechanical strength.

Actin Networks and Filamin

• Fimbrin cross-links actin filaments into closely packed, parallel bundles.
• Filaments are approximately 14 nm apart.
• A-actinin cross-links filaments to form more loosely spaced contractile bundles.
• Filaments are approximately 40 nm apart.
• Both proteins have multiple Ca^2^+ binding domains.

Association of Actin Filaments with the Plasma Membrane

• Actin filaments are highly concentrated at the periphery of the cell.
• They form a three-dimensional network below the plasma membrane (cell cortex).
• The network determines cell shape and is involved (through actin-binding proteins) in various cell surface activities, like movement.

Protrusions of the Cell Surface

• Cell surfaces have protrusions involved in movement, phagocytosis, and specialized functions (e.g., nutrient absorption).
• Most are based on actin filaments.
• Actin filaments organize into relatively permanent or rapidly rearranging bundles/networks for these protrusions.

Organization of Microvilli

• Microvilli are finger-like projections of the plasma membrane, abundant in cells involved in absorption (e.g., intestinal cells).
• Core actin filaments are cross-linked into closely packed bundles by fimbrin and villin.
• Microvilli are anchored to the plasma membrane via lateral arms of myosin I and calmodulin.
• Actin filament barbed ends are in a cap of unidentified proteins at the tip of the microvillus.

Pseudpodia, Lamellipodia, and Filopodia

• Pseudopodia are extensions of moderate width, important for cell movement and phagocytosis.
• Lamellipodia are broad sheet-like extensions at the leading edge of fibroblasts and other cells.
• Filopodia extend projections of the plasma membrane supported by actin bundles.

Formation of Protrusions and Cell Movement

• Cell movement across a surface is a basic form of cell locomotion in various cell types.
• Examples include amoeba crawling, embryonic cell migration, white blood cell invasion of tissues, wound healing, and cancer cell metastasis.

Actin, Myosin, and Cell Movement

• Actin filaments, often associated with myosin, are responsible for many cell movements.
• Myosin converts chemical energy (ATP) into mechanical energy for force and movement.
• Muscle contraction is a prominent example of actin-myosin movement.

Muscle Contraction

• Muscle cells are specialized for contraction.
• Muscle cells became the prototype for studying cell and molecular movement.
• Three types of muscle cells exist: skeletal, cardiac, and smooth muscles.

Skeletal Muscles

• Skeletal muscles are bundles of muscle fibers formed through the fusion of individual cells during development.
• Muscle fibers are large single cells.
• Their cytoplasm mostly comprises myofibrils; these are cylindrical bundles of:
  • thick myosin filaments (about 15 nm in diameter)
  • thin actin filaments (about 7 nm in diameter).
• Myofibrils are organized as a series of contractile units called sarcomeres.

Sarcomere Structure

• Sarcomeres are the contractile units of skeletal muscle.
• Sarcomeres contain:
  • I bands (thin filaments only)
  • A bands (thick and thin filaments overlap)
  • Z disc (attaches thin filaments)
  • M line (attaches thick filaments)
• Specialized proteins (titin and nebulin) contribute to sarcomere structure and stability.


Sliding Filament Model of Muscle Contraction

• Contraction results from interactions between actin and myosin filaments that generate relative movement.

• Myosin binds to actin filaments driving filament sliding; shortening the sarcomere.
• The length of the filaments does not change during contraction, but the sarcomere shortens.


Contractile Assemblies in Nonmuscle Cells

• Actin-myosin interactions are crucial for nonmuscle cell processes like cytokinesis, cell transport of membrane vesicles/organelles, phagocytosis, and pseudopod extension.
• Cytokinesis is a dramatic example, dividing a cell into two after mitosis.

Intermediate Filaments

• Intermediate filaments have diameters (8-11 nm) between actin filaments and microtubules (25 nm).
• They provide cells and tissues with mechanical strength, not directly involved in cell movements.
• Intermediate filaments are formed through the polymerization of various proteins (e.g., keratins, vimentin, neurofilaments, nuclear lamins).
• These proteins have a central rod domain, and variable head and tail regions giving them diverse shapes.
• Polymerization creates dimers, then tetramers and finally protofilaments wound around in a rope-like structure.

Intracellular Organization of Intermediate Filaments

• A network extends from a ring around the nucleus to the plasma membrane in most cells.
• The network in epithelial cells is anchored at specialized cell contacts (desmosomes and hemidesmosomes).
• These structures play specialized roles in muscle/nerve cells.

Microtubules

• Microtubules are rigid hollow rods (~25 nm) and dynamic structures.
• They help determine cell shape and are involved in various cell movements, like cell locomotion, organelle transport, and chromosome separation (mitosis).

Assembly of Microtubules

• In animal cells, microtubules originate from the centrosome, located near the nucleus in non-dividing cells.
• Microtubules extend outward to the cell periphery.

Microtubule Structure

• Microtubules (MTs) are polymers built from tubulin dimers (α and β tubulin).
• 13 protofilaments assemble around a hollow core to form an MT.
• Y-tubulin initiates MT assembly at centrosomes.

Microtubules (Polarity)

• Microtubules have two ends: fast-growing plus ends and slow growing minus ends, providing polarity.
• Polarity is crucial to directional transport of materials.

Intracellular Organization of Microtubules

• The minus ends of microtubules are towards the centrosome.
• Microtubules extend outward to the cell periphery in interphase cells.
• Duplicated centrosomes, separate during mitosis, and reorganize to form the mitotic spindle, essential for chromosome separation in mitosis.


Microtubule Motors and Movement

• Microtubules facilitate various intracellular movements, including transport of membrane vesicles/organelles, movement of cilia/flagella and the separation of chromosomes during mitosis.
• Movement along microtubules relies on motor proteins utilizing ATP hydrolysis.

Cargo Transport and Intracellular Organization

• Microtubules play a significant role in transporting macromolecules, membrane vesicles, and organelles throughout the cytoplasm of eukaryotic cells.

Reorganization of Microtubules during Mitosis

• Microtubules reorganize at the beginning of mitosis, forming the mitotic spindle to separate chromosomes.
• Daughter chromosomes separate and move to opposite poles of the mitotic spindle.


Test your knowledge on the role and structure of actin filaments in cell biology. This quiz covers topics such as actin polymerization, bundling proteins, and the functions of various cellular extensions. Challenge yourself and see how well you understand the mechanics of cell movement and shape!