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Components of Cells
The Macromolecules
Proteins: Self assembly

Combinations of Polypeptides

Protein molecules are capable of coming together and forming complex arrangements or structures that are composed of more than one polypeptide subunit. Quite often, these larger structures also contain extra, non-protein material such as carbohydrate, lipid or even polynucleotides.

These giant, "super-molecules", can take the form of multicomponent enzyme complexes, strong filaments that hold the contents of cells in place or are specialized for such roles as contraction (in muscles), ribosomes, membranes and hybrid structures such as viruses.

Hemoglobin, for example, is an important oxygen carrying protein that consists of four polypeptides (two alpha-chains and two beta-chains) and four non-protein, iron containing, ring-like structures where the oxygen molecules are bound and carried.

Such proteins are said to have quaternary structure, and these larger polypeptide arrangements can sometimes reach very large sizes. Probably the record quaternary structure protein is found in the bacterium Escherichia coli, where a cytoplasmic enzyme called pyruvate dehydrogenase is composed of 88 polypeptide subunits organized into three massive groups of catalytic enzymes.

Cells and organisms take advantage of the fact that it is both easier, and needs less genetic information, to assemble very large complexes from smaller, easier to make subunits. For example, only one gene coding for a single polypeptide can produce enough small subunits to assemble into a very large protein filament that can stretch all the way across the interior of a cell. This is a great saving in the amount of genetic information which must be stored.

When the cell needs a particular structure, it is easy to assemble one from the component subunits, and then dissemble those same subunits when the structure is no longer needed. This needs less energy than any other method.

If a protein subunit has a region on its surface that is complementary to a different region on the same subunit, then a collection of these subunits will spontaneously unit and form larger and larger structures, without any external direction or control.

Some protein subunits are capable of assembling into flat sheets of material, others into tubes, yet others into spheres, but the champions of self assembly must be those tiny particles of disease and destruction known as viruses.

Assembly of Viruses

Viruses are not "alive", and cannot perform all of the "signs of life" seen in even the most primitive bacterial cell. They are parasites that need either prokaryotic or eukaryotic cells in which to reproduce, and outside such cells are little more than very complicated complexes of two types of macromolecule; nucleic acids and proteins.

The polynucleotides the viruses carry contain all the coded information the virus needs to take over the host cell protein synthesizing machinery and produce more of its own protein component. Once synthesized, these virus proteins can self-assemble into new virus particles once more, and the life-cycle of the virus continues.

These simple virus particles, therefore, illustrate how some proteins carry within their three dimensional structure all the properties they need to direct macromolecular assembly of even larger structures.

Tobacco Mosaic Virus

The tobacco mosaic virus, as its name suggests, infects tobacco plants causing considerable distress to the invaded plant cells and to the productivity of the whole plant.

A virus particle, when viewed under the electron microscope, appears as a cylinder that is 16 nm wide and about 300 nm long. It has a mass of 40 x 106 daltons and contains a single strand of RNA. This is its genetic material, in which 6500 nucleotides hold the codes necessary for producing more virus particles.

In an intact particle, the viral RNA is wound into a helix that has a radius of 4 nm, and stretches all the way from one end of the cylinder to the other.

Associated with this polynucleotide helix are 2130 identical polypeptides, folded into 2130 identical globular protein shapes. These protein molecules bind and complex with three adjacent nucleotides along RNA molecule, and with other proteins on either side of them.

As the two components of the virus are synthesized in the infected cell, they come together, unite, and assemble into a new virus particle without any other assistance. In fact, when taken out of the cell, pure protein and RNA can be made to re-assemble and reform an active virus without any special assistance or other enzymes, etc.

It appears that the proteins (and the RNA) hold enough information in their three dimensional structures to direct the supra-assembly of a larger, more complex structure.

Bacteriophages

These are more complex virus particles that invade, take over and destroy bacteria during their active reproductive cycle.

One such bacteriophage attacks the bacterium Bacillus subtilis by injecting a double stranded DNA molecule (about 5.7 um long) through the bacterial cell wall and into the cytoplasm of the host.

Once inside the host cell, the DNA takes over and directs the synthesis of more bacteriophage proteins (and DNA) which then assemble into new intact virus particles for release and further infection.

These bacteriophage particles consist of 145 identical protein molecules which assemble into the "head" or DNA containing capsid, and six other proteins that have roles to play in the "tail" of the phage particle and help in the DNA injection process.

It is possible to gently disrupt the complete bacteriophage particles and examine how the proteins assemble into such large and complex structures. It appears that the proteins themselves, once again, contain all the properties they need for their own self assembly.


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© 2003, Professor John Blamire