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TOPIC 8: THE CYTOSKELETON: INTERMEDIATE FILAMENTS & ACTIN FILAMENTS Cytoskeleton : a complex network of interconnected filaments and tubules that extends throughout the cytosol, from the nucleus to the inner surface of the plasma membrane This network: 1. Provides an architectural framework for eukaryotic cells. 2. Plays an important role in cell movement and cell division 3. Positions and actively moves membrane-bounded organelles within cytosol 4. Involved in cellular movement and other processes like cell signaling and cell- adhesion The Major Structural Elements of the Cytoskeleton Three major structural elements of the cytoskeleton: microtubules , microfilaments , and intermediate filaments Electron microscopes help with the recognition of specific proteins in the cytoskeleton, but they can be seen using a light microscope Prokaryotes, such as rod-shaped bacteria, have proteins that function very similar to microfilaments Each structural element is formed by the polymerization of a different kind of subunit Microtubules are composed of tubulin and are about 25 nm in diameter Microfilaments are polymers of the protein actin and are about 7nm in diameter Intermediate filaments have a diameter that ranges from 8-12 nm and differ depending on cell type Microtubules Microfilaments Intermediate Filaments Structure Hollow tube 13 protofilaments 2 intertwined chained of F-actin 8 protofilaments joined end-to- end with staggered over laps Diameter Outer – 25nm Inner – 15nm 7nm 8-12nm Monomers Alpha/beta tubulin G- Actin Over 50 types of proteins Polarity +/- end +/- end No polarity Function Cell motility Organization/maintenance of cell shape Chromosome movement Movement of organelles Muscle contraction Cell locomotion/cell crawling Cell division Cell shape Cytoplasmic streaming Cytokinesis Structural support Formation of nuclear lamina Strengthening of axons Desmin – muscle fibers
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Intermediate Filaments Intermediate in size between microtubules and microfilaments More predominant in animal cells where they play a structural or tension-bearing role Most stable and least soluble constituents of the cytoskeleton Support the entire cytoskeletal framework In contrast to other structural elements, IFs do not appear to be polarized Intermediate Filament Proteins Are Tissue-Specific Only found in multicellular organisms In contrast to other structural elements, IFs differ remarkably in amino acid composition from tissue to tissue Keratins are proteins that are found in epithelial cells that cover the body surfaces and line its cavities Desmin is found in muscle cells and used for structural support for muscular contractions Nuclear lamins form a filamentous scaffold along the inner surface of the nuclear membrane of virtually all eukaryotic cells which is necessary in order for mitosis to occur phosphorylation causes them to disassemble --- Mitosis dephosphorylation causes them to assemble
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