Biology Reference
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composed of a pair of centrioles and their surrounding pericentriolar material
(PCM) (Bettencourt-Dias and Glover
2007
). The centriole duplication cycle, as
shown by electron microscopy, implies that there is a fundamental intrinsic
asymmetry in the age of centrioles. The centrosome cycle includes four main
phases: disengagement of the two centrioles, nucleation of the daughter centrioles,
elongation of the daughter centrioles, and separation of the new centrosomes.
Therefore, within one centrosome, there will always be one older centriole, which
was assembled at least one cell cycle prior to, and one younger centriole that was
assembled during, the current cell cycle. Furthermore, the ''age'' of the centrosome
can be denoted by the ''age'' of older centrioles within the centrosome: ''the
mother centrosome'' contains a centriole that was assembled at least two cell
cycles prior and a centriole that was assembled in the current cell cycle, while ''the
daughter centrosome'' contains a centriole that was assembled in one cell cycle
prior and a centriole that was assembled in the current cell cycle. The age of the
centrioles, and by extension the age of the centrosome, affect their maturity and
function (Yamashita
2009a
). In addition to the asymmetric composition of mother/
daughter centrosomes, there are cellular effectors that concentrate on the centro-
some in a cell cycle-dependent manner (Bornens
2002
).
The centrosomes display functional and behavioral asymmetry, primarily due to
structural asymmetry in the mother centrioles. Prior to centrosome duplication, the
mother centriole has distal and surrounding subdistal appendages along its proximo-
distal axis, while similar appendages are not found in the daughter centriole (Sluder
2005
). Therefore, after centrosome duplication the mother centriole of the mother
centrosome, which contains the appendages, is more mature compared to the mother
centriole of the daughter centrosome throughout most of the cell cycle (Vorobjev and
Chentsov Yu
1982
). As the distal and subdistal appendages are major MT-anchoring
sites, the mother centrosome has a greater capability to initiate and organize
cytoplasmic MTs (Bornens
2002
). Furthermore, vertebrate primary cilia can only
grow from the mature mother centriole in the centrosome, and after cell division the
cell that inherited the oldest centriole (i.e. the cell that inherited the mother centro-
some) usually grows a primary cilium first (Anderson and Stearns
2009
). Proteins
that specifically localize to the appendages have been identified. For example,
e-tubulin and ODF2 asymmetrically localize to the mother centriole, while centrobin
asymmetrically localizes to the daughter centrosome (Chang et al.
2003
; Chang and
Stearns
2000
; Nakagawa et al.
2001
; Zou et al.
2005
) (Fig.
6.1
).
A group of proteins are known to play important roles in centriole-duplication
and PCM recruitment during centrosome biogenesis. In C. elegans, the kinase ZYG-1
(a functional ortholog of protein kinase PLK4) is involved in centrosome formation
through phosphorylating SAS-6 (Kitagawa et al.
2009
). It has been shown that SAS-4
and SAS-6 are required for centriole duplication in C. elegans (Dammermann et al.
2004
; Kirkham et al.
2003
), Drosophila melanogaster (Basto et al.
2006
) and humans
(Leidel and Gonczy,
2005
), and polo kinase and centrosomin (Cnn) are localized to
the centrosome and important for PCM recruitment (Yamashita
2009b
). Cnn is
asymmetrically incorporated into the PCM and the rate of Cnn incorporation is
responsible for regulating the size of the centrosome (Conduit et al.
2010
). Therefore,
