Posted by: Dan | July 11, 2007

Basic Concepts: The Cytoskeleton

Despite the irregularity of some cell types, all varieties of the eukaryotic cell have defined shapes and movements that are visible under the microscope. In each case, a cell’s shape and its functional polarity are provided by a three-dimensional filamentous protein network called the Cytoskeleton. The cytoskeleton extends throughout the cell and is attached to the plasma membrane and internal organelles, providing a framework for organization and structure in the cell. Surprisingly, the cytoskeleton is not a static structure at all, but is highly dynamic, constantly changing over the length of the cell (microns) and over subcellular lengths (10’s or 100’s of nanometers).


(Actin filaments are shown in red, microtubules in green, and the nuclei are in blue.)

The cytoskeleton is composed of three major filament types, each of which has a distinct set of organizations and functions. Each filament system is composed of a polymer of assembled subunits, which undergo regulated assembly and disassembly, giving the cell the flexibility to construct or remove specialized structures as needed. The three filament types, actin/microfilaments, microtubules, and intermediate filaments, are described below.


Image:
Metzler, David E.: Biochemistry
Harcourt/Academic Press, 2001
ISBN/ISSN 012492543X

Actin filaments / Microfilaments

Around 7 nm in diameter, this filament is composed of two actin chains oriented in a circular shape. They are mostly concentrated just beneath the cell membrane, as they keep cellular shape, form cytoplasmatic protuberances (like pseudopodia and microvilli), and participate in some cell-to-cell or cell-to-matrix junctions and in the transduction of signals. They are also important for cytokinesis and, along with myosin, muscular contraction. Actin/Myosin interactions also help reduce cytoplasmic streaming in most cells.

Intermediate filaments

These filaments, 8 to 11 nanometers in diameter, are more stable (strongly bound) than actin filaments, and heterogeneous constituents of the cytoskeleton. They organize the internal tridimensional structure of the cell (they are structural components of the nuclear envelope or the sarcomeres for example). They also participate in some cell-cell and cell-matrix junctions.

Different intermediate filaments are:

  • made of vimentins, being the common structural support of many cells.
  • made of keratin, found in skin cells, hair and nails.
  • neurofilaments of neural cells.
  • made of lamin, giving structural support to the nuclear envelope.

Microtubules

They are hollow cylinders of about 25 nm in diameter, most commonly comprised of 13 protofilaments which, in turn, are polymers of alpha and beta tubulin. They have a very dynamic behaviour, binding GTP for polymerization. They are organized by the centrosome.

They play key roles in:

  • intracellular transport (associated with dyneins and kinesins they transport organelles like mitochondria or vesicles).
  • the axoneme of cilia and flagella.
  • the mitotic spindle.
  • synthesis of the cell wall in plants.

For more reference:


Responses

  1. Ah, the cytoskeleton, I sure miss that field …

    How about the “lesser” cytoskeletal elements? Septins, spectrin. And the bacterial cytoskeleton (MreB, FtzZ) although these are mostly homologs of eukaryotic cytoskeletal genes.

  2. Hopefully I wasn’t keeping it too basic! I’ll have to get to those other cytoskeleton-related issues sooner or later, to be sure. I’m shifting more towards MT-associated proteins and centrosome duplication though, and was thinking of using this as a setup for a post on that topic sometime in the next few days…

  3. […] 13th, 2007 by Dan Following my review of the major eukaryotic cytoskeleton components, and microtubules in […]

  4. […] Alex had asked for a mention of the bacterial cytoskeleton, which does deserve mention. As Shih and […]

  5. […] 8th, 2007 by Dan The cell’s skeleton, or cytoskeleton, is involved in just about every major activity that cells perform, and the actin cytoskeleton in […]

  6. Hi all I know this discussion date back from 2007 but can someone tell me if the pics of the actin filament, Mts and intermdiate filaments are from electron microscopy or from Xray microscopy ?

  7. Hi Bastien,
    Those images are electron microscopy. I’m not sure whether it’s SEM, but that is more commonly used than TEM.


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