Protein structure: Tubulin

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The function of proteins depends on the structure of the proteins. There are four basic levels of protein structure that you need to be familiar with. Primary structure, secondary structure, tertiary structure and quaternary structure. I illustrate them here using a common protein in your cells, tubulin. I decided to use this particular protein to illustrate another basic concept namely that structures inside of cells are made of supermolecular assemblies. Tubulin, for instance is a major component of a super molecular assembly called a microtubule.

Primary structure: Part of the amino acid sequence of the tubulin molecule. The amino acid sequence of a polypeptide represents the polypeptide's primary structure:

MET ARG GLU CYS ILE SER ILE HIS VAL GLY GLN ALA GLY VAL GLN ILE GLY ASN ALA CYS TRP GLU LEU TYR CYS LEU GLU HIS GLY ILE GLN PRO ASP GLY GLN MET PRO SER LYS THR ILE GLY GLY GLY ASP ASP SER PHE ASN THR PHE PHE SER GLU THR GLY ALA GLY LYS HIS VAL PRO ARG ALA VAL PHE VAL ASP LEU GLU PRO THR VAL ILE ASP GLU VAL ARG THR GLY THR TYR ARG GLN

Each three letter abbreviation represents a particular amino acid. For example VAL stands for an amino acid called valine. A tubulin molecule has 862 amino acids.

Secondary structure. This picture shows how neighboring amino acids twist in relation to each other to form a variety of three dimensional structures including sheets and helices.

The pink and yellow ribbons and the grey strings represent different types of structures formed by interactions between neighboring amino acids. Plus this is a really cool picture.

Tertiary structure.

The secondary structure will interact with water and with distant amino acids to cause a complex folding of the protein. This is called tertiary structure.Some of this is seen in the illustration of secondary folding. This diagram shows the tubulin molecule from a distance away. Notice how the helices and other structures of secondary folding have in turn folded in complex ways. The colors represent diffent large scale groupings within the protein. Notice that it looks like there are two molecules side by side. Indeed, you're seeing an indication of tubulin's quaternary structure.

Quaternary structure. Quaternary structure happens when two or more protein subunits join together to make a larger protein. In tubulin, two almost identical subunits are joined together to make what's called a dimer. In the case of some proteins there may be three four or more subunits making up the larger protein. For instance nornmal hemoglobin in the human red blood cell has four protein subunits. For tubulin there's more.....

Supermolecular assemblies.

Tubulin is an important component of microtubules. A microtubule is a hollow tube the wall of which is made largely from a spiral assembly of tubulin dimers. Microtubules may contain hundreds of thousands of microtubules. The diagram shows one way tubulin dimers can assemble to make a microtubule. Each "ring" of the microtubule consists of 13 tubulin dimers.
Illustration modified from M. Maaloum, D. ChrÈtien, E. Karsenti and J. K. H. H?rbe(1994) Approaching microtubule structure with the scanning tunneling microscope (STM), Journal of Cell Science 107, 3127-3131

In the cell microtubules serve as "conveyor belts" to move organelles around the cell. They are also important in mitosis as part of the spindle fiber apparatus involved in moving the chromosomes around. The arrow in the accompanying micrograph shows a microtubule conveyer which is carrying some chloroplasts in Elodea.

Finally, microtubules are important components of flagella and cilia both in single celled animal like protists, and in our lungs where the cilia on some of the cells on the lining of our lungs are important in cleaning our lungs.

So, we see that if we really want to get picky even though when we talk about the organization of the living world we tend to go from molecules to organelles as the next level up, really there is yet another level of organiszation namely the supermolecular assembly. Further when we talk about proteins as being components of organelles we must also consider the four levels of protein structure.

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