cytoskeleton (Microtubules, microfilaments intermediate filaments)

Microtubules,
microfilaments
intermediate filaments

1. Cytoskeleton of the cell Asma Saleh Alilesh

2. Cytoskeleton The cytoskeleton is composed of three well-defined filamentous structures: M icrotubules, microfilaments intermediate filaments Each of the three types of cytoskeletal filaments is a polymer of protein subunits held together by weak , noncovalent bonds . They together form an elaborate interactive network.

3. Cytoskeletal Functions 1- structural support that can determine the shape of the cell and resist forces that tend to deform it . 2- responsible for positioning the various organelles within the interior of the cell. 3- direct the movement of materials and organelles within cells. 4- The force-generating apparatus that moves cells from one place to another 5- An essential component of the cell’s division machinery. Cytoskeletal elements make up the apparatus responsible for separating the chromosomes during mitosis and meiosis and for splitting the parent cell into two daughter cells during cytokinesis

4. Microtubules Hollow , relatively rigid , tubular structures. They found in nearly every eukaryotic cell . Microtubules have an outer diameter of 25 nm and a wall thickness of approximately 4 nm. The wall of a microtubule is composed of globular proteins (tubulin) arranged in rows , termed protofilaments , that are aligned parallel to the long axis of the tubule. Each protofilament is assembled from dimeric building blocks consisting of one α -tubulin and one β -tubulin subunit. Tubulin is a GTPase ,

5. Has polarity.One end of a microtubule is known as the plus end and is terminated by a row of β -tubulin subunits The opposite end is the minus end and is terminated by a row of α -tubulin subunits Microtubules function as structural supports and organizers: (structure of cilia and flagella and the axons of nerve cells.) Microtubules as agents of intracellular motility.

6. Microfilaments Flexible helical filament composed of globular subunits of the protein actin In the presence of ATP, actin monomers polymerize to form . They also called actin filament , F-actin . 8 nm diameters Actin filaments have polarity, which is denoted as a plus and minus end. Actin is an ATPase. They found in nearly every eukaryotic cell.  functioning in  cytokinesis ,  amoeboid  movement, and changes in cell shape and involved in intracellular motile processes .

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8. Intermediate filaments ( IFs ) Intermediate filaments are strong, flexible ropelike filaments. Have only been identified in animal cells . 10 nm diameter provide mechanical strength to cells that are subjected to physical stress, including neurons, muscle cells, and the epithelial cells that line the body’s cavities. IFs are a chemically heterogeneous polypeptide (Keratin and Keratin- like proteins), in humans, are encoded by approximately 70 different genes . Have no enzymatic activity or polarity

9. Motor proteins The motor proteins of a cell are proteins convert chemical energy ( stored in ATP) into mechanical energy, which is used to generate force to move cellular cargo attached to the motor. Types of cellular cargo transported by these proteins include vesicles , mitochondria, lysosomes, chromosomes. A single cell may contain a hundred different motor proteins, specialized for a different activity. motor proteins can be grouped into three broad superfamilies : kinesins , dyneins , and myosins . Kinesins and dyneins move along microtubules. Myosins move along microfilaments. No motor protein is known that uses intermediate filament. Why???

10. Kinesins : Kinesin is a tetramer constructed from two identical heavy chains and two identical light chains. Heads that bind a microtubule and act as ATP-hydrolyzing unit (engines). Move along microtubules toward their plus end. Kinesin is therefore said to be a plus end-directed microtubular motor. kinesins tend to move vesicles and organelles (e.g., peroxisomes and mitochondria) in an outward direction toward the cell’s plasma membrane .

11. Dynein: Cytoplasmic dynein is a huge protein (molecular mass of approximately 1.5 million daltons ) composed of two identical heavy chains and a variety of intermediate and light chains. Dynein moves along a microtubule toward the minus end—opposite that of kinesin . - Function as: 1. As a force-generating agent in positioning the spindle and moving chromosomes during mitosis 2 . As a minus end–directed microtubular motor with a role in positioning the centrosome and Golgi complex and moving organelles, vesicles, and particles through the cytoplasm .