Contractile vacuole complex--its expanding protein inventory.

The contractile vacuole complex (CVC) of some protists serves for the osmotic equilibration of water and ions, notably Ca(2+), by chemiosmotic exploitation of a H(+) gradient generated by the organelle-resident V-type H(+)-ATPase. Ca(2+) is mostly extruded, but there is also some reflux into the cytosol via Ca(2+)-release channels. Most data available 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 complemented by Rab-type GTPases (shown in Tetrahymena) and exocyst components (Chlamydomonas). All this reflects a multitude of membrane interactions and fusion processes. Ca(2+)/H(+) and other exchangers are to be postulated, as are aquaporins and mechanosensitive Ca(2+) channels. From the complexity of the organelle, many more proteins may be expected. For instance, the pore is endowed with its own set of proteins. 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 manifold regulators currently known from CVC suggest the cooperation of widely different mechanisms to maintain its dynamic function and to drive its biogenesis.

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