Wk 7 - 1st half
Actin Components and Dynamics
Cortical Nature of Actin
Actin filaments are primarily localized at the cell cortex, which is the outer part of the cytoplasm, playing a crucial role in maintaining the cell's shape, facilitating cell movement, and enabling cellular interactions with the environment.
This cortical actin plays a key role in the mechanics of cell motility and stability, influencing processes such as cytokinesis, cell adhesion, and endocytosis.
Properties of Globular Actin and Polar Microfilaments
Globular Actin (G-actin): The monomeric form of actin, comprising a globular structure that can polymerize into filamentous actin (F-actin).
Filamentous Actin (F-actin): The polymerized form of G-actin that forms long, helical structures.
F-actin exhibits polarity with distinct physiological properties at the plus (+) end and minus (-) end, where:
The plus end is where polymerization occurs more rapidly, while the minus end is where depolymerization typically occurs faster.
This polarity is critical for various actin-mediated cellular processes and the directionality of myosin motor proteins.
Role of Nucleus in Elongation
The formation of a nucleus, a stable aggregate of actin monomers, is essential because it provides a template for further G-actin addition. This accelerates the polymerization process:
Once a critical nucleus size is attained, the rate of actin filament growth at the plus end increases significantly due to a reduction in the energy barrier for further additions.
This initial nucleation step is considered a rate-limiting step in the assembly of actin filaments.
Critical Concentrations and Treadmilling
Critical Concentration (Cc): The concentration of free G-actin at which the rate of addition to the filament equals the rate of loss from the filament.
Treadmilling occurs when the concentration of G-actin is between the critical concentrations of the plus and minus ends, where:
G-actin is added at the plus end while simultaneously being lost from the minus end, resulting in a dynamic equilibrium as the filament length remains constant, which is essential for maintaining cellular shape and function during movement and growth.
Actin Binding Proteins and Regulation
Regulation of Polymerization
Abundance of G-actin: The availability of G-actin in the cytoplasm is essential for maintaining the actin dynamics necessary for cellular activity.
Key Actin-Binding Proteins:
Thymosin: Binds G-actin, preventing its polymerization and thereby regulating the pool of free G-actin available for filament formation.
Profilin: Facilitates the conversion of ADP-G-actin to ATP-G-actin, enhancing the rate of polymerization by promoting the addition of ATP-bound G-actin to the growing filament, ensuring a rapid response to cellular needs.
Cofilin: Binds to ADP-actin filaments, enhancing their depolymerization and promoting turnover, thereby regulating actin filament dynamics in response to cellular signaling.
Formin and Arp2/3 Complex
Formin: Promotes the nucleation and elongation of unbranched actin filaments, enabling rapid filament formation in specific cellular contexts, such as during the formation of stress fibers.
Arp2/3 Complex: Initiates the formation of branched actin networks:
Binds to the side of existing filaments, creating new filaments at an angle, which is essential for the formation of lamellipodia in motile cells.
This branching is vital for processes such as endocytosis, where it helps to engulf cellular material, and phagocytosis, where it aids in immune cell function.
Bundling and Branching
Actin-binding proteins, such as fimbrin and filamin, assist in bundling and branching of actin filaments, enhancing structural integrity and facilitating specialized functions:
Bundled actin filaments provide mechanical strength and support, while branched networks are crucial for motility and energy-efficient cellular movements.
Actin's Role in Membrane Support
Actin filaments provide mechanical support to cellular membranes, playing an essential role in maintaining cell shape and facilitating membrane dynamics during processes like endocytosis and exocytosis:
The interaction of actin with membrane proteins aids in the stability and functioning of cell junctions, maintaining the integrity and the barrier functions of epithelial tissues.
Myosin and the Sarcomere
Functions of Myosin
Myosin comprises several classes, each varying in structure and function:
Myosin Class I: Involved in cellular transport mechanisms; facilitates vesicle movement.
Myosin Class II: Primarily responsible for muscle contraction, interacting directly with actin filaments in sarcomeres.
Myosin Class V: Functions in cargo transport along actin filaments in non-muscle cells; these myosins enable the transport of organelles and vesicles within the cell.
Myosin Movement Towards Actin's Plus End
Myosin heads attach to the actin filament's plus end, and through a series of steps powered by ATP hydrolysis, they pull the filaments closer together, enabling muscle contraction and other cellular movements.
This process, known as the cross-bridge cycle, involves:
Attachment: Myosin heads bind to actin, forming a cross-bridge.
Power Stroke: The myosin head pivots, pulling the actin filament toward the center of the sarcomere.
Release: ATP attaches to the myosin head, causing it to detach from the actin filament.
Re-cocking: The hydrolysis of ATP re-cocks the myosin head into its high-energy state in preparation for another power stroke.
Labeling a Sarcomere
A sarcomere is the structural and functional unit of a muscle fiber, composed of:
Thin Filaments: Primarily made of actin.
Thick Filaments: Composed of myosin.
Z-lines: Define the boundaries of each sarcomere and anchor the actin filaments.
M-line: The center of the sarcomere that holds the thick filaments together.
I-band: Region containing only thin filaments, appearing lighter.
A-band: Region containing thick filaments and overlapping thin filaments, responsible for the darker appearance.
Relationship Between Sarcoplasmic Reticulum and T-tubules
The sarcoplasmic reticulum (SR) is a specialized organelle that stores and releases calcium ions into the cytoplasm to initiate muscle contraction.
T-tubules are extensions of the plasma membrane that penetrate into the muscle fiber, providing a pathway for electrical signals from the neuromuscular junction to quickly reach the interior of the muscle cell.
The interaction between the SR and T-tubules is crucial for effective signal transduction:
When action potentials travel down the T-tubules, they trigger the release of calcium from the SR into the cytoplasm.
Calcium's Role in Muscle Contraction
Calcium ions are central to the regulation of muscle contraction. When released from the sarcoplasmic reticulum:
Calcium binds to troponin, a regulatory protein on the thin filaments.
This binding causes a conformational change that moves tropomyosin, another regulatory protein, away from the binding sites on actin, allowing myosin heads to attach to actin.
The cycle of myosin binding to actin and performing power strokes leads to the contraction of the muscle filament, shortening the sarcomere and generating force.
After contraction, calcium ions are pumped back into the sarcoplasmic reticulum, allowing the muscle to relax and prepare for subsequent contractions.