The protein from which microtubules
are made. Tubulin is a heterodimer protein,
meaning it is comprised of a pair of polypeptide
chains, called monomers, that differ in the sequence of their amino
acids. When tubulin polymerizes it does so into long chains or filaments
that form microtubules – hollow fibers which serve as a skeletal system
for living cells. The ability of microtubules to shift through various formations
is what enables a cell to undergo mitosis
or to regulate intracellular transport. The formation-shifting of microtubules
is made possible by the flexibility of tubulin, which is why scientists
have sought to understand the protein's molecular structure since its discovery
in the 1950s.
|A model of tubulin developed by Berkeley Lab scientists,
showing the protein to be a dimer consisting of two monomers that
are almost identical in structure. Each monomer is formed by a core
of two beta sheets (blue and green) surrounded by helices, and each
binds to a guanine nucleotide (pink). In addition to a nucleotide
binding site, each monomer also has two other binding sites, one for
proteins, and one for another substance
Interest in tubulin structure intensified in the 1990s when taxol, a natural
substance found in the bark of the Pacific
yew tree (the name "taxol" has been trademarked by Bristol-Myers-Squibb),
was shown in clinical tests to be an effective treatment for a number of
cancers including ovarian, breast, and lung. By binding to tubulin and causing
the protein to lose its flexibility, taxol prevents a cell from dividing.
With better knowledge of tubulin structure and its interaction with taxol,
scientists believe that an even more effective anti-cancer drug, one that
interacts only with the tubulin of cancerous cells, may be synthesized.
The detailed structure of the taxon dimer was announced by a team of researchers
at the Lawrence Berkeley Laboratory in 1998 (see accompanying photo).