Microtubules, Intermediate Filaments and Actin
Eukaryotic Cytoskeleton Structure and Function
Water molecules in cells move rapidly and collide with other molecules millions of times a second. Proteins are ten thousand times larger than water molecules and merely vibrate under the impact of collisions. Bacteria that are thousands of times smaller than cells with nuclei, eukaryotic cells, function with their proteins, nucleic acids and other molecules all diffusing within a single cytoplasmic compartment. The comparatively gigantic size of a eukaryotic cell requires molecules that must come in contact, e.g. enzymes and their metabolites, to be moved to the same compartments or regions within the cell. The cytoskeleton provides the tracks and scaffold for the structure and function of the cell.
Cytokeleton is Composed of Three Types of Protein Filaments:
- Microtubules are large, hollow tubes of proteins that radiate from the center of the cell.
- Intermediate filaments are multiple strand threads of proteins that provide the physical strength of cells.
- Actin microfilaments are single threads of proteins responsible for cellular changes of shape and muscular contractions.
Microtubules Provide Transport to and from the Cell Nucleus
Microtubules originate and radiate from a pair of centrioles that form the microtubule organizing center (MTOC). Protein subunits, microtubulin, rapidly assemble and elongate the microtubules that extend from the MTOC to cell surface, the cytoplasmic membrane. Microtubules are stable for as long as they extend, but if they stop, they immediately disassemble back to the MTOC.
Proteins serve as ATP-powered engines to move cargos of organelles, other proteins or messenger RNA to or from the MTOC. The largest organelle moved is the nucleus. Proteins attached to the inner membrane (and underlying intermediate filaments) of the nucleus move on microtubules toward the MTOC. Other proteins embedded in the outer nuclear membrane are attached to the microtubules with an opposite orientation and extend the nuclear membrane to form the endoplasmic reticulum (ER). Vesicles move between the ER and the Golgi Apparatus using the microtubules as tracks.
Intermediate Filaments Make Cells Tough
The membranes that surround the cell, nucleus and other organelles are relatively weak. Proteins embedded in these membranes would be pulled out of the membranes if a force was applied to them. To resist stretching, layers of cells, such as the epithelium that lines the intestines, are connected to each other by tough proteins that penetrate the cell membrane and attach to fibers of intermediate filaments. The intermediate filaments span the cell and connect to proteins penetrating the membrane and attached to the cell on the opposite side. In this way tough fibers of protein with the tensile strength of steel provide an intracellular mesh to help cell layers resist mechanical stretching.
Actin Microfilaments Can Extend and Contract
The cortical region of a cell, just below the cytoplasmic membrane is the location for the assembly of actin filaments. These filaments are extended under the cytoplasmic membrane and are attached to proteins that anchor the filament to the cytoplasmic membrane. Motor proteins, e.g. myosin, attach to adjacent filaments and force the filaments to slide past each other. In muscle cells, bundles of actin filaments are organized in opposite directions with myosin between to produce muscle contractions.
The Cytoskeleton Provides Cell Structure and Function
The interplay between the microtubules, intermediate filaments and actin microfilaments is fundamental to the organization of molecular activities, e.g. metabolism, secretion, endocytosis, within a cell.
Cytoskeleton Microtubules
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