Contractile Vacuole Complex—Its Expanding Protein Inventory - International Review of Cell and Molecular Biology - page 366

are from Dictyostelium and Paramecium. In Paramecium, the major parts of CVC contain
several v-/R-SNARE (synaptobrevins) and t-/Q-SNARE (syntaxins) proteins. This is com-
plemented by Rab-type GTPases (shown in Tetrahymena) and exocyst components
(Chlamydomonas). All this reflects a multitude of membrane interactions and fusion pro-
cesses. Ca 2þ /H þ and other exchangers are to be postulated, as are aquaporins and
mechanosensitive Ca 2þ channels. From the complexity of the organelle, many more
proteins may be expected. For instance, the pore is endowed with its own set of pro-
teins. We may now envisage the regulation of membrane dynamics (reversible
tubulation) and the epigenetic control of organelle shape, size and positioning. New
aspects about organelle function and biogenesis are sketched in Section 7. The man-
ifold regulators currently known from CVC suggest the cooperation of widely different
mechanisms to maintain its dynamic function and to drive its biogenesis.
ABBREVIATIONS
CRC Ca 2 þ -release channel
CVC contractile vacuole complex
SNARE soluble NSF [ N -ethylmaleimide sensitive factor] attachment protein receptor
1. INTRODUCTION
Freshwater protists constantly have to cope with the permeation of
water into the cell. They have developed a complex organelle, the contrac-
tile vacuole complex (CVC), for the extrusion of water and an excess of ions,
notably Ca 2 þ . There are general principles in the design of CVC, though
with considerable variation ( Allen, 2000; De Chastellier et al., 1978;
Frankel, 2000; Hausmann and Patterson, 1984; Patterson, 1980;
Schneider, 1960 ). One of the most important feature is the endowment with
a V-type H þ -ATPase as a primary active transporter ( Fok et al., 1993;
Heuser et al., 1993; Nolta et al., 1993 ). It forms a proton gradient ( D H þ )
which chemiosmotically drives the transfer of water and ions into the con-
tractile vacuole (CV). The CV expels its contents by exocytosis at preformed
sites, the “pore” ( Allen, 2000; Allen and Naitoh, 2002; Frankel, 2000;
McKanna, 1973, 1976 ).
2. BASIC STRUCTURAL AND FUNCTIONAL ELEMENTS
OF CVC
The CVCs of different organisms share some important structural and
molecular elements. Although some CVC proteins are known only from