The cytoskeleton, mitochondrial bioenergetics and apoptosis Professor Daniel C. Hoessli March 2013.

The cytoskeleton, mitochondrial bioenergetics and apoptosis

Professor Daniel C. HoessliMarch 2013

The red cell cytoskeleton

The red cells (erythrocytes) contain a cytoskeleton which is integrated in the plasma membrane, conferring to the red cell its biconcave shape. The red cell is extremely resilient and made to resist the very strong shear forces of the blood circulation.

Figure 11-30 Essential Cell Biology (© Garland Science 2010)

The major protein of the red cell cytoskeleton is spectrin, which is tethered totwo membrane proteins:1) Glycophorin, its main attachment point, where spectrin ends contact the cytoskeletal proteins actin and tropomyosin2) Band 3, where the red cell specific protein ankyrin anchors band 3 to the side of spectrin filaments

The cytoskeleton of nucleated cells

The cytoplasm is structured and organized by 3 different polymeric filamentous structures, the cytoskeleton.

All cells contain actin filaments, intermediate filaments and microtubules.

Movement of organelles and intracellular macromolecules occurs along microtubules, and is powered by motor proteins (kinesins and dyneins).

Cytoskeleton of nucleated cells

This is the typical arrangement in nucleated cells, showing the actin cytoskeleton in red and the mi-crotubules in green.

The two types of filaments coincide in membraneprotrusions (yellow).

The nucleus appears in blue and microtubulesradiate from the nucleus, while the actin filamentsare peripherally disposed, without connectionswith the nucleus.

The dynamic and the structural components of the cytoskeleton

The microtubules (MT) are highly dynamic. The MT networks are constantly modified: MT are shortened or elongated. They organize the cyoplasmic space.

The actin filaments (or microfilaments, MF) are highly dynamic as well and power the cellular movements.

The intermediate filaments (IF) are purely structural and provide epithelial sheets with resilient properties. In epithelial cells, IF consist of keratin and connect the cells in the sheet. In mesenchymal cells (i.e. lymphocytes), IF consist of another protein, vimentin, which is devoid of connecting properties

Figure 17-18a Essential Cell Biology (© Garland Science 2010)

Figure 17-18b, c Essential Cell Biology (© Garland Science 2010)


Microtubules originatefrom the centrosome

Figure 17-8b Essential Cell Biology (© Garland Science 2010)

Figure 17-8c Essential Cell Biology (© Garland Science 2010)

Actin filaments make thebackbone of cilia and Other membrane protrusions

Figure 17-10a, b Essential Cell Biology (© Garland Science 2010)

How to move along microtubules ?

A variety of motor proteins can move along MTs.The two main categories of moto proteins are:

The kinesins, andThe dyneins

Different dyneins and different kinesins have distinct tails that accomodate different „cargoes“.

Dyneins and kinesins move in opposite directions on the polarized microtubule


Plectin (green) in a connecting protein that joins MT (actin filaments have been removed)

Epithelial cells utilize IF and MF as structural components

IF (keratin) constitute large bundles of filaments that run through the entire cell and connect the cell with the next one at the desmosomes and

hemidesmosomes.

MF in form of filamentous actin belts, also run through the cell from one adherens junction to the other junction. These belts allow anchoring of other MF bundles that run at 90° into microvilli.

Summary

The cytoskeletal organization of a cell differs between strongly associated cells (epithelial, nerve cells) and individual ones (mesenchymal). The structural role of tubules and filaments is essential in epithelial cells (intercellular adhesion) and the functional cytoskeleton supports mesenchymal cells' various functions (movement and phagocytosis).

In all cells, the MT network is needed for internal organization, cell division and movement of organelles and smaller objects.

The cytoskeleton, mitochondrial bioenergetics and apoptosis Professor Daniel C. Hoessli March 2013.

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