Microtubules of a huge amount of research in the

Microtubules are hollow tubular structures that are present
in almost all eukaryotic cells. Due to the fact that they play a role in a huge
number of cellular processes they are a fascinating organelle to study, and
have been the subject of a huge amount of research in the past number of years.

The first discovery of microtubules came over sixty
years ago in 1954 by Fawcett and Porter’s use of transmission electron
microscopy.i  As at that time specimens and samples could
not be preserved well the research was hindered and a huge number of questions
triggered: “Which cells contain these microtubules? What are the components of
these structures? How do these microtubules perform work in the cell? How are
they involved in cell division and movement? How do they assemble?” These
questions piqued the interest of many and the search for answers began.

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By 1963, Sabatini et al, having used more successful
structural preservation techniques, had seen a universal presence of
microtubules in almost all eukaryotic cells.ii This
triggered even further interest in these cellular components and their working
mechanisms and structure.

The discovery of tubulin didn’t occur until years
later in 1967, through the pioneering work by E. W. Taylor.iii This
discovery solved the mystery of the microtubule components.

The mechanism of the self-assembly of these structures
was of huge interest, they are known as nature’s best example of a dynamic
“smart” material. Microtubules show a spontaneous assembly from different parts
which a man-made machine would never be able to do.

Due to the numerous functions and attractive
mechanisms of these microtubules, the scientific world was inspired to recreate
these structures from different materials. A further question was asked; “what
can we learn from these extraordinary structures that can be utilised in
materials science and bio nanotechnology?” This biomimicry has been tested
using a huge number of different materials. Mentioning them all would be an
immeasurable task, so I focus on tubular structures that show self-assembly.
These include some types of carbon nanotubes, organic tubules, DNA nanotubes, “annular-ring”
microtubes and microtubules that use colloidal building blocks.

In general, microtubes have an inner diameter in the
micron region and nanotubes have a diameter in the order of nanometers.
However, for cellular based microtubules the diameters are within the order of
nanometers. Typically they have an inner diameter of 12nm and an outer diameter
of 24nm. Although the naming of these structures can be slightly confusing, the
use of “micro” is merely to show the very small scale of these structures.

I will focus on the growth mechanisms, structure,
properties and applications of these biomimetic materials. Also, how they
compare to the original cellular based microtubule.

Microtubules

The
cell’s cytoskeleton main functions are to maintain the shape and organisation
of the cell’s components; to give a cell mechanical support; and to enable
specific functions, for example transport, movement and cell division. The cytoskeleton’s
components can be classified into three different types of filament-like proteins:
microtubules, intermediate filaments and actin filaments. Microtubules (MTs),
which are of interest to us, are the largest of the three filaments with a
diameter of around 24nm. Intermediate filaments have a diameter of around 10nm
and actin filaments are the smallest with diameters of approximately 6nm. The
scale of these filaments in relation to each other is shown in Figure 1.
Microtubules consist of tubulin dimers and their structure will be discussed in
detail below. Intermediate filaments are made up of proteins that vary from
cell to cell. They have a high tensile strength that can be utilised in the
strengthening of microtubules in the cell.iv Actin
filaments (also known as microfilaments) consist of free monomers of the
globular protein actin organised into long helical chains. These microfilaments
allow the cell to take on many different structures and are involved in
movement and cell division. Microtubules presence has been reported in almost
all eukaryotic cells, with their absence only observed in Nanochlorumv, a
species of algae.

Figure 1: Three filamentous structures present in cells; (a) actin filaments,
(b)microtubules and (c)intermediate filaments. (Open University Course- A Tour
of the Cell)

Microtubules
are made up of tubulin monomers (? and ?) which dimerise to form heterodimers
which then polymerise to form protofilaments. These protofilaments assemble
into sheets which curve to form the hollow tubular structure characteristic of
these filaments.

The
smallest building block of MTs are  tubulin monomers. Two almost identical structures
of the tubulin monomer exists, ?- and ?-tubulin, with each being composed of a
core of 2? sheets surrounded by ? helices.

In
the first step of MT growth an ?-monomer and a ?-monomer dimerise together spontaneously
to form a heterodimer, a protein comprising of two different macromolecules.
These heterodimers then polymerise end-to-end in a polar fashion to form a protofilament
with a linear structure – each 5nm in diameter. The tubulin dimer and the size
of the protofilament can be seen in Figure 1.  Protofilaments assemble parallel to each other
and as lateral interactions occur a sheet is formed. This sheet has
characteristic inward curvature which leads to the formation of a tubular
structure. This structure indicates that a single microtubules has been formed.
The length of the MT is extended by the addition of more heterodimers to the
positive end of the protofilament. The full schematic mechanism for the
formation of microtubules is shown in Figure 2.

Generally
the structure formed is a left-handed three-start helical structure. The three-start
nomenclature refers to the three tubulin monomers at the bottom of the helix
that are not interacting with a neighbouring protofilament. The left-handed
helix, shown by the anticlockwise twisting of the structure, is a requirement
for the self-assembly of microtubules.vi Microtubules
generally consist of 13 protofilaments in
vivo, however the actual amount can vary from 12 protofilaments,vii
in nerve cells of crayfish, to 15 protofilaments, in cockroach epidermal cellsviii.
The hollow centre of the MT structure has an approximate diameter of 12-14 nm.

i Fawcett, D. W., and Porter, K. R. (1954) A study of the fine
structure of ciliated epithelia, J. Morphol. 94, 221–28

ii Sabatini, D. D., Bensch, K., and Barrnett, R. J. (1963)
Cytochemistry and electron microscopy: The preservation of cellular ultrastructure
and enzymatic activity by aldehyde fixation, J. Cell Biol. 17, 19–58.

iii Borisy, G. G., and Taylor, E. W. (1967a) The mechanism of action of
colchicine: Binding of colchicine-3H to cellular protein, J. Cell Biol. 34,
525–533.

Borisy, G. G., and Taylor, E. W.
(1967b) The mechanism of action of colchicine: Colchicine binding to sea urchin
eggs and the mitotic apparatus. J. Cell Biol. 34, 535–548.

Shelanski, M. L., and Taylor, E. W.
(1967) Isolation of a protein subunit from microtubules, J. Cell Biol. 34,
549–554.

Weisenberg, R. C., Borisy, G. G., and
Taylor, E. W. (1968) The colchicine binding protein of mammalian brain and its
relation to microtubules, Biochemistry 7, 4466–4467

ivO’Connor, C. M. & Adams, J. U. Essentials of Cell Biology.
Cambridge, MA: NPG Education, 2010

v Zahn, R K. 1984. A green alga with minimal eukaryotic features:
Nanochlorum eukaryoticum. Origins Life 13:289-303

vi Hunyadi, V., Chrétien, D., Flyvbjerg, H. and Jánosi, I. M. (2007),
Why is the microtubule lattice helical?. Biology of the Cell, 99: 117–128.

vii Hinkley, R. E., and P. R Burton. 1976. Tannic acid staining axonal
microtubules. J. Cell Biol. 63:139a.

viii Nagaro, H. C., and F. Suzuki. 1975. Microtubules with 15 subunits
in cockroach epidermal cells. J. Cell Biol. 64:242-245.