Question

What are the molecular mechanisms regulating actin filaments, microtubules and intermediate filaments?

Answer 1

Microtubules, actin filaments, and intermediate filaments are much more dynamic in cells than they are in the test tube. The cell regulates the length and stability of its cytoskeletal filaments, as well as their number and the geometry.

Microtubules Are Nucleated by a Protein Complex Containing γ-tubulin

While α- and β-tubulins are the regular building blocks of microtubules, another type of tubulin, called γ-tubulin, has a more specialized role. Present in much smaller amounts than α- and β-tubulin, this protein is involved in the nucleation of microtubule growth in organisms

Microtubules Emanate from the Centrosome in Animal Cells

In most animal cells, there is a single, well-defined MTOC called the centrosome, located near the nucleus. From this focal point, the cytoplasmic microtubules emanate in a star-like, “astral” conformation. Microtubules are nucleated at the centrosome at their minus ends, so the plus ends point outward and grow toward the cell periphery. A centrosome is composed of a fibrous centrosome matrix that contains more than fifty copies of γ-TuRC. Most of the proteins that form this matrix, remain to be discovered, and it is not yet known how they recruit and activate the γ-TuRC.

Actin Filaments Are Often Nucleated at the Plasma Membrane

In contrast to microtubule nucleation, which occurs primarily deep within the cytoplasm near the nucleus, actinfilament nucleation most frequently occurs at the plasma membrane. Consequently, the highest density of actin filaments in most cells is at the cell periphery. These actin filaments in the layer underlying the plasma membrane, called the cell cortex, determine the shape and movement of the cell surface. For example, depending on their attachments to one another and to the plasma membrane, actin structures can form many strikingly different types of cell surface projections. These include spiky bundles such as microvilli or filopodia, flat protrusive veils called lamellipodia that help move cells over solid substrates, and the phagocytic cups in macrophages.

Filament Elongation Is Modified by Proteins That Bind to the Free Subunits

Once cytoskeletal filaments have been nucleated, they generally elongate by the addition of soluble subunits. In most nonmuscle vertebrate cells, approximately 50% of the actin is in filaments and 50% is soluble.

Proteins That Bind Along the Sides of Filaments Can Either Stabilize or Destabilize Them

Once a cytoskeletal filament is formed by nucleation and elongated from the subunit pool, its stability and mechanical properties are often altered by a set of proteins that bind along the sides of the polymer. Different filament-associated proteins use their binding energy to either lower or raise the free energy of the polymer state, and they thereby either stabilize or destabilize the polymer, respectively.

Intermediate Filaments Are Cross-linked and Bundled Into Strong Arrays

Each individual intermediate filament forms as a long bundle of tetrameric subunits . Many intermediate filaments further bundle themselves by self-association; for example, the neurofilament proteins NF-M and NF-H contain a C-terminal domain that extends outward from the surface of the assembled intermediate filament and binds to a neighboring filament. Thus groups of neurofilaments form robust parallel arrays that are held together by multiple lateral contacts, giving strength and stability to the long cell processes of neurons.

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