Cell Structure and Function Lecture 15

Kinesins

Motor proteins that move toward the plus ends of microtubules, transporting cargo such as organelles and vesicles, powered by ATP hydrolysis.

Dyneins

Motor proteins that move toward the minus ends of microtubules, involved in organelle transport, centrosome positioning, and cilia/flagella beating, powered by ATP hydrolysis.

Myosins

Motor proteins that interact with actin filaments to facilitate cellular movements, muscle contraction (Myosin II), vesicle transport, and cell division, powered by ATP hydrolysis.

Intermediate Filaments (IFs)

Rope-like, stable cytoskeletal components (\sim10 nm diameter) that provide mechanical strength and support to cells and tissues, without using ATP or GTP for assembly.

Axoneme

The structural core of cilia and flagella, characterized by a "9+2" arrangement of nine outer doublet microtubules and two central single microtubules, responsible for movement.

Basal Body

A centriole-like structure (9+0 triplet microtubule arrangement) that anchors the axoneme of cilia and flagella to the cell and initiates their assembly.

Cilia

Short, numerous, hair-like cell surface projections. Motile cilia move fluids, while primary (non-motile) cilia function as sensory antennae.

Flagella

Longer, whip-like structures that propel cells through fluid environments via an undulating, wave-like motion (e.g., sperm tails), containing an axoneme.

ATP Hydrolysis

The chemical reaction (ATP \to ADP + P_i) that releases energy, utilized by motor proteins (kinesins, dyneins, myosins) to drive conformational changes and mechanical movement.

Processivity

The ability of a motor protein to remain bound to its filament and take multiple steps, traveling long distances before detaching, enhancing efficient cargo transport.

Intermediate Filament Typing

A diagnostic technique used in pathology to identify the cellular origin of tumors by analyzing the specific types of intermediate filament proteins expressed.

Cargo Transport

The intracellular movement of vesicles, organelles, and proteins along cytoskeletal tracks, facilitated by motor proteins (kinesins, dyneins, myosins).

Motor Proteins Structure

Composed of globular head domains (for ATP and filament binding), a flexible linker region (for conformational change), and a tail domain (for cargo binding).

Vimentin

An intermediate filament protein found in mesenchymal cells (e.g., fibroblasts, endothelial cells), contributing to cell integrity, migration, and signaling.

Desmin

A muscle-specific intermediate filament protein that forms a scaffold in muscle cells, integrating myofibrils and maintaining structural integrity, especially at Z-discs.

Spectraplankins

Large, multi-domain proteins (e.g., Plectin) that cross-link and integrate different cytoskeletal components (actin, microtubules, intermediate filaments), providing mechanical resilience.

Intraflagellar Transport (IFT)

Bidirectional transport of proteins and subunits along the axoneme of cilia/flagella, with kinesin-2 for anterograde (tip-directed) and dynein-2 for retrograde (base-directed) movement, essential for assembly and maintenance.

Nexin

An elastic protein that forms interdoublet links between adjacent outer doublet microtubules in the axoneme, converting dynein-mediated sliding into bending motion.

AAA+ Domains

ATPase domains found in proteins like dyneins that bind and hydrolyze ATP, providing the energy for conformational changes and movement along microtubules.

Actin Filaments

Dynamic, helical polymers of actin protein (\sim7 nm diameter) with polarity, critical for cell shape, muscle contraction (with myosin), cell migration, and cytokinesis.

Keratins

The most diverse family of intermediate filament proteins, forming obligate heterodimers in epithelial cells to provide mechanical strength; also form hair and nails.

Myosin II

A two-headed, conventional myosin that self-assembles into bipolar filaments. It drives muscle contraction and cytokinesis.

Microtubules (MTs)

Hollow, dynamic cylinders (\sim25 nm diameter) made of α- and β-tubulin heterodimers, critical for cell shape, intracellular transport, and chromosome segregation.

Tubulin

The globular protein subunit that polymerizes to form microtubules. It exists as α-tubulin and β-tubulin, which form stable heterodimers.

Dynamic Instability

The rapid alternation between growth (polymerization) and shrinkage (depolymerization) at microtubule ends, primarily driven by GTP hydrolysis on β-tubulin.

Centrosome

The primary microtubule-organizing center (MTOC) in animal cells, consisting of two centrioles and pericentriolar material, where microtubule nucleation occurs.

Actin Treadmilling

A dynamic state of actin filaments where subunits are added at the plus (barbed) end while simultaneously dissociating from the minus (pointed) end, leading to filament movement.

ARP2/3 Complex

A protein complex that nucleates new actin filaments as branches from existing ones, crucial for the formation of branched actin networks (e.g., in lamellipodia).

Formins

A class of actin-binding proteins that nucleate and continuously elongate unbranched actin filaments at their plus ends, forming structures like stress fibers and filopodia.

Profilin

An actin-binding protein that binds to G-actin-ATP monomers, promoting their addition to the plus end of growing actin filaments by facilitating ADP/ATP exchange.

Cofilin

An actin-binding protein that binds to ADP-actin filaments, severing them and promoting their depolymerization from the minus end, increasing actin turnover.

MAPs (Microtubule-Associated Proteins)

A diverse group of proteins that bind to microtubules, regulating their stability, assembly, dynamics, spacing, and interactions with other cellular components.

F-actin

Filamentous actin, the polymerized form of G-actin monomers, which constitutes microfilaments and is responsible for many cell functions involving movement and structure.

Sarcomere

The fundamental contractile unit of striated muscle, composed of organized bundles of actin (thin) and myosin (thick) filaments, which slide past each other during contraction.

G-actin

Globular actin, the monomeric (unpolymerized) form of actin that binds ATP or ADP and polymerizes to form F-actin filaments.

\gamma-tubulin

A specialized tubulin isoform found in the γ-tubulin ring complex (\gamma TRC) within the pericentriolar material, critical for nucleating (starting) microtubule assembly.

Microtubule-Organizing Centers (MTOCs)

Specialized cellular structures (like the centrosome) that nucleate, anchor, and organize microtubules, controlling their number, polarity, and location.

Centrioles

Cylindrical structures within the centrosome, composed of nine triplet microtubules arranged in a '9+0' pattern, acting as templates for basal body formation and playing a role in spindle pole organization.

Stathmin/Op18

A microtubule-destabilizing protein that binds to tubulin heterodimers, preventing their assembly into microtubules and promoting microtubule depolymerization.

Plus-end tracking proteins (+TIPs)

Proteins that specifically associate with the growing plus ends of microtubules, regulating their dynamics, mediating interactions with other structures, and linking them to various cellular components.

Gelsolin

An actin-binding protein that severs actin filaments and caps their plus ends, reducing filament length and inhibiting further elongation, thereby regulating actin network fluidity.

Thymosin \beta4

A small actin-sequestering protein that binds to G-actin monomers (preventing polymerization) and acts as a buffer of available actin for rapid filament assembly.

Tropomyosin

A coiled-coil protein that binds along the length of actin filaments. In muscle, it regulates myosin binding by blocking myosin attachment sites on actin, often in conjunction with troponin.

Myosin V

A highly processive, two-headed myosin motor that moves along actin filaments towards the plus end, primarily involved in long-range transport of vesicles and organelles.

Nuclear Lamina

A fibrous meshwork of intermediate filaments (lamins) underlying the inner nuclear membrane, providing structural support to the nucleus and organizing chromatin.

Plectin

A prominent spectraplankin that cross-links all three cytoskeletal networks (actin, microtubules, IFs) and anchors them to cell junctions, enhancing mechanical integrity.

Colchicine

A drug that binds to free tubulin dimers, preventing their polymerization into microtubules and thereby depolymerizing existing microtubules; used in research and as an anti-inflammatory.

Taxol (Paclitaxel)

A chemotherapy drug that binds to and stabilizes microtubules, preventing their depolymerization and thus arresting cell division, particularly in mitosis.

Fimbrin

An actin-binding protein that cross-links actin filaments into tightly packed, non-contractile parallel bundles, commonly found in filopodia and microvilli.

Filamin

An actin cross-linking protein that forms flexible, orthogonal networks of actin filaments, providing structural support to the cell cortex and facilitating cell migration.

Neurofilaments

Type IV intermediate filaments found in the cytoplasm of neurons, providing mechanical strength to axons and regulating axonal diameter.

GFAP (Glial Fibrillary Acidic Protein)

A type of intermediate filament protein expressed in astrocytes and other glial cells, forming structural networks that support neural tissue and respond to injury.