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Questions and Answers
What is one of the primary functions of the cytoskeleton in eukaryotic cells?
What is one of the primary functions of the cytoskeleton in eukaryotic cells?
Which type of protein filament is NOT a principal component of the cytoskeleton?
Which type of protein filament is NOT a principal component of the cytoskeleton?
Which protein is primarily responsible for muscle contraction in relation to the cytoskeleton?
Which protein is primarily responsible for muscle contraction in relation to the cytoskeleton?
What is a function of intermediate filaments within the cytoskeleton?
What is a function of intermediate filaments within the cytoskeleton?
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What is the approximate diameter of actin filaments?
What is the approximate diameter of actin filaments?
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What role do actin filaments play in cell function?
What role do actin filaments play in cell function?
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What is the first step in the assembly of actin filaments?
What is the first step in the assembly of actin filaments?
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What distinguishes the ends of actin filaments?
What distinguishes the ends of actin filaments?
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What type of actin structure is characterized by closely packed, parallel arrays?
What type of actin structure is characterized by closely packed, parallel arrays?
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Which proteins are involved in cross-linking actin filaments into networks?
Which proteins are involved in cross-linking actin filaments into networks?
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What distinguishes microvilli from other actin-based protrusions?
What distinguishes microvilli from other actin-based protrusions?
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Which protein is primarily associated with the cross-linking of actin filaments in microvilli?
Which protein is primarily associated with the cross-linking of actin filaments in microvilli?
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What is a key feature of lamellipodia?
What is a key feature of lamellipodia?
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Which type of cell surface extension is primarily involved in engulfing other cells during phagocytosis?
Which type of cell surface extension is primarily involved in engulfing other cells during phagocytosis?
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What are microspikes, also known as filopodia, characterized by?
What are microspikes, also known as filopodia, characterized by?
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Study Notes
Cytoskeleton and Cellular Motility
- The cytoskeleton is a network of protein filaments throughout the cytoplasm of eukaryotic cells.
- It provides structural support, determining cell shape, positioning of organelles, and overall cytoplasm organization.
- The cytoskeleton is responsible for cellular movements.
- The cytoskeleton comprises three main types of filaments: actin filaments, intermediate filaments, and microtubules.
- Accessory proteins hold these filaments together and link them to subcellular organelles and the plasma membrane.
Actin Filaments
- Actin, the major cytoskeletal protein in most cells, polymerizes to form thin, flexible fibers (7 nm in diameter).
- These fibers can reach several micrometers in length.
- Actin filaments (also called microfilaments) are abundant beneath the plasma membrane forming a network for mechanical support, maintaining cell shape, and enabling cell surface movement.
- Enabling cells to migrate, engulf particles, and divide.
- Actin filaments have two ends: barbed and pointed, differing in assembly rates and functions.
Structure and Organization of Actin Filaments
- Actin monomers (G-actin) polymerize to form filaments (F-actin).
- Actin monomers rotate by 166 degrees in the filaments.
- The filaments' polarity (barbed/pointed ends) distinguishes them from one another.
- Actin bundles are characterized by closely packed parallel arrays.
- Actin cross-linking proteins, like fimbrin and a-actinin, assemble actin filaments into bundled or networked structures.
Actin Bundles and Networks
- Actin filaments form bundles and networks, crucial for cell shape and movement.
- Electron micrographs demonstrate these structures beneath the plasma membrane of cells like macrophages, supporting projections (e.g., filopodia).
Actin-Bundling Proteins
- Fimbrin and a-actinin, for example, create parallel bundles and cross-link filaments into networks in different ways.
- Fimbrin generates tighter and more closely spaced bundles (approx. 14 nm apart).
- a-actinin generates sparser, loosely spaced bundles (approx. 40 nm apart).
- These proteins consist of related structural units (e.g., Ca2+-binding domains).
Actin Networks and Filamin
- Filamin, a large V-shaped protein, crosslinks actin filaments into orthogonal networks for three-dimensional meshworks.
- Its carboxy-terminal dimerization domain separates from the amino-terminal actin-binding domain by several β-sheet spacer domains.
Association of Actin filaments with the Plasma Membrane
- Actin filaments are highly concentrated at the cell periphery, forming a 3D network beneath the plasma membrane.
- This network (cell cortex) is essential for cell shape and activities like movement.
Protrusions of the Cell Surface
- Most cells have protrusions like microvilli (finger-like projections), involved in movement, phagocytosis, and specialized functions.
- Microvilli, based on actin filaments, form permanent or rapidly rearranging bundles for specialized functions.
Intestial Microvilli
- Villin, a 95 kd actin-bundling protein, plays a key role in these structures
- Microvilli are an important structure in intestinal and kidney tubules.
Organization of Microvilli
- Core actin filaments are cross-linked into tightly packed bundles by fimbrin and villin.
- Lateral arms (containing myosin I and calmodulin) link actin filaments to the plasma membrane.
- Barbed ends of actin filaments are embedded in a protein cap at the microvillus tip.
Other Surface Protrusions
- Pseudopodia, broad and sheetlike (e.g., lamellipodia), and thin projections (e.g., filopodia) are examples of transient cell surface projections formed in response to environmental stimuli.
- These protrusions are involved in cell locomotion and phagocytosis.
Formation of Protrusions and Cell Movement
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Cell movement is a fundamental process crucial for multicellular organisms.
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Locomotion involves various processes like amoeba crawling, embryonic cell migration, immune cell invasion during infection, wound healing, and cancer metastasis.
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These processes are driven by the dynamic nature of the actin cytoskeleton.
Actin, Myosin, and Cell Movement
- Interactions between actin and myosin are essential for various types of cell movements, including muscle contractions.
- Myosin, a molecular motor protein, converts chemical energy (ATP) into mechanical energy, generating force and movement.
Muscle Contraction:
- Muscle cells are specialized for contraction, and their structure facilitates the study of movement at the cellular and molecular levels.
Types of Muscle Cells
- Vertebrates exhibit three major types of muscle cells: skeletal, cardiac, and smooth.
- Skeletal muscle is responsible for voluntary movements.
- Cardiac muscle powers heart function.
- Smooth muscle governs involuntary organ movements.
Skeletal Muscle Structure
- Skeletal muscles are bundles of muscle fibers.
- Muscle fibers develop through the fusion of many individual cells.
- Myofibrils, which consist of thick myosin and thin actin filaments, compose the majority of the cytoplasm.
- Organization into highly organized sarcomeres gives skeletal and cardiac muscle its striated appearance.
Structure of Sarcomeres
- Myofilaments (actin and myosin) are organized into functional units.
- The arrangement of thin and thick filaments determines the striated appearance.
- The I-bands contain only actin filaments, while the A-bands contain both actin and myosin filaments.
Titin and Nebulin
- Titin acts as a spring, centering myosin filaments within the sarcomere.
- Nebulin determines the length of actin filaments.
Sliding Filament Model
- Actin filaments slide past myosin, shortening the sarcomere and generating movement without changing filament length.
- This interaction's basis is myosin's binding to actin filaments to act as a motor driving sliding.
Contractile Assemblies in Nonmuscle Cells
- Myosin's role extends to cytokinesis (cell division) and other nonmuscle processes such as membrane vesicle/organelle transport, phagocytosis, and pseudopod extension.
Intermediate Filaments
- Intermediate filaments lie in diameter between actin and microtubules (8-11nm).
- They primarily provide mechanical strength to cells and tissues, unlike actin and microtubules which are involved in movement.
- Intermediate filament proteins (over 65 identified) have similar amino acid sequences and organize as polymers through a coiled-coil structure, building up into protofilaments until filaments are made.
Assembly of Intermediate Filaments
- Intermediate filaments assemble using a polypeptide that winds around each other to form dimers, then they associate into tetramers, then protofilaments, and finally, intermediate filaments which consist of eight protofilaments.
Intracellular Organization of Intermediate Filaments
- They form an extensive network through the cytoplasm, extending from a ring-like structure surrounding the nucleus to the plasma membrane.
- In epithelial cells, intermediate filaments are anchored to the cell membrane where specialized contacts (desmosomes and hemidesmosomes) are formed.
Modifications of Intermediate Filaments
- Intermediate filaments can be modified by phosphorylation, impacting assembly and disassembly.
- This modification can influence the function of these filaments by impacting behavior like during cell division.
- Keratin and vimentin are examples of these filaments that connect with the membrane and nucleus, maintaining position and stability.
Microtubules
- Microtubules are rigid hollow rods approximately 25nm wide.
- Like actin filaments, they are dynamic, constantly assembling and disassembling inside the cell.
- They play a vital role in determining cell shape, many cell movements (like locomotion), intracellular transport, and chromosome segregation during mitosis.
Assembly of Microtubules
- Microtubules, in animal cells, emanate from the centrosome near the nucleus.
- Microtubules are polymers of the single type of globular protein tubulin, which polymerizes into protofilaments.
Microtubules: Structure and Dynamic Organization
- α-tubulin and β-tubulin are the two types of closely related protein polypeptides that form the building blocks of microtubules.
- 13 protofilaments arranged around a hollow core create a microtubule.
- γ-tubulin is involved in initiating microtubule assembly at the centrosome.
Microtubule Polarity
- Microtubules' distinct ends define their polarity (fast-growing plus end, slow-growing minus end).
- Just like actin filaments, this polarity defines the direction of movement along microtubules.
Intracellular Organization of Microtubules
- In non-dividing cells, microtubules radiate outward from the centrosome to the periphery.
- In dividing cells, the centrosome's duplicate parts separate, with microtubules reorganizing to form the mitotic spindle.
Microtubule Motors and Movement
- Microtubules are pivotal for intracellular transportation and movement of vesicles and organelles, chromosome separation in mitosis, and the movement of cilia and flagella.
Cargo Transport and Intracellular Organization
- Transport of macromolecules, membrane vesicles, and organelles through the cytoplasm of eukaryotic cells is a crucial function of microtubules.
Reorganization of Microtubules During Mitosis
- Microtubules reorganize at the start of mitosis to form the mitotic spindle.
- This spindle is essential for separating chromosomes during cell division.
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Description
Explore the fundamental concepts of the cytoskeleton and its role in cellular motility. Learn about the different types of filaments, their functions, and how they contribute to the structure and movement of eukaryotic cells. This quiz will test your knowledge of the essential components of the cytoskeleton.
