Actin Filaments in Cell Division and Junctions

Actin Filaments

• Form the contractile ring that separates the two daughter cells at mitosis and meiosis
• Involved in cell-to-cell junctions, specifically in zonula occludens (tight junction) and zonula adherens junction
• Attach to occludin or claudin transmembrane proteins via ZO-1 proteins in tight junctions
• Attach to the cytoplasmic domain of transmembrane protein called cadherins in adherens junctions
• Involved in focal adhesions, which are dynamic attachments that attach cells to extracellular matrix proteins
• Necessary for cell motility/crawling, which involves three interrelated processes: actin polymerization, formation of protrusions, and depolymerization of actin filaments

Intermediate Filaments

• The toughest and most stable component of the cytoskeleton
• Do not undergo polymerization-depolymerization
• About 10 nm in diameter, intermediate in size between actin microfilaments and thicker myosin of smooth muscle cells
• Main function is to enable cells to withstand mechanical stress
• Form a network throughout the cytoplasm and extend out to the cell periphery
• Often anchored to the plasma membrane at cell-to-cell junctions
• Prominent in cells that are subject to mechanical stress, such as nerve cells, muscle cells, and epithelial cells

Microtubule Organizing Center (MTOC)

• A nucleating site responsible for the polymerization of tubulins
• Two types: Centrosome and Basal bodies

Centrosome

• The dominant form of MTOC, found in most cells
• Located on the side of the nucleus in non-dividing cells
• Composed of two short cylindrical structures called centrioles, which are at right angle to each other
• Surrounded by a cloud of proteins called Pericentriolar material
• Each centriole has nine triplets of microtubules

Microtubules

• Found in the cytoplasm of all eukaryotic cells
• Long hollow tubes, 25 nm in outer diameter
• Subunits are heterodimers of alpha (α) tubulin and beta (β) tubulin
• Function: Provide the “tracks” for intracellular transport, form cilia and flagella, and form the mitotic spindle
• Orientation of assembly gives a (+) plus and (-) minus ends
• Highly dynamic in length, with MTs growing and shrinking by the addition or loss of tubulin heterodimers at the + end
• The rate of microtubule growth is regulated by microtubule-associated proteins (MAPs)
• Involved in the transportation of vesicles and organelles within the cell, using microtubule-associated motor proteins such as kinesin and dynein
• Used by some viruses, such as herpes virus, to get from the plasma membrane to the nuclear envelope during infection

Clinical Correlation: Intermediate Filaments

• The type of cytoplasmic intermediate filaments produced by cells in a tumor can tell the tissue type of origin of the tumor
• Used by pathologists to identify the tissue of origin of a tumor cell that may have metastasized