Cytoskeleton & Muscle Contraction (Ch. 17)

What is the cytoskeleton?

A network of protein filaments that provides structural support, organization, and movement for the cell.

What are the three main types of cytoskeletal filaments?

Intermediate filaments, microtubules, and actin filaments.

What are the main functions of the cytoskeleton?

It provides mechanical strength, determines cell shape, enables motility, organizes organelles, and drives intracellular transport and cell division.

What are intermediate filaments made of?

Fibrous proteins that form rope-like structures providing tensile strength.

What is the main role of intermediate filaments?

They provide mechanical stability and resist stretching forces, particularly in cells under mechanical stress.

Describe the structure of an intermediate filament.

Each filament is composed of staggered antiparallel tetramers of fibrous subunits twisted together into a rope-like structure about 10 nm in diameter.

How do intermediate filaments differ from microtubules and actin filaments?

They are symmetrical and do not have polarity; they are more stable and provide strength rather than movement.

What are the four major classes of intermediate filaments?

Keratins (epithelial cells), vimentin/vimentin-related (connective, muscle, glial cells), neurofilaments (neurons), and nuclear lamins (nucleus).

What disease results from keratin gene mutations?

Epidermolysis bullosa simplex, where skin cells rupture easily, causing blistering.

What disease involves abnormal accumulation of neurofilaments?

Amyotrophic lateral sclerosis (ALS), leading to axon degeneration and muscle weakness.

What supports and strengthens the nuclear envelope?

A meshwork of nuclear lamins (intermediate filaments) beneath the inner nuclear membrane.

What disorder results from defects in nuclear lamins?

Progeria, a premature aging disease caused by defective lamina assembly.

What is plectin and what does it do?

A cross-linking protein that connects intermediate filaments to microtubules, actin filaments, and cell junctions.

What connects the nucleus to the cytoskeleton?

Protein complexes spanning the nuclear envelope link the nuclear lamina to cytoplasmic filaments.

What are microtubules?

Hollow cylindrical filaments about 25 nm in diameter made of α- and β-tubulin dimers.

How do microtubules assemble?

Tubulin dimers polymerize end-to-end into protofilaments; 13 parallel protofilaments form a hollow tube.

What is the polarity of a microtubule?

The plus (+) end grows faster, the minus (–) end grows slower or is anchored at the centrosome.

Where do microtubules originate in most cells?

From the centrosome, which contains a pair of centrioles surrounded by a matrix of γ-tubulin ring complexes that nucleate microtubules.

What is dynamic instability?

The rapid switching between growth and shrinkage of individual microtubules.

What controls dynamic instability?

GTP hydrolysis; GTP-bound tubulin promotes growth, while GDP-bound tubulin causes shrinkage.

What happens to a microtubule when the GTP cap is lost?

The filament becomes unstable and depolymerizes catastrophically.

What happens when a new GTP cap forms?

The microtubule resumes growth.

How do microtubules help organize the cell?

They form tracks along which organelles, vesicles, and macromolecules are transported.

What are motor proteins?

ATP-powered proteins that move along cytoskeletal filaments carrying cellular cargo.

What are the two major microtubule motor proteins?

Kinesins (move toward the plus end) and dyneins (move toward the minus end).

What is the function of kinesin?

It transports organelles and vesicles outward from the cell center, helping position the ER.

What is the function of dynein?

It moves cargo toward the cell center, positioning the Golgi apparatus and pulling vesicles inward.

How do motor proteins generate movement?

ATP hydrolysis drives conformational changes in their heads that cause a “walking” motion along the filament.

What cellular structures contain stable microtubules for movement?

Cilia and flagella.

Where are cilia found and what do they do?

On epithelial cells of the respiratory tract and oviduct; they move fluid or mucus and help transport eggs.

What is the function of flagella?

Long whip-like structures that propel single cells such as sperm through fluid.

Describe the microtubule arrangement in a cilium or flagellum.

A “9 + 2” arrangement: nine outer doublets surrounding two central microtubules.

What motor protein drives bending of cilia and flagella?

Ciliary dynein, which slides adjacent microtubule doublets against each other to produce bending.

What anchors cilia and flagella to the cell?

The basal body, structurally similar to a centriole.

What drugs affect microtubule dynamics?

Taxol stabilizes microtubules and prevents depolymerization; colchicine and vinblastine bind tubulin and inhibit polymerization.

What are actin filaments also known as?

Microfilaments.

What is the diameter of actin filaments?

Approximately 7 nm, thinner and more flexible than microtubules.

What is the main role of actin filaments?

They enable cell movement, determine cell shape, and provide structural support beneath the plasma membrane.

How are actin filaments organized in cells?

Into structures such as microvilli, contractile bundles, lamellipodia, filopodia, and contractile rings.

Describe actin filament polarity.

The plus end (barbed end) grows faster, while the minus end (pointed end) grows slower.

What controls actin filament polymerization?

The concentration of free actin monomers and actin-binding regulatory proteins.

What is treadmilling?

A steady-state condition where actin subunits add to the plus end and disassemble from the minus end at the same rate.

What proteins regulate actin polymerization?

Formin and ARP (actin-related proteins) promote nucleation; thymosin and profilin regulate monomer availability.

What are actin-binding proteins and what do they do?

They control filament length, organization, and dynamics (e.g., cross-linking, capping, severing, anchoring).

Where is actin concentrated in cells?

In the cell cortex just beneath the plasma membrane, supporting cell shape and surface motility.

What is the cell cortex?

A dense network of actin filaments linked to the plasma membrane that provides mechanical strength and flexibility.

What cellular processes depend on cortical actin?

Cell crawling, phagocytosis, and cytokinesis.

How does cell crawling occur?

Cells extend protrusions (lamellipodia and filopodia), attach via integrins to the substrate, and contract the rear using myosin.

What are lamellipodia?

Sheet-like extensions of the plasma membrane filled with a network of branched actin filaments.

What are filopodia?

Thin, finger-like protrusions formed by bundles of parallel actin filaments that explore the environment.

What are integrins?

Transmembrane proteins that link actin filaments to the extracellular matrix, enabling traction during movement.

What are myosins?

Actin-based motor proteins that use ATP hydrolysis to move along actin filaments.

What is myosin I?

A simple myosin found in all cells that moves vesicles or the plasma membrane toward the plus end of actin filaments.

What is myosin II?

A motor protein that forms bipolar filaments in muscle and non-muscle cells, enabling contractile movements.

In what direction do most myosins move along actin filaments?

Toward the plus end, except myosin VI, which moves toward the minus end.

What is the sliding-filament mechanism?

Myosin heads bind to actin and, through ATP hydrolysis, pull the filaments past each other, causing contraction.

How does myosin II produce movement?

Each myosin head binds actin, performs a power stroke upon releasing ADP + Pi, then detaches after binding new ATP.

What are sarcomeres?

Repeated contractile units within myofibrils of muscle cells, composed of actin (thin) and myosin II (thick) filaments.

What are myofibrils?

Long cylindrical arrays of sarcomeres that fill most of the cytoplasm of a muscle fiber.

What happens during muscle contraction?

Myosin II heads slide actin filaments toward the center of the sarcomere, shortening it and contracting the muscle.

What triggers skeletal muscle contraction?

An action potential from a motor neuron causes Ca²⁺ release from the sarcoplasmic reticulum into the cytosol.

What are transverse (T) tubules?

Invaginations of the plasma membrane that conduct action potentials into the interior of muscle fibers.

What is the sarcoplasmic reticulum (SR)?

A specialized smooth ER in muscle cells that stores Ca²⁺ and releases it during excitation.

How does Ca²⁺ initiate muscle contraction?

Ca²⁺ binds to troponin, causing tropomyosin to move away from actin’s myosin-binding sites, allowing cross-bridge formation.

What proteins regulate actin-myosin interaction in muscle?

Tropomyosin and the troponin complex.

What is the role of tropomyosin?

A long, fibrous protein that blocks myosin-binding sites on actin in the absence of Ca²⁺.

What is the role of troponin?

A complex of three proteins that controls the position of tropomyosin and binds Ca²⁺ to initiate contraction.

How is skeletal muscle relaxation achieved?

Ca²⁺ is pumped back into the SR by a Ca²⁺-ATPase pump, tropomyosin re-covers the binding sites, and cross-bridges detach.

What ensures that contraction is rapid and uniform along a muscle fiber?

T-tubules transmit the depolarization quickly to all sarcomeres, synchronizing Ca²⁺ release from the SR.

How is energy supplied for muscle contraction?

ATP hydrolysis by myosin powers each power stroke; additional ATP is regenerated by creatine phosphate and mitochondria.

How do actin and microtubules coordinate intracellular transport?

Microtubules provide long-distance tracks, while actin filaments handle short-range transport and local positioning.

How is the cytoskeleton dynamic?

Filaments constantly assemble and disassemble in response to cellular needs and signaling pathways.

Why is cytoskeletal regulation essential?

It allows cells to adapt their shape, movement, and internal organization to different environments and functions.