Biologia Celular-Ensino Superior-Aula 21:Citoesqueleto II- Microfilamentos

Summary

In this 21st biology lesson, Professor Wesley delves into actin filaments, integral components of the cytoskeleton, crucial for cellular movement and flexibility. The video explores the structure of actin filaments, their formation through polymerization, and their role in processes like cell division, endocytosis, and muscle contraction. Actin interacts with several accessory proteins, regulating cell shape and movement, including during amoeboid movement and immune cell migration. The lesson also covers muscle contraction mechanisms, specifically how actin and myosin filaments work together in muscle fibers to produce contraction, with a focus on ATP hydrolysis and calcium signaling in both skeletal and smooth muscle cells.

Takeaways

• 😀 Actin filaments (microfilaments) are thin, flexible protein structures crucial for cellular movement and flexibility.
• 😀 Actin filaments have a diameter of 5 to 8 nanometers and are found in all eukaryotic cells.
• 😀 These filaments organize into bundles or networks, providing structural support and playing roles in processes like cell division and movement.
• 😀 Actin filaments are associated with various accessory proteins that modulate their polymerization, location, and interactions.
• 😀 Thymosin and profilin are key regulatory proteins that control actin filament formation and prevent free actin monomers from polymerizing.
• 😀 Actin filaments are involved in important physiological functions like cell shape maintenance, movement, and intracellular transport.
• 😀 Actin filaments are essential for cellular processes such as endocytosis, exocytosis, and positioning of macromolecules.
• 😀 Actin filaments play a critical role in cell migration, including amoeboid movement and neutrophil/macrophage migration towards infection sites.
• 😀 Muscle contraction relies on the interaction between actin and myosin, which slide past each other, shortening muscle fibers.
• 😀 The process of muscle contraction is triggered by ATP hydrolysis and the release of calcium ions, which activate myosin's interaction with actin.
• 😀 In smooth muscle, actin filaments form a less organized network than in skeletal muscle, and contraction is slower and regulated by different signals.

Q & A

What are actin filaments and what is their primary function in the cell?

Actin filaments, also known as microfilaments, are protein structures found in all eukaryotic cells. Their primary function is to maintain cell shape, facilitate cell movement, and support various cellular processes such as endocytosis, exocytosis, and muscle contraction.

What is the structural characteristic of actin filaments?

Actin filaments are thin and flexible structures with a diameter ranging from 5 to 8 nanometers. They are composed of actin monomers (actin G) that polymerize to form long, thin filaments (actin F), and they organize into bundles or networks within the cell.

How does actin filament polarity contribute to their function?

Actin filaments have two distinct ends with different polarity: a positive end where new actin monomers are added more rapidly, and a negative end where dissociation of monomers is more frequent. This polarity allows for the dynamic growth and shrinkage necessary for cellular movement and changes in cell shape.

What role do accessory proteins play in actin filament dynamics?

Accessory proteins like thymosin and profilin regulate actin filament dynamics by preventing the binding of actin monomers to existing filaments, thus controlling polymerization. Other proteins, such as cofilin, assist in filament disassembly, ensuring proper turnover and organization of actin filaments in response to cellular needs.

How do actin filaments contribute to muscle contraction?

In muscle cells, actin filaments interact with myosin, a motor protein, to produce contraction. Myosin heads bind to actin filaments and, using energy from ATP hydrolysis, slide the actin filaments over myosin, resulting in muscle shortening and contraction.

What is the role of actin filaments in cell division?

During cell division, actin filaments form a contractile ring at the site of cytokinesis. This ring contracts, leading to the physical separation of the two daughter cells.

What is the significance of actin filaments in cell motility?

Actin filaments are essential for cell motility, including processes like amoeboid movement. They enable cells to extend protrusions, such as lamellipodia or filopodia, and move in response to chemical signals, which is critical for processes like immune cell migration and wound healing.

How do actin filaments interact with other cellular components like microvilli?

Actin filaments provide structural support to microvilli, specialized cell membrane protrusions. The actin network stabilizes these structures, enhancing their ability to increase the surface area for absorption, particularly in epithelial cells.

What are the differences between muscle contraction in smooth muscle and skeletal muscle?

In smooth muscle, actin filaments interact with myosin in a less organized manner, resulting in slower contractions. In skeletal muscle, actin and myosin are highly organized into sarcomeres, allowing for rapid and strong contractions in response to nerve signals.

What is the relationship between actin filaments and calcium in muscle contraction?

In muscle contraction, an influx of calcium ions into the muscle cell activates the troponin-tropomyosin complex, which exposes binding sites on actin filaments for myosin. This interaction allows myosin heads to bind to actin and, using ATP, produce movement and contraction.