Essential Cell Biology 5th edition

Microtubules
581
α
tubulin dimer
(= microtubule subunit)
β
protofilament
plus
end
50 nm
(B)
lumen
(D)
25 nm
Figure 17–12 Microtubules are hollow
tubes made of globular tubulin subunits.
(A) One tubulin subunit (an αβ dimer) and
one protofilament are shown schematically,
together with their position within a
microtubule. Note that the tubulin dimers
in the protofilament are all arranged with
the same orientation. (B and C) Schematic
diagrams of a microtubule, showing
how tubulin dimers pack together in the
microtubule wall. At the top, 13 β-tubulin
molecules are shown in cross section.
Below this, a side view of a short section
of a microtubule shows how the dimers
are aligned in the same orientation in all
the protofilaments; thus, the microtubule
has a definite structural polarity—with
a designated plus and a minus end. (D)
Electron micrograph of a cross section of
a microtubule with its ring of 13 distinct
subunits, each of which corresponds to
a separate tubulin dimer. (E) Electron
micrograph of a microtubule viewed
lengthwise. (D, courtesy of Richard Linck;
E, courtesy of Richard Wade.)
minus
end
(A)
(C)
microtubule
(E)
25 nm
Microtubules Are Hollow Tubes with Structurally
Distinct Ends
Microtubules are built from subunits—molecules of tubulin—each of
which is a dimer composed of two very similar globular proteins called
α-tubulin and β-tubulin, bound tightly together by noncovalent interactions.
The tubulin dimers ECB5 stack e17.11/17.12
together, again by noncovalent bonding,
to form the wall of the hollow, cylindrical microtubule. This tubelike structure
is made of 13 parallel protofilaments, each a linear chain of tubulin
dimers with α- and β-tubulin alternating along its length (Figure 17–12).
Each protofilament has a structural polarity, with α-tubulin exposed at
one end and β-tubulin at the other, and this polarity is the same for all
the protofilaments in the microfilament. Thus the microtubule as a whole
has a structural polarity: the end with β-tubulin showing is called its plus
end, and the opposite end, which contains exposed α-tubulin, is called
the minus end.
In a concentrated solution of pure tubulin in a test tube, tubulin dimers
will add to either end of a growing microtubule. However, they add
more rapidly to the plus end than to the minus end, which is why the
ends were originally named this way—not because they are electrically
charged. The polarity of the microtubule—the fact that its structure has
a definite direction, with the two ends being chemically and functionally
distinct—is crucial, both for the assembly of microtubules and for their
role once they are formed. If microtubules had no polarity, they could not,
for example, guide directional intracellular transport.
The Centrosome Is the Major Microtubule-organizing
Center in Animal Cells
Inside cells, microtubules grow from specialized organizing centers that
control the location, number, and orientation of the microtubules. In
most animal cells, for example, the centrosome—which is typically close