Biology Reference
In-Depth Information
FIGURE 5.10
The assembly of tubulin into hollow cylindrical associations of protofilaments: microtubules.
In vivo
, microtubules grow by addition of tubulin dimers to their ends so that their constituent protofilaments
elongate, not by coming together of protofilaments.
and tubulin will be described as 'GTP-tubulin' or 'GDP-tubulin' according to the status of
the
b
-subunit only.
Microtubules form when GTP-tubulin molecules come together by self-assembly. Linear
protofilaments formed in this way associate into hollow cylinders of, usually, 13 protofilaments
with a diameter of about 25 mn. This cylinder is called the 'microtubule' (
Figure 5.10
). The
arrangement of tubulin protofilaments as a hollow tube produces a structure of great stiffness.
)
The flexural rigidity of a microtubule has been measured as about 2
10
23
Nm
2
compared
10
26
Nm
2
for actinmicrofilaments;
43
microtubules are therefore about 300 times
more rigid. Thismakes themsuitable for bearing compressive loadswithout buckling (buckling
is never a problem for tensile loads, of course). The Young's moduli (resistances to stretching) of
microtubules andmicrofilaments are 1.2 GPa and 2.6 GPa, respectively,
43
emphasizing that it is
the arrangement of tubulin as a broad cylinder, and not inherent properties of the protofila-
ments, that givesmicrotubules their rigidity. Having a higher Young'smodulus, microfilaments
are better suited to withstanding tensile loads without stretching far than are microtubules. It is
interesting to note, in passing, that the Young's modulus of microtubules is similar to that of
ordinary engineering steel, whereas that of microfilaments is similar to that of piano wire,
which has been developed to withstand extremely high tension.
44
In vivo
, the formation of newmicrotubules is nucleated by microtubule-organizing centres,
MTOCs, which contain short cylinders of
g
-tubulin that are stabilized by their associated
proteins. These act as nucleation sites on which
aeb
tubulin dimers can construct mature
microtubules. In animal cells, MTOCs are typically found in the immediate environment
of the centrosomes near the nucleus; they may be found in basal bodies of cilia as well.
The main microtubule network that is relevant to tensegrity originates from the pericentriolar
zone, and it therefore radiates through the cell from one point (see
Figure 5.3
).
The binding of a new GTP-tubulin unit to the growing microtubule is stable. Once further
growth has buried the unit into a microtubule, so that it is no longer at the end, hydrolysis of
its GTP to GDP has no effect and it remains stable (the energy of hydrolysis is stored as strain
with about 7
)
Classroom demonstration: cut a 5 cm (2 inch) square out of A4 paper. Attempt to use it to lift a coffee cup
and you will fail
d
it bends. Then roll it into a cylinder, taping the edges together; used this way and poked
through the cup handle, it can easily lift the weight of the cup without collapsing. (Note: use an
empty
cup).
