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Components of Cells
The Macromolecules
Cytoskeleton Proteins

Combinations of Polypeptides

Small as it is, a human eukaryotic cell holds inside it about 1 billion protein molecules, which, when the water is removed, make up about 60% of its total mass.

There may be as many as 10,000 different proteins in a typical human body cell, all of which have to be organized, placed in the right locations and be regulated. This is no easy task.

The volume and shape of a eukaryotic cell can vary a lot, but if an average diameter of 10 um is assumed, that is a very large space in which a protein molecule (about 2000x smaller) can get lost.

Some proteins are organized into teams, or complexes, based on their role or function. Ribosomes, for example, cluster about 83 different proteins into highly structured aggregates for the purpose of joining together strings of amino acids.

Many proteins are bound to membranes, or encapsulated into organelles such as the mitochondrion or nucleus, and yet others are organized, moved and controlled by an internal highway called the cytoskeleton.

Fillaments and Tubes

All the major elements of the internal cytoskeleton are either filaments or tubes that are constructed by combining specialized, identical protein subunits into larger polymers, or chains.

Thousands of these protein subunit molecules are united end-to-end in longer and longer arrays that is some cases can stretch all the way across a cell, and back again. As well as providing structural support for the interior of the cell, these filaments of protein also serve as trackways along which the internal contents of the cell can be moved, or not moved, as the metabolism of the cell requires.

They do not function alone, however. There are many accessory or ancillary proteins that either link cytoskeleton elements together into larger complexes, join other cell components to the filaments, or simply control the whole process.

The Players

Actin filaments (also known as microfilaments) consist of subunits of actin protein joined together in a double or two-stranded twisted rope-like polymers. They are about 5-10 nm in diameter with great linear flexibility and form into a variety of either two- or three-dimensional networks.

While actin-based filaments can be found almost anywhere inside the cell, they are most often concentrated around the edges of the cell, just below the surface or plasma membrane. Here they stiffen the liquid contents of the cytoplasm into a cell cortex, and often give the plasma membrane of animals cells its characteristic shape(s).

  • thin, pointy extensions or microspikes,

  • sheets or lamellipodia,

  • tucks, or infoldings of the outer membrane such as might occur during the process of cell division.

The position of the actin filaments in the cortex, and their direction, is controlled by tiny "nucleation sites" in the plasma membrane itself. Different sites and different parts of the membrane organize the actin filaments in different ways, and the whole cortex is very sensitive to external signals arriving at the outer surface of the plasma membrane.

Microtubules are made of the protein tubulin, which are assembled into hollow tubes or cylinders about 25 nm in diameter.

These tubes of protein are very rigid, long, straight and usually have one end firmly anchored into a centrosome, or microtubule organizing center (MTOC).

Although each microtubule has two ends, they are not the same. One end (the plus end) is where the subunits of tubulin protein are added, and thus is capable of elongating the tubule. The other end (the minus end) is where the protein subunits are removed unless prevented by the centrosome.

Microtubules begin as short rods with their minus ends embedded in a centrosome, and their plus ends out in the surrounding cytoplasm. As more and more protein subunits are added and added to the plus end, the rod or tubule begins to extend and grows outwards, often ending up close to the plasma membrane.

But at any moment this process can be reversed, and the centrosome begins to removed subunits from the minus end of the tubule. This causes the whole microtubule to shorten, often rapidly and dramatically until it completely disappears.

As well as providing structural support for the interior of the cell, the microtubules also provide a highway along which the contents of the cell can travel. Motor proteins bind to either the microtubules or actin filaments and use the energy stored in ATP molecules to move organelles and other internal structures from one location to the next.

One of the first motor proteins to be discovered was myosin, which binds to, and moves along, the actin filaments (this protein is also a major component in contracting muscle cells).

Motor proteins have at least two important regions; a "motor" region which both binds to the microtubule and creates the movement as the ATP energy is used, and an "adaptor" region that binds the motor to the cargo it is carrying. Since there are many different possible types of cargo, there must be many different types of adaptor regions.

Intermediate filaments are a mixed bag of fibers about 10 nm in diameter made up of a range of different protein subunits and serving a variety of functions including providing the cell with mechanical strength.

In some cells these intermediate fiber-like filaments lay just beneath the surface of the nuclear membrane where they form the nuclear lamina. In others they span the whole cytoplasm, from side to side, apparently linking the junctions between one cell and another.

The study

Although the basic principles and components of the cytoskeleton have been identified and studied, and lots and lots of the accessory proteins have been isolated and their composition determined, it has not been easy to tease out how all of these components and properties come together to produce the dynamic entity that is the cytoskeleton.

It is not easy to study a complex that is constantly changing in both its protein makeup and structural diversity. While it is not difficult, for example, to isolate and study an accessory protein for its chemical composition, it is much, much harder to study the role of that protein plays in the constantly changing storm of activity that is taking place minute by minute and second by second in the cell's interior.

© 2003, Professor John Blamire