Cytoskeleton

The cytoskeleton ( ancient Greek κύτος kytos - "cell" ) (also cytoskeleton or cytoskeleton ) is a structured network of proteins in the cytoplasm of eukaryotic cells. It consists of dynamically assembled and disassembled, thin, thread-like cell structures ( filaments). It is responsible for the mechanical stabilization of the cell and its exterior shape for active movement of the cell as a whole, as well as movements and transport within the cell.

In fluorescence microscopic images, the cytoskeleton may actually look like a skeleton, the name is misleading because it is not the cytoskeleton to a rigid skeleton or framework, but to a dynamic network of structures. We now know also that Zytoskelettelemente are not only essential for the mechanical stability, but also for sensory functions, such as the transmission of signals between cells.

The word cytoskeleton in 1931 marked by the embryologist Paul Wintrebert, who speculated about how it could succeed oocytes to maintain their internal organization, if they have to walk through the narrow reductive female tract. From the strong deformation forces acting on the egg passes Wintrebert from the necessity of the existence of a cytoskeleton ( " cytosquelette ").

The eukaryotic cytoskeleton

In the eukaryotic cell, there are three classes of cytoskeletal filaments, which are each formed of different proteins or protein classes with specific accompanying proteins and participate in different ways in each of the tasks of the cytoskeleton:

• Actin filaments,
• Intermediate filaments,
• Microtubules.

All three classes are involved in the mechanical stability of the cell. Surface differentiations are supported by actin filaments and microtubules. Also, all forms of active exercise done along these two types of filaments, as they have specific motor proteins.

In general, the structures of the cytoskeleton with information specific to the task and the type of protein secondary proteins (including adapter proteins and motor proteins ) are associated, which stabilize the filaments, move them or connect them with other structures ( see, eg, profilin ).

Microtubules

The most striking elements of the cytoskeleton are microtubules, hollow cylinders with a diameter of 25 nm, which are composed of the protein tubulin. Intracellularly, they are with their motor proteins dynein and kinesin responsible for longer transport operations and the movements or fastening of the organelles in the cytosol. In the case of the mitotic spindle, the replicated chromosomes are pulled to the two core poles. Microtubules contribute only little to the mechanical stabilization, but they represent the characteristic internal structure of the moving cilia.

The assembly and disassembly of microtubules can be performed very dynamic and is based on animal cells from the centrosome.

Actin filaments

Actin ( microfilaments ) are 7 nm in diameter with fibers composed of actin. Especially in grid- like arrangements under the plasma membrane and in Membranausbuchtungen ( microvilli, pseudopodia ) to stabilize the outer shape of the cell, hold membrane-bound proteins in place and pull in a certain Zelljunktionen ( adherens junction). They can also dynamically to be assembled and disassembled.

The motor proteins actin form the protein class of myosins. ( While the long- distance transport of microtubule / dynein and kinesin taken ) on the actin -myosin interaction is based not only the movement of the muscles, but myosins brace the actin filaments to stabilize and ensure the short-distance transport, for example, of vesicles to the plasma membrane.

Intermediate filaments

The term intermediate filaments one summarizes a number of protein filaments, which they all have very similar properties. Its diameter is about 10 nm ( 8-11 nm), and they can, since they are significantly more stable than microtubules and actin filaments, best absorb mechanical tensile forces. For this reason, they are mainly used for mechanical stabilization of cells. They form the supporting structure and radiate in certain cell compounds ( desmosomes, hemidesmosomes ).

The prokaryotic cytoskeleton

Today we know that prokaryotic cells have proteins that are considered to be homologous or analogous to the proteins of all three eukaryotic protein classes. Although they are evolutionarily so far apart that a comparison of the amino acid sequence alone does not reveal any relationship. However, the prokaryotic proteins form structures whose similarity in structure and function is with the eucaryotic a clear sign of the relationship.

As a tubulin homologue FtsZ was found when actin homolog FTSA. These proteins are involved in cell division processes in particular. FtsZ protein is encoded by a small gene family in plants in the nucleus. Your proteins are mostly imported into the chloroplast, where they participate in the sharing of these cell organelles. Experiments on the moss Physcomitrella patens is known that the FtsZ proteins can also form a complex network in chloroplasts. Since this network is strongly reminiscent of the cytoskeleton, Reski 2000 coined the term " Plastoskelett " for this structure and postulated that it fills similarly complex functions in the plastids such as the cytoskeleton for the entire cell. In addition, in the bacterium Caulobacter crescentus the Crescentin was found which is similar to the intermediate filaments in its function.