Biology Reference
In-Depth Information
19.2 Centrosomes in Apicomplexa
In most metazoans, the centrosome functions as the main microtubule-organizing
center (MTOC) during interphase as well as during cell division, where it is
responsible for mitotic spindle formation and chromosome segregation after dis-
assembly of the nuclear membrane. Centrosomes are formed around a pair of
centrioles that are arranged perpendicular to each other. Each centriole is a barrel-
shaped structure with nine triplets of microtubules forming the barrel wall
(Azimzadeh and Marshall
2010
). In apicomplexans, like in yeast, the nuclear
membrane continues to be present throughout the cell cycle. The spindle structure
and the centrosomes are formed from an electron-dense plaque named centrocone
or spindle pole plaque (Aikawa and Beaudoin
1968
; Kelley and Hammond
1972
;
Schrevel et al.
1977
; Dubremetz and Elsner
1979
; Sibert and Speer
1981
;
Morrissette and Sibley
2002a
; Gerald et al.
2011
) (Fig.
19.1
b). Also, the centrioles
present a unique ''9 ? 1'' microtubule structure, with a sole central microtubule
within the nine microtubule triplets (Dubremetz and Elsner
1979
) (Fig.
19.1
c).
Curiously, it has been observed in intranuclear spindles of Eimeria that some
microtubules span between and connect both centrocones, while other microtu-
bules terminate at kinetochores (Sibert and Speer
1981
). However, in another
parasitic protozoan, Lecudina tuzetae, a gregarine living in the intestine of a
marine polychaete annelid, only a few or no astral microtubules were observed
(Kuriyama et al.
2005
). Because no cytokinesis occurs, the spindle may not require
astral microtubules for mitosis. Curiously, Lecudina induces microtubule assem-
blies in different developmental stages that originate from different MTOCs
containing c-tubulin, pericentrin, Cep135, and mitosis-specific phosphoproteins
(Kuriyama et al.
2005
).
In contrast to spindle microtubules, the subpellicular microtubules that subtend
the pellicle of the parasite are mostly not susceptible to microtubule-depolymer-
izing drugs. These microtubules are often cited as being responsible for main-
taining parasite shape or might be involved in vesicular transport (Bannister et al.
2000
; Morrissette and Sibley
2002a
; Schrével et al.
2008
). They are located ca.
20 nm beneath the inner leaflet of the inner membrane complex (IMC), a mem-
brane structure formed from flattened vesicles and unique to this clade of organ-
isms. The IMC encircles the entire parasite at a distance of about 30 nm from the
plasma membrane. The space between plasma membrane and IMC is containing
the actin-myosin machinery that drives parasites through tissues and into host cells
(Heintzelman
2006
; Baum et al.
2008
). The microtubules may be connected to the
IMC with linker proteins (Morrissette et al.
1997
; Kudryashev et al.
2010
) and do
not seem to undergo continuous assembly and disassembly steps. Curiously, these
subpellicular microtubules in some apicomplexan parasites contain what could be
a protein associated with the luminal part of the microtubule wall (Cyrklaff et al.
2007
). This could be important for the unusual stability of these microtubules.
After host cell invasion the subpellicular microtubules slowly disassemble.
During formation of the parasite progeny when the subpellicular microtubules are
