Contractile Vacuole Complex—Its Expanding Protein Inventory - International Review of Cell and Molecular Biology - page 386

vesicles inside the CV lumen. Accordingly, the CV of Dictyostelium would
participate in an unconventional mode of secretion. In Dictyostelium , muta-
tional analysis of targeting of a membrane-integrated CV-specific protein,
Rh50 (a homolog to the mammalian Rhesus protein), revealed the involve-
ment of a cluster of acidic amino acids as targeting motifs, and of clathrin and
adaptor protein AP-1 ( Mercanti et al., 2006 ).
4.2.3 Lvs proteins and Nramp/Slc11 protein
Lvs (large volume sphere) proteins are related to lysosomal trafficking reg-
ulator proteins identified in Beige mice whose mast cells display excessively
large secretory lysosomes due to abnormal fusion processes. Of the six Lvs
genes occurring in D. discoideum , LvsA localizes to the CV and is required
for osmoregulation ( Gerald et al., 2002 ). LvsA binds in a calmodulin-
dependent fashion ( Malchow et al., 2006 ) and null cells display abnormal
CV membranes ( Harris et al., 2002 ). This has been interpreted as an indi-
cation of the involvement of the CVC in Dictyostelium in a recycling/
phagosomal activity ( Harris et al., 2002 ), particularly also because expression
of a dominant negative RabD GTPase alters phagocytic activity and mor-
phology of the CV network ( Harris and Cardelli, 2002 ). Yet, this is in con-
trast to data discussed in Section 5.2 .
Nramp (natural resistance-associated macrophage protein) proteins are
orthologs to mammalian Slc11 (solute carrier) proteins which are proton-
coupled transporters of divalent cations ( Nevo and Nelson, 2006 ). Nramp1
mediates resistance to infection by Legionella pneumophila . Nramp2 is exclu-
sively localized to the CVC in Dictyostelium where it regulates iron homeo-
stasis and may serve for the storage of cations ( Peracino et al., 2013 ).
4.2.4 Drainin and Disgorgin
Drainin, a peripheral membrane protein, is a homolog of proteins occurring
from yeast to man. In Dictyostelium , where it has been discovered, it is essen-
tial for CV discharge by formation of the pore, probably by acting along a
signaling cascade ( Becker et al., 1999 ). Recent analyses identified Drainin as
a Rab11a effector and indicate sequential recruitment of Drainin, Rab8a,
and the exocyst ( Essid et al., 2012 ). Drainin binding is paralleled by binding
Disgorgin and followed by recruitment of LvsA (see above). Disgorgin, a
GAP for Rab8a, is also required for CV-content release by fusion with
the cell membrane ( Du et al., 2008 ). Disgorgin and LvsA, in concert with
GTP hydrolysis by Rab8a, is also thought to initiate detachment of the
empty CV after contents release ( Essid et al., 2012 ).