Contractile Vacuole Complex—Its Expanding Protein Inventory - International Review of Cell and Molecular Biology - page 375

Dictyostelium , CVC-resident SNAREs await characterization, but

the

SNARE-associated protein SecA ( Munc18) has been found ( Essid

et al., 2012; Sriskanthadevan et al., 2009 ) to be mandatory for CV contents
release ( Zanchi et al., 2010 ). Three SNAREs—still to be characterized—
have been assigned to the CVC in Trypanosoma cruzi ( Ulrich et al., 2011 ),
as specified in Table 9.2 .

¼

2.3.2 Exocyst, Rab-type GTPases, and additional regulators
of vesicle trafficking
The multimeric exocyst tethers donor and acceptor membranes to each
other ( Guo et al., 2000; Heider and Munson, 2012 ). Next to join the
intermembrane link, in concert with SNAREs, are Rab-type monomeric
GTP-binding proteins (small G-proteins or GTPases). Together with their
regulators, such as GAP (GTPase-activating proteins), GEF (guanine nucle-
otide exchange factor), and GDI (guanosine nucleotide dissociation inhib-
itor), they serve for improvement and acceleration of membrane-specific
interactions ( Bustelo et al., 2007; Zerial and McBride, 2001 ). Several types
have been found in the CVC of different protists, for example, Tetrahymena
( Bright et al., 2010 ). In summary, the succession of steps are as follows: Mul-
timeric protein complexes, called the exocyst, tether vesicles to a target
membrane, followed by interaction with Rab-GTPases and formation of
a t-(Q-)/v-(R-) SNARE trans-complex which mediates vesicle docking
for subsequent fusion ( Jahn and Fasshauer, 2012 ). The latter generally
requires a very local Ca 2 þ signal ( Neher, 2012 ).
Generally, the different Rab-type small GTPases can be attributed to dif-
ferent sites and stages of intracellular membrane transport ( Galvez et al.,
2012; Stenmark, 2009; Zerial and McBride, 2001 ). Among the small
GTPases found experimentally in the CVC of Dictyostelium are RabD
( Harris and Cardelli, 2002; Knetsch et al., 2001 ), Rab4 ( Bush et al.,
1994, 1996 ), Rab8 ( Essid et al., 2012 ), and Rab11 ( Harris et al., 2001 ).
( Dd RabD previously been considered identical with Rab4 is now consid-
ered rather close to Rab 14; see below.) RabD clearly regulates CV activity
in Dictyostelium , in conjunction with Dd RacGap1, a protein containing
both, Rho-GEF and Rho-GAP domains, similar to mammalian Bcr/Abr
( Knetsch et al., 2001 ). Also in Dictyostelium , the RhoGDI homolog
GD11 appears to be relevant for CV function ( Rivero et al., 2002 ). The lat-
ter two proteins aim at Rho- and Rac-type GTPases, basically discussed in
the context of actin organization; this is remarkable since no actin is known
from the CVC ( Section 5.1 ).