32. Cytoskeleton. Actin, intermediate and myosin filaments.

Actin Filaments (Microfilaments) structure

• Actin filaments are thin, dynamic, double-stranded structures with a diameter of about 7 nm, making them the thinnest filaments of the cytoskeleton.

• They are composed of linear polymers of actin monomers (G-actin), forming filamentous actin (F-actin).

• Actin filaments are highly flexible and shorter compared to microtubules.

Actin Filaments (Microfilaments) dynamics

Assembly: G-actin monomers add to the plus (+) or barbed end of the filament, while they are removed from the minus (−) or pointed end. This dynamic process, known as treadmilling, involves a balance between polymerization and depolymerization.

Actin Filaments (Microfilaments) functions

• Transport: Facilitate the transport of organelles, vesicles, and granules within the cell through processes like cytoplasmic streaming.

• Cell Structure: Contribute to maintaining cell shape and structural integrity.

• Muscle Contraction: Actin filaments interact with myosin filaments to generate forces that drive muscle contraction.

• Cell Migration: Rapid assembly and disassembly of actin filaments enable cell movement and migration.

intermediate filaments structure

• Intermediate filaments are more stable than microfilaments and microtubules, with a diameter of 8-10 nm.

• They are composed of a variety of proteins, forming polymers that provide mechanical support to cells.

intermediate filaments types

• Keratin: Found in epithelial cells, providing structural support.

• Vimentin: Present in mesenchymal cells, including fibroblasts and endothelial cells.

• Desmin: Located in muscle cells, contributing to the structural integrity of muscle fibers.

• GFAP (Glial Fibrillary Acidic Protein): Found in astrocytes, supporting the structure of glial cells in the nervous system.

• Neurofilaments: Present in neurons, helping to maintain the structural stability of nerve cells.

• Lamins: Found in the nuclear lamina, supporting the nuclear envelope and contributing to nuclear structure.

myosin filaments structure

• Myosin filaments are thick filaments with a diameter of approximately 15 nm.

• They consist of heavy and light chains:

  • Heavy Meromyosin: Includes the head and the proximal 60% of the tail.

  • Light Meromyosin: Comprises the distal 40% of the tail.

• Each myosin filament is composed of two heavy chains and two pairs of light chains.

myosin filaments function

Myosin interacts with actin filaments to produce contractile forces, playing a key role in muscle contraction and various types of cellular motility.