Which organelles form like the cytoskeleton



The Cytoskeleton (Greek kytoscell) (also Cytoskeleton written) is a network made up of proteins in the cytoplasm of every cell and consists of thin, thread-like cell structures (filaments) that can be dynamically built up and broken down. It is responsible for the mechanical stabilization of the cell and its external shape, for active movements of the cell as a whole, as well as for movements and transports within the cell.

The name "cell skeleton" certainly comes from the appearance of these structures in the fluorescence microscope, but it is misleading because the cytoskeleton is not a rigid skeleton or framework, but an extremely flexible network of structures. This applies to all three types of cytoskeletal components. It is now also known that cytoskeletal elements are essential not only for mechanical stability, but also for sensory functions such as signal transmission between cells.

The eukaryotic cytoskeleton

 

The word cytoskeleton was coined in 1931 by the embryologist Paul Wintrebert, who speculated on how egg cells might manage to maintain their internal organization when they have to wander through the narrow female reductive tract. From the strong deformation forces acting on the egg cell, Wintrebert derives the necessity of the existence of a cytoskeleton ("cytosquelette") [1].

In the eukaryotic cell, a distinction is made between three classes of cytoskeletal filaments, which are each formed by different proteins or protein classes, have specific accompanying proteins and each participate in the tasks of the cytoskeleton in different ways:

All three classes are involved in the mechanical stabilization of the cell. Surface differentiations are supported by actin filaments and microtubules. All forms of active movement also take place along these two filament types, as they have specific motor proteins.

In general, the structures of the cytoskeleton are specific to the task and type of protein Accompanying proteins (i.a. Adapter proteins and Motor proteins) that stabilize the filaments, move on them or connect them to other structures (see e.g. Profilin).

Microtubules

The most noticeable components of the cytoskeleton are the microtubules, hollow cylinders with a diameter of 25 nm, which are made up of the protein tubulin. Intracellularly, with their motor proteins dynein and kinesin, they are responsible for longer transport processes and the movements and attachment of the organelles in the cytosol. In the case of the mitotic spindle, the replicated chromosomes are drawn to the two nuclear poles. Microtubules only play a minor role in mechanical stabilization, but they represent the characteristic internal framework of the mobile kinocilia.

The assembly and dismantling of the microtubules can be carried out very dynamically and starts from the centrosome.

Actin filaments

Actin filaments (also called microfilaments) are fibers with a diameter of 7 nm that are made of actin. Especially in net-like arrangements below the plasma membrane and in membrane bulges (microvilli, pseudopodia), they stabilize the outer shape of the cell, hold membrane-bound proteins in place and move into certain cell functions (adherence contact). They can also be built up and taken down dynamically.

The motor proteins of actin form the protein class of myosins. The actin-myosin interaction is not only based on the movement of the muscles, but myosins also tighten the actin filaments for stabilization and ensure short-distance transport, for example from vesicles to the plasma membrane (while long-distance transport is taken over by microtubules / dynein and kinesin).

Intermediate filaments

The term intermediate filaments covers a number of protein filaments, all of which have very similar properties. Their diameter is around 10 nm (8 to 11 nm), and because they are significantly more stable than microtubules and actin filaments, they can best absorb mechanical tensile forces. For this reason, they mainly serve to mechanically stabilize the cells. They form their supporting structure and radiate into certain cell connections (desmosomes, hemidesmosomes).

There are no intermediate filaments in arthropods and plants.

credentials

Wintrebert P, La rotation immediate de l-Oeuf pondu et al rotation d'activation chez Discoglassus picus, Orth Compres Rend Soc Biol 106, 439-442

The prokaryotic cytoskeleton

Today we know that prokaryotic cells also have proteins that are regarded as homologous or analogous to the proteins of all three eukaryotic protein classes. They are so far apart from one another in evolution that a comparison of the amino acid sequence alone does not reveal any relationship. However, the prokaryotic proteins form structures whose similarity to the eukaryotic proteins in structure and function is a clear sign of the relationship.

FtsZ was found as the tubulin homologue and FtsA as the actin homologue. These proteins are particularly involved in cell division processes. In addition, crescentin was found in the bacterium Caulobacter crescentus, which is similar in function to the intermediate filaments.

See also

Adducin

Category: Structural Protein