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Delivering Nanoparticles to Cancer Cells 215
Once established upon and across the C microtubule the Plk4
recruits SAS6, a protein forming the structural base for the growth of
a new centriole: Specifically, the SAS6 protein then emits nine radial
spokes angled at 40° to each other where each spoke provides support
for the growth of a triplet of microtubules — thus forming a micro
tubule blade of the newly growing centriole.
The conventional understanding13–28 for the growth of the micro
tubules, and thus also of the centrioles, is that the a /b tubulin dimers
are recruited to the SAS6 spokes by a γtubulin protein. The SAS6
spokes provide platforms for triplets of γtubulin rings. These rings
have a series of 13 pores through which the a /b dimers are recruited
from the MTOC and pushed upward to form microtubule filaments.
Figure 9 provides a sketch of the envisioned γtubulin ring together
with its pores.
The pores used for inserting a /b tubulin are sometimes called the
“γtubulin small complex” (γTu–SC).
Interestingly, the γtubulin ring is not quite planar. Instead, its
ends overlap so that it has the form of a lockwasher. This in turn
causes the microtubule filaments to be inclined at a small angle.
The centriole host is known as the “mother” and the growing
centriole via the growing microtubules is known as the “daughter”.
In the original motherdaughter centriole pair, each centriole hosts a
new centriole. The mother has a new daughter, and the daughter has
a daughter of its own — a “granddaughter”.
The microtubules of a daughter grow to only about 80% of the
length of the mother’s microtubule.
Fig. 9. A γTubulin ring complex (γTu–RC).