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Cell News 2/2015
Building the meat of pericentriolar material
surrounding the skeleton of a centriole
Anand Ramani, Li Ming Gooi and Jay Gopalakrishnan
1*
1
Center for Molecular Medicine and Institute for Biochemistry I of the University of Cologne,
Robert-Koch-Str. 21 50931 Cologne, Germany.
*
Correspondence should be addressed to JG (
)
Centrosome structure and cellular functions
Centrosomes are conserved eukaryotic organelles, essential for
fundamental cellular functions such as microtubule nucleation,
accurate cell division and cilium formation
1-4
. At its core, a cen-
trosome consists of a pair of orthogonally positioned centrioles
surrounded by a protein network of pericentriolar material (PCM).
Within a centrosome, the centriole templates cilia formation du-
ring interphase, while assembling PCM to form spindle poles that
nucleate astral and spindle microtubules during mitosis (Fig. 1).
The PCM is composed of numerous proteins including
γ
-tubulin
ring complexes (
γ
-TuRC), which are essential for catalyzing mi-
crotubule nucleation
5,6
. Thus, in order for centrosomes to form
functional microtubule-organizing centers (MTOCs), they must
first successfully recruit PCM around centrioles.
In this context, one could ask why it is so important for a centri-
ole to undergo the complex event of regulated PCM recruitment?
This question especially arises as microtubule nucleation is spon-
taneous when free tubulin concentration is high enough and in
the presence of sufficient GTP, a nucleotide that confers the right
conformation for tubulin to polymerize. If this has to occur in
cells, there should be at least two mechanisms at play. The first
focuses microtubule nucleation to a confined space within a cell
to form an array of asters. The second overcomes the kinetic
barrier of spontaneous microtubule nucleation, due to the
in
vivo
tubulin concentration that does not encourage spontaneous
nucleation. The organization of PCM harboring
γ
-TuRCs surroun-
ding centrioles seems to facilitate these processes in an orderly
manner.
The MTOC activity of a centrosome is proportional to the amount
of PCM it contains
7-9
. Accordingly, in interphase, centrosomes
usually contain little PCM and thus the MTOC activity of inter-
phase centrosomes is minimal. However, during mitosis, the PCM
of a centrosome expands leading to enhanced MTOC activity
7-9
(Fig. 1). The complexity of PCM assembly, recruitment to centro-
somes and their spatiotemporal regulation in tandem with the
cell cycle remains a mysterious yet important question in cell
biology. Recent studies applying different techniques in a wide
variety of organisms have started demystifying the complexity
of PCM assembly and recruitment. In this review, we state the
recent developments on PCM assembly in the cytoplasm, how
it is recruited to a developing centrosome and the mechanisms
that spatiotemporally regulate these processes.
Historical perspective of centrosomes
Since Boveri’s monograph on the nature of centrosomes dating
back to 1900, centrosomes have been described as a pair of cen-
trioles surrounded by amorphous PCM. The “amorphous” nature
of PCM was generally accepted for decades due to the lack of
knowledge on the molecular composition of PCM and appropria-
te tools that could distinguish PCM composition at sub-diffrac-
tion resolution. This possibly led to confusion on where a cen-
triole ends and where the PCM begins, which was delete nicely
highlighted by Wheatly in 1982 stating that “discovery of the
discreet tubular cylinder was not an end to the problem, for even
today we have yet to decide exactly where the centriole starts
and ends”
10
. The recent advent of super resolution microscopy
allowed the Glover, Nigg, Agard and Pelletier labs to define the-
se boundaries in both humans and flies, demonstrating that key
centrosomal proteins are organized into distinct spatial com-
partments
11-14
. The core centriolar proteins of proteins of Sas-6,
Ana-2 (STIL) and Cep135 (Bld10) form the innermost layers fol-
lowed by Sas-4 (CPAP), Spd-2 and polo at the centriolar wall
that is further surrounded by pericentrin (PCNT), Asl (Cep152),
Plk4, Cnn (CDK5RAP2) and
γ
-tubulin
11-14
.
Despite these studies unequivocally dissecting the different PCM
layers, the question still remains of what layers or components
are essential for priming further PCM organization and microtu-
bule nucleation. In our opinion, this fundamental question has
been answered by elegant biochemical studies that use high
concentrations of chaotropic salts to strip the PCM leaving be-
hind a salt resistant protein matrix surrounding the centriole
15,16
.
Interestingly, this salt-resistant structure could functionally re-
constitute
γ
-TuRC-mediated microtubule nucleation, indicating
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