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The near-Golgi Rab11 is indicative of involvement of an endosomal
recycling compartment in mammalian cells (
Galvez et al., 2012;
Takahashi et al., 2012
). Its occurrence in the CVC of some species may sup-
port an emerging concept inferring the contribution of recycling processes
although no further details are known. In
T. cruzi
, GTPases attributed to the
CVC encompass not only Rab11 but also Rab32 (
Ulrich et al., 2011
). In
T. thermophila
, RabD2, RabD10, and RabD14 are restricted to the CVC
(
Bright et al., 2010
). These
Tt
Rab proteins are lineage specific and thus dif-
ferent from Rab14 described for the CVC of
Dictyostelium
and from several
related, but not yet localized Rabs in
Paramecium
(
Pt
31649,
Pt
47027,
Pt
53159, and
Pt
RabC49) (
Bright et al., 2010
). In
Dictyostelium
, some Rab
effector and regulator proteins are also known from the CVC, that is, the
Rab11 effector Drainin (
Becker et al., 1999
) and the Rab8a-GAP Disgorgin
(
Du et al., 2008
; see
Section 4.2
).
The occurrence of these Rab proteins in the CVC of different species
underscores the importance of membrane trafficking in the CVC. Previ-
ously, this has been concluded from the occurrence of NSF (
Kissmehl
et al., 2002
) and of SNAREs in the
Paramecium
CVC (
Plattner, 2010a,b
).
Organelle specificity of both, Rabs and SNAREs supports evolution sepa-
rately from other vesicular pathways, making the CVC an organelle newly
“invented” during evolution.
2.3.3 Possible involvement of Golgi apparatus in CVC biogenesis
In
Dictyostelium
, RabD is now considered related to Rab14 (rather than to
Rab4 as in earlier claims;
Bush et al., 1996; Knetsch et al., 2001; Rivero
et al., 2002
) which in mammalian cells serves for Golgi
endosome trans-
port (
Junutula et al., 2004
). This would go along with the importance of the
clathrin-adaptor protein, AP-1, a vesicle budding-mediator operating in
mammalian cells in the Golgi, for the biogenesis of the CVC in
Dictyostelium
(
Essid et al., 2012; Lefkir et al., 2003
). In
Tetrahymena
, the CVC harbors
RabD subtypes D2 (around CV), D10 (on tubular extensions), and D14
(CVC-associated large vesicles), whereas Rab4 is associated with the
phagosomal system (
Bright et al., 2010
), thus supporting the lack of identity
of Rab4 with RabD-type GTPases.
The V0 part of the H
þ
-ATPase requires a glycoprotein, Voa1p in yeast,
which drives its assembly (
Ryan et al., 2008
), before this complex dissociates
for further transport to the Golgi apparatus. For the CVC, the pathway(s) of
delivery is unknown. In the EM, with
Paramecium
, one can see a close asso-
ciation of the decorated spongiome with cisternae of the rough ER; besides
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