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Fundamentals of Cell Biology

Cytoskeletal Systems and Polymer Dynamics

Overview of Cellular Organization

• Cells exhibit spatial organization and mechanical interaction with their environments, showcasing the intricate relationship between cellular structure and function.

• Key Functions:

  • Correctly shaping cells and structuring internal components is crucial for their viability.

  • Transporting molecules efficiently within the cytoplasm is fundamental for cellular metabolism and signaling.

  • Mobility is essential for various functions including movement, migration, and response to stimuli.

  • The ability to rearrange internal components during cell growth, division, and adaptation plays a pivotal role in maintaining homeostasis.

• Examples of Cells:

  • Red Blood Cells (Erythrocytes): Specialized for oxygen transport, exhibit a biconcave shape that increases surface area for gas exchange.

  • S. cerevisiae (Baker's Yeast): A model organism used for studying cellular processes and genetics, understanding how cytoskeletal dynamics affect yeast cell division and morphology.

  • Motor Neurons in the Spinal Cord: Cells that transmit signals throughout the nervous system, dependent on cytoskeletal components for maintaining structure and facilitating transport of organelles.

• Eukaryotic Cell Overview of Cytoskeleton:

  • Actin filaments: Stained red, crucial for cellular mobility and structure.

  • Microtubules: Stained green, involved in transporting materials within the cell and maintaining shape.

  • Nuclei: Stained blue, central command centers of cellular activity.

The Cytoskeleton

• The cytoskeleton is a complex network of protein filaments that underpins cell behavior and morphology. It is crucial for cellular structure, facilitating a variety of functions.

• Major Systems Include:

  • Microtubules: Tubulin polymers that help organize intracellular structures and serve as tracks for motor proteins.

  • Actin Filaments: Essential for muscle contraction and shape changes, forming networks beneath the plasma membrane.

  • Intermediate Filaments: Provide tensile strength to cells and structural support to nuclear membranes.

Functions of the Cytoskeleton

• The cytoskeleton is indispensable for several key cellular processes:

  • Mitosis: Microtubules pull chromosomes apart and assist in cell division by facilitating the separation of daughter cells.

  • Intracellular Transport: Organelles and vesicles are moved along microtubule tracks, ensuring proper cellular function and organization.

  • Mechanical Support: The cytoskeleton supports the plasma membrane under stress, enabling cells to maintain their shape during external pressure.

  • Cellular Movements: Swimming seen in spermatozoa and crawling methods used by fibroblasts and leukocytes rely on actin filament dynamics for motility.

  • Signal Transmission: The cytoskeleton is involved in neuronal signaling by extending axons and dendrites, essential for nerve signal transmission.

  • Plant Cell Growth: The cytoskeleton directs the expansion and differentiation of plant cell walls.

  • Control of Cell Shape: Actin filaments control the diverse shapes of cells, impacting their functionality and interaction with other cells.

Classes of Cytoskeletal Filaments

• There are three primary classes of cytoskeletal filaments identifiable by microscopy:

  • Microtubules: ~25 nm diameter, with a hollow structure that facilitates intracytoplasmic transport.

  • Intermediate Filaments: ~10 nm diameter, known for their role in mechanical resilience against stress.

  • Microfilaments: ~5-8 nm diameter, composed mainly of actin, crucial for shape and motility.

Characteristics of Filaments

• Intermediate Filaments (IFs):

  • Extend throughout the cytoplasm, forming a supportive lamina beneath the nuclear membrane that maintains nuclear integrity.

  • Provide mechanical strength and resist shear stress exerted on cells during physical activity.

• Microtubules:

  • Emanate from the centrosome, essential for positioning organelles and directing intracellular transport of vesicles.

• Microfilaments:

  • Organized into networks closely associated with the cell cortex, influencing the shape of the cell and playing a crucial role in locomotion.

Accessory Proteins

• Cytoskeletal filaments interact with various accessory proteins that play crucial roles in their assembly, positioning, and function:

  • Linking Proteins: Attach filaments to other cell components and to each other.

  • Motor Proteins: Facilitate the movement of organelles along filaments, integrating cytoskeletal dynamics with cellular function.

Functions of Microtubules

• Microtubules are pivotal for cellular operations, contributing to:

  • The organization of cytoplasmic organelles like the Golgi apparatus and endoplasmic reticulum.

  • Providing tracks for motor proteins to transport cellular components.

  • Generating forces necessary for the movement of cilia and flagella, essential for cell motility and fluid movement.

  • Forming the mitotic spindle that plays a critical role in chromosome alignment and separation during cell division.

Functions of Actin Filaments

• Actin filaments exhibit versatility by taking part in various structural formations:

  • Schedule into linear bundles, two-dimensional sheets, or three-dimensional gels primarily located beneath the plasma membrane, known as the cell cortex.

  • Change cell shapes by exerting force on the plasma membrane, crucial during processes like cytokinesis.

  • In muscle cells, actin filaments contribute to contraction by forming structures that allow interaction with myosin.

Functions of Intermediate Filaments

• Intermediate filaments serve multiple functions including:

  • Serving as components of the nuclear lamina, providing structural support for chromatin organization during interphase.

  • Controlling the dynamics of nuclear disassembly and reassembly during the mitotic phase of the cell cycle.

  • Providing mechanical integrity to tissues that routinely experience stress, like epithelial tissues.

  • Displaying tissue-specific types: keratins are primarily found in epithelial cells, vimentin in fibroblasts, desmin in muscle tissue, glial filaments in glial cells, and neurofilaments in neurons.

Stability of Cytoskeletal Structures

• Cytoskeletal structures demonstrate unique characteristics:

  • Some exhibit stability, strengthening cellular architecture, while others are highly dynamic, allowing for quick adaptation to environmental changes.

  • Composed of polymers that can rapidly undergo assembly and disassembly, indicating their adaptability in response to cellular needs and signaling.

Rapid Cytoskeletal Reorganization

• During cell division, the dynamic actin cytoskeleton of fibroblasts is reorganized:

  • A polarized arrangement of filaments pushes the leading edge of the cell forward, facilitating movement.

  • Microtubules rearrange into a bipolar mitotic spindle, essential for proper chromosome alignment and separation.

  • Actin also forms a contractile ring which aids in the physical division of the daughter cells.

Stable Cytoskeletal Structures

• Certain structures of the cytoskeleton maintain stability:

  • Features such as microvilli at the apical surfaces enhance surface area for absorption and signaling.

  • The terminal web of actin filaments along with junction types (such as adherens, desmosomes, and hemidesmosomes) ensures structural integrity throughout the cell, contributing to tissue cohesiveness.

Assembly of Cytoskeletal Filaments

• Cytoskeletal filaments are composed of assemblies of specific proteins:

  • In vitro experiments reveal the dynamics of assembly and disassembly, providing insights into cellular behavior and function.

  • Understanding assembly dynamics can reveal mechanisms through which cells adapt their cytoskeleton to changing conditions, enabling efficient intracellular transport and structural integrity.

In Vitro Models of Cytoskeletal Assembly

• Assembly reactions occur at the filament ends, highlighting the dynamic nature of the cytoskeleton:

  • The dynamics involve rapid addition and loss of subunits, showcasing the flexibility and adaptability of cytoskeletal structures.

  • Nucleation processes, which are energetically unfavorable, require specific protein-protein interactions to initiate filament formation.

Critical Concentration in Assembly Dynamics

• Critical concentration (Cc) is vital for understanding filament assembly:

  • It represents the steady-state concentration of subunits necessary to maintain filament stability.

  • The addition rate of subunits is proportionate to the concentration of free subunits (K_ON * C), while the leaving rate is independent of the concentration (K_OFF).

  • The net assembly/disassembly balance is governed by Cc:

    • When above Cc, net assembly occurs.

    • When below Cc, net disassembly occurs.

Polarity of Microtubules and Microfilaments

• Microtubules and microfilaments are inherently polarized structures:

  • They have distinct assembly and disassembly rates at different filament ends: the plus end typically grows faster, whereas the minus end grows slower.

Differential Kinetics of Assembly and Disassembly

• Assembly at both ends involves conformational changes:

  • Kinetics observed vary based on structural differences, affecting the rates of growth and shrinkage at the respective ends.

  • While both ends may show similar critical concentrations at equilibrium, their rates of assembly and disassembly differ above or below Cc.

Stability of Protofilaments

• Protofilaments, the building blocks of microtubules:

  • Exhibit stability through interactions with multiple neighboring protofilaments, which enhance resilience against breakage.

  • Removal of subunits from one end mainly affects a single bond, preserving overall filament integrity.

Structure of Microtubules

• Microtubules are constructed from a heterodimer of α-tubulin and β-tubulin:

  • The β-subunit contains GTP, which hydrolyzes to GDP, initiating conformational changes during assembly.

  • Thirteen protofilaments coalesce to shape a fully functional microtubule, contributing to the stability of the cellular structure.

Structure of Microfilaments

• Microfilaments are primarily composed of monomers of actin:

  • They exhibit plus and minus ends, with ATP binding predominantly at the plus end, while ADP-bound states characterize the filament form.

Role of Nucleotide Hydrolysis in Microtubule Assembly

• Nucleotide binding is crucial for microtubule assembly:

  • Hydrolysis is essential for exhibiting dynamic properties, such as treadmilling and dynamic instability.

Assembly Dynamics vs Hydrolysis Rate

• Unpolymerized subunits primarily bind ATP/GTP without hydrolysis:

  • Assembly promotes hydrolysis, resulting in nucleotide diphosphate (NDP) binding, which influences disassociation under varying conditions.

Microtubule Treadmilling

• The distinct assembly and disassembly processes of GTP-tubulin versus GDP-tubulin:

  • Treadmilling is enabled due to varying equilibrium constants, permitting simultaneous assembly at the plus end while disassembly occurs at the minus end.

Treadmilling Mechanics

• GTP-tubulin demonstrates different equilibrium constants as a result of hydrolysis rates:

  • This creates non-equilibrium dynamics that allow plus and minus end activities to vary independently, enhancing cellular adaptability.

GTP Hydrolysis Impact on Microtubules

• GTP hydrolysis impacts microtubule dynamics:

  • A GTP cap at the plus end stabilizes assembly; loss of the GTP cap can trigger rapid disassembly, impacting overall cellular structure and response.

Conformational Changes Induced by GTP Hydrolysis

• Hydrolysis of GTP-tubulin leads to conformational changes that weaken polymer bonds:

  • Protofilaments transition from a straight, stable form into curved structures as GDP-tubulin predominates, enhancing the dynamic behavior of microtubules.

Microtubule Instability Video

• For a deeper visualization of microtubule dynamics:

  • Video Link: Microtubule Dynamics Overview

Additional Dynamics of Microtubules

• Further explorations into microtubule behaviors can be found here:

  • Video Link: Microtubule Behaviors

Similarities Between Actin and Tubulin

• Tubulin and actin demonstrate similarities in assembly properties:

  • Both require specific ions for assembly (e.g., Mg++ for tubulin; K+, Mg++, ATP for actin).

  • Rapid polymerization and depolymerization are essential for achieving steady-state conformations during cellular processes.

Evolutionary Conservation of Tubulin and Actin

• Actin and tubulin proteins exhibit a high degree of evolutionary conservation across eukaryotes:

  • Numerous isoforms exist in humans; a significant 75% identity of tubulin is found in yeast compared to human versions.

  • Variability among isoforms affects interactions with accessory proteins, influencing cytoskeletal dynamics.

Intermediate Filaments Overview

• Intermediate filaments consist of various proteins specific to certain cell types:

  • They provide vital mechanical properties to cells particularly subjected to stress and deformation.

  • Found prominently in metazoans, they are not universally required across all cellular systems.

Types of Intermediate Filaments

• Nuclear Lamins: Form the nuclear lamina, anchoring chromosomes and ensuring structural integrity during cell division.

• Cytoplasmic Types:

  • Keratins: Found in epithelial cells, crucial for maintaining cell structure and barrier functions.

  • Vimentin: Present in fibroblasts, plays roles in cellular integrity and signaling.

  • Desmin: Found in muscle cells, involved in maintaining muscle fiber integrity.

  • Neurofilament Proteins: Essential for maintaining the architecture of neurons.

Assembly Unit of Intermediate Filaments

• Intermediate filaments form from tetramers of IF proteins:

  • These elongated molecules with helical domains form stable coiled-coil structures, lacking structural polarity, allowing for diverse arrangements in cells.

Structure and Stability of Intermediate Filaments

• Intermediate filaments exhibit strong lateral interactions, providing flexibility and resilience:

  • Displaying strong yet bendable properties akin to rope, they underpin the mechanical strength of cells against various stresses.

Mechanical Properties Comparison

• Intermediate filaments contrast with microtubules and microfilaments:

  • Their fibrous characteristics and resilience allow them to withstand significant mechanical stress without losing structural integrity.

Intermediate Filament Protein Diversity

• Intermediate filament proteins display a coiled-coil structure with variability in terminal regions:

  • Keratins, for example, show extensive diversity across different epithelial cell populations.

Keratin Filaments in Epithelial Cells

• The organization of keratin filaments is crucial for the resilience of epithelial tissues:

  • Their structure is linked to diagnostic and therapeutic approaches concerning epithelial cancers, shedding light on interventions that could enhance cell adhesion and integrity.

  • Genetic mutations affecting keratin can lead to conditions such as Epidermolysis Bullosa Simplex, characterized by skin fragility.

Epidermolysis Bullosa Simplex

• This condition arises from mutations in cytokeratin, leading to heightened vulnerability of the epidermis to mechanical stress:

  • Clinical manifestations involve severe blistering upon minor trauma, highlighting the importance of keratin's structural role in maintaining skin integrity.

Neurofilaments in Neurons

• Neurofilaments are abundant in neuronal axons, impacting growth dynamics:

  • Expression levels significantly affect axon diameter, which plays a vital role in signaling speed across neurons.

  • Abnormal accumulation of neurofilaments is linked to various neurodegenerative diseases, such as Amyotrophic Lateral Sclerosis (ALS).

Differences Between Intermediate and Other Filaments

• The assembly properties of intermediate filaments diverge from those exhibited by microtubules and microfilaments:

  • Their assembly mechanisms are less understood and indicate a highly stable state with minimal amounts of unassembled protein present.

Dynamic Regulation of IF Assembly

• The assembly of intermediate filaments can also be dynamic:

  • The networks can disassemble in response to mechanical stress when cells detach from their respective substrates, followed by reassembly upon reattachment.

Summary of Cytoskeletal Dynamics

• Microtubules, microfilaments, and intermediate filaments manifest distinct characteristics:

  • Each class is assembled from specific proteins, with dynamic behaviors during assembly and disassembly processes.

  • Their inherent polarization affects assembly rates and effectiveness in fulfilling cellular functions.

  • Nucleotide hydrolysis significantly contributes to dynamic behaviors observed in tubulin and actin, while intermediate filaments exhibit notable stability and nuanced regulatory mechanisms in response to cellular demands.

Fundamentals of Cell Biology

Slide 1: Overview of Cellular Organization

• Cells exhibit spatial organization and mechanical interaction with their environments, showcasing the intricate relationship between cellular structure and function.

Slide 2: Key Functions

• Correctly shaping cells and structuring internal components is crucial for their viability.

• Transporting molecules efficiently within the cytoplasm is fundamental for cellular metabolism and signaling.

• Mobility is essential for various functions including movement, migration, and response to stimuli.

• The ability to rearrange internal components during cell growth, division, and adaptation plays a pivotal role in maintaining homeostasis.

Slide 3: Examples of Cells

• Red Blood Cells (Erythrocytes): Specialized for oxygen transport, exhibit a biconcave shape that increases surface area for gas exchange.

• S. cerevisiae (Baker's Yeast): A model organism used for studying cellular processes and genetics, understanding how cytoskeletal dynamics affect yeast cell division and morphology.

• Motor Neurons in the Spinal Cord: Cells that transmit signals throughout the nervous system, dependent on cytoskeletal components for maintaining structure and facilitating transport of organelles.

Slide 4: Eukaryotic Cell Overview of Cytoskeleton

• Actin filaments: Stained red, crucial for cellular mobility and structure.

• Microtubules: Stained green, involved in transporting materials within the cell and maintaining shape.

• Nuclei: Stained blue, central command centers of cellular activity.

Slide 5: The Cytoskeleton

• The cytoskeleton is a complex network of protein filaments that underpins cell behavior and morphology. It is crucial for cellular structure, facilitating a variety of functions.

• Major Systems Include:

  • Microtubules: Tubulin polymers that help organize intracellular structures and serve as tracks for motor proteins.

  • Actin Filaments: Essential for muscle contraction and shape changes, forming networks beneath the plasma membrane.

  • Intermediate Filaments: Provide tensile strength to cells and structural support to nuclear membranes.

Slide 6: Functions of the Cytoskeleton

• The cytoskeleton is indispensable for several key cellular processes:

  • Mitosis: Microtubules pull chromosomes apart and assist in cell division by facilitating the separation of daughter cells.

  • Intracellular Transport: Organelles and vesicles are moved along microtubule tracks, ensuring proper cellular function and organization.

  • Mechanical Support: The cytoskeleton supports the plasma membrane under stress, enabling cells to maintain their shape during external pressure.

  • Cellular Movements: Swimming seen in spermatozoa and crawling methods used by fibroblasts and leukocytes rely on actin filament dynamics for motility.

  • Signal Transmission: The cytoskeleton is involved in neuronal signaling by extending axons and dendrites, essential for nerve signal transmission.

  • Plant Cell Growth: The cytoskeleton directs the expansion and differentiation of plant cell walls.

  • Control of Cell Shape: Actin filaments control the diverse shapes of cells, impacting their functionality and interaction with other cells.

Slide 7: Classes of Cytoskeletal Filaments

• There are three primary classes of cytoskeletal filaments identifiable by microscopy:

  • Microtubules: ~25 nm diameter, with a hollow structure that facilitates intracytoplasmic transport.

  • Intermediate Filaments: ~10 nm diameter, known for their role in mechanical resilience against stress.

  • Microfilaments: ~5-8 nm diameter, composed mainly of actin, crucial for shape and motility.

Slide 8: Characteristics of Filaments

• Intermediate Filaments (IFs):

  • Extend throughout the cytoplasm, forming a supportive lamina beneath the nuclear membrane that maintains nuclear integrity.

  • Provide mechanical strength and resist shear stress exerted on cells during physical activity.

• Microtubules:

  • Emanate from the centrosome, essential for positioning organelles and directing intracellular transport of vesicles.

• Microfilaments:

  • Organized into networks closely associated with the cell cortex, influencing the shape of the cell and playing a crucial role in locomotion.

Slide 9: Accessory Proteins

• Cytoskeletal filaments interact with various accessory proteins that play crucial roles in their assembly, positioning, and function:

  • Linking Proteins: Attach filaments to other cell components and to each other.

  • Motor Proteins: Facilitate the movement of organelles along filaments, integrating cytoskeletal dynamics with cellular function.

Slide 10: Functions of Microtubules

• Microtubules are pivotal for cellular operations, contributing to:

  • The organization of cytoplasmic organelles like the Golgi apparatus and endoplasmic reticulum.

  • Providing tracks for motor proteins to transport cellular components.

  • Generating forces necessary for the movement of cilia and flagella, essential for cell motility and fluid movement.

  • Forming the mitotic spindle that plays a critical role in chromosome alignment and separation during cell division.

Slide 11: Functions of Actin Filaments

• Actin filaments exhibit versatility by taking part in various structural formations:

  • Schedule into linear bundles, two-dimensional sheets, or three-dimensional gels primarily located beneath the plasma membrane, known as the cell cortex.

  • Change cell shapes by exerting force on the plasma membrane, crucial during processes like cytokinesis.

  • In muscle cells, actin filaments contribute to contraction by forming structures that allow interaction with myosin.

Slide 12: Functions of Intermediate Filaments

• Intermediate filaments serve multiple functions including:

  • Serving as components of the nuclear lamina, providing structural support for chromatin organization during interphase.

  • Controlling the dynamics of nuclear disassembly and reassembly during the mitotic phase of the cell cycle.

  • Providing mechanical integrity to tissues that routinely experience stress, like epithelial tissues.

  • Displaying tissue-specific types: keratins are primarily found in epithelial cells, vimentin in fibroblasts, desmin in muscle tissue, glial filaments in glial cells, and neurofilaments in neurons.

Slide 13: Stability of Cytoskeletal Structures

• Cytoskeletal structures demonstrate unique characteristics:

  • Some exhibit stability, strengthening cellular architecture, while others are highly dynamic, allowing for quick adaptation to environmental changes.

  • Composed of polymers that can rapidly undergo assembly and disassembly, indicating their adaptability in response to cellular needs and signaling.

Slide 14: Rapid Cytoskeletal Reorganization

• During cell division, the dynamic actin cytoskeleton of fibroblasts is reorganized:

  • A polarized arrangement of filaments pushes the leading edge of the cell forward, facilitating movement.

  • Microtubules rearrange into a bipolar mitotic spindle, essential for proper chromosome alignment and separation.

  • Actin also forms a contractile ring which aids in the physical division of the daughter cells.

Slide 15: Stable Cytoskeletal Structures

• Certain structures of the cytoskeleton maintain stability:

  • Features such as microvilli at the apical surfaces enhance surface area for absorption and signaling.

  • The terminal web of actin filaments along with junction types (such as adherens, desmosomes, and hemidesmosomes) ensures structural integrity throughout the cell, contributing to tissue cohesiveness.

Slide 16: Assembly of Cytoskeletal Filaments

• Cytoskeletal filaments are composed of assemblies of specific proteins:

  • In vitro experiments reveal the dynamics of assembly and disassembly, providing insights into cellular behavior and function.

  • Understanding assembly dynamics can reveal mechanisms through which cells adapt their cytoskeleton to changing conditions, enabling efficient intracellular transport and structural integrity.

Slide 17: In Vitro Models of Cytoskeletal Assembly

• Assembly reactions occur at the filament ends, highlighting the dynamic nature of the cytoskeleton:

  • The dynamics involve rapid addition and loss of subunits, showcasing the flexibility and adaptability of cytoskeletal structures.

  • Nucleation processes, which are energetically unfavorable, require specific protein-protein interactions to initiate filament formation.

Slide 18: Critical Concentration in Assembly Dynamics

• Critical concentration (Cc) is vital for understanding filament assembly:

  • It represents the steady-state concentration of subunits necessary to maintain filament stability.

  • The addition rate of subunits is proportionate to the concentration of free subunits (K_ON * C), while the leaving rate is independent of the concentration (K_OFF).

  • The net assembly/disassembly balance is governed by Cc:

    • When above Cc, net assembly occurs.

    • When below Cc, net disassembly occurs.

Slide 19: Polarity of Microtubules and Microfilaments

• Microtubules and microfilaments are inherently polarized structures:

  • They have distinct assembly and disassembly rates at different filament ends: the plus end typically grows faster, whereas the minus end grows slower.

Slide 20: Differential Kinetics of Assembly and Disassembly

• Assembly at both ends involves conformational changes:

  • Kinetics observed vary based on structural differences, affecting the rates of growth and shrinkage at the respective ends.

  • While both ends may show similar critical concentrations at equilibrium, their rates of assembly and disassembly differ above or below Cc.

Slide 21: Stability of Protofilaments

• Protofilaments, the building blocks of microtubules:

  • Exhibit stability through interactions with multiple neighboring protofilaments, which enhance resilience against breakage.

  • Removal of subunits from one end mainly affects a single bond, preserving overall filament integrity.

Slide 22: Structure of Microtubules

• Microtubules are constructed from a heterodimer of α-tubulin and β-tubulin:

  • The β-subunit contains GTP, which hydrolyzes to GDP, initiating conformational changes during assembly.

  • Thirteen protofilaments coalesce to shape a fully functional microtubule, contributing to the stability of the cellular structure.

Slide 23: Structure of Microfilaments

• Microfilaments are primarily composed of monomers of actin:

  • They exhibit plus and minus ends, with ATP binding predominantly at the plus end, while ADP-bound states characterize the filament form.

Slide 24: Role of Nucleotide Hydrolysis in Microtubule Assembly

• Nucleotide binding is crucial for microtubule assembly:

  • Hydrolysis is essential for exhibiting dynamic properties, such as treadmilling and dynamic instability.

Slide 25: Assembly Dynamics vs Hydrolysis Rate

• Unpolymerized subunits primarily bind ATP/GTP without hydrolysis:

  • Assembly promotes hydrolysis, resulting in nucleotide diphosphate (NDP) binding, which influences disassociation under varying conditions.

Slide 26: Microtubule Treadmilling

• The distinct assembly and disassembly processes of GTP-tubulin versus GDP-tubulin:

  • Treadmilling is enabled due to varying equilibrium constants, permitting simultaneous assembly at the plus end while disassembly occurs at the minus end.

Slide 27: Treadmilling Mechanics

• GTP-tubulin demonstrates different equilibrium constants as a result of hydrolysis rates:

  • This creates non-equilibrium dynamics that allow plus and minus end activities to vary independently, enhancing cellular adaptability.

Slide 28: GTP Hydrolysis Impact on Microtubules

• GTP hydrolysis impacts microtubule dynamics:

  • A GTP cap at the plus end stabilizes assembly; loss of the GTP cap can trigger rapid disassembly, impacting overall cellular structure and response.

Slide 29: Conformational Changes Induced by GTP Hydrolysis

• Hydrolysis of GTP-tubulin leads to conformational changes that weaken polymer bonds:

  • Protofilaments transition from a straight, stable form into curved structures as GDP-tubulin predominates, enhancing the dynamic behavior of microtubules.

Slide 30: Microtubule Instability Video

• For a deeper visualization of microtubule dynamics:

  • Video Link: Microtubule Dynamics Overview

Slide 31: Additional Dynamics of Microtubules

• Further explorations into microtubule behaviors can be found here:

  • Video Link: Microtubule Behaviors

Slide 32: Similarities Between Actin and Tubulin

• Tubulin and actin demonstrate similarities in assembly properties:

  • Both require specific ions for assembly (e.g., Mg++ for tubulin; K+, Mg++, ATP for actin).

  • Rapid polymerization and depolymerization are essential for achieving steady-state conformations during cellular processes.

Slide 33: Evolutionary Conservation of Tubulin and Actin

• Actin and tubulin proteins exhibit a high degree of evolutionary conservation across eukaryotes:

  • Numerous isoforms exist in humans; a significant 75% identity of tubulin is found in yeast compared to human versions.

  • Variability among isoforms affects interactions with accessory proteins, influencing cytoskeletal dynamics.

Slide 34: Intermediate Filaments Overview

• Intermediate filaments consist of various proteins specific to certain cell types:

  • They provide vital mechanical properties to cells particularly subjected to stress and deformation.

  • Found prominently in metazoans, they are not universally required across all cellular systems.

Slide 35: Types of Intermediate Filaments

• Nuclear Lamins: Form the nuclear lamina, anchoring chromosomes and ensuring structural integrity during cell division.

• Cytoplasmic Types:

  • Keratins: Found in epithelial cells, crucial for maintaining cell structure and barrier functions.

  • Vimentin: Present in fibroblasts, plays roles in cellular integrity and signaling.

  • Desmin: Found in muscle cells, involved in maintaining muscle fiber integrity.

  • Neurofilament Proteins: Essential for maintaining the architecture of neurons.

Slide 36: Assembly Unit of Intermediate Filaments

• Intermediate filaments form from tetramers of IF proteins:

  • These elongated molecules with helical domains form stable coiled-coil structures, lacking structural polarity, allowing for diverse arrangements in cells.

Slide 37: Structure and Stability of Intermediate Filaments

• Intermediate filaments exhibit strong lateral interactions, providing flexibility and resilience:

  • Displaying strong yet bendable properties akin to rope, they underpin the mechanical strength of cells against various stresses.

Slide 38: Mechanical Properties Comparison

• Intermediate filaments contrast with microtubules and microfilaments:

  • Their fibrous characteristics and resilience allow them to withstand significant mechanical stress without losing structural integrity.

Slide 39: Intermediate Filament Protein Diversity

• Intermediate filament proteins display a coiled-coil structure with variability in terminal regions:

  • Keratins, for example, show extensive diversity across different epithelial cell populations.

Slide 40: Keratin Filaments in Epithelial Cells

• The organization of keratin filaments is crucial for the resilience of epithelial tissues:

  • Their structure is linked to diagnostic and therapeutic approaches concerning epithelial cancers, shedding light on interventions that could enhance cell adhesion and integrity.

  • Genetic mutations affecting keratin can lead to conditions such as Epidermolysis Bullosa Simplex, characterized by skin fragility.

Slide 41: Epidermolysis Bullosa Simplex

• This condition arises from mutations in cytokeratin, leading to heightened vulnerability of the epidermis to mechanical stress:

  • Clinical manifestations involve severe blistering upon minor trauma, highlighting the importance of keratin's structural role in maintaining skin integrity.

Slide 42: Neurofilaments in Neurons

• Neurofilaments are abundant in neuronal axons, impacting growth dynamics:

  • Expression levels significantly affect axon diameter, which plays a vital role in signaling speed across neurons.

  • Abnormal accumulation of neurofilaments is linked to various neurodegenerative diseases, such as Amyotrophic Lateral Sclerosis (ALS).

Slide 43: Differences Between Intermediate and Other Filaments

• The assembly properties of intermediate filaments diverge from those exhibited by microtubules and microfilaments:

  • Their assembly mechanisms are less understood and indicate a highly stable state with minimal amounts of unassembled protein present.

Slide 44: Dynamic Regulation of IF Assembly

• The assembly of intermediate filaments can also be dynamic:

  • The networks can disassemble in response to mechanical stress when cells detach from their respective substrates, followed by reassembly upon reattachment.

Slide 45: Summary of Cytoskeletal Dynamics

• Microtubules, microfilaments, and intermediate filaments manifest distinct characteristics:

  • Each class is assembled from specific proteins, with dynamic behaviors during assembly and disassembly processes.

  • Their inherent polarization affects assembly rates and effectiveness in fulfilling cellular functions.

  • Nucleotide hydrolysis significantly contributes to dynamic behaviors observed in tubulin and actin, while intermediate filaments exhibit notable stability and nuanced regulatory mechanisms in response to cellular demands