The contractile vacuole complex of protists - New cues to function and biogenesis

The contractile vacuole complex (CVC) of freshwater protists sequesters the excess of water and ions (Ca2+) for exocytosis cycles at the pore. Sequestration is based on a chemiosmotic proton gradient produced by a V-type H+-ATPase. So far, many pieces of information available have not been combined to a comprehensive view on CVC biogenesis and function. One main function now appears as follows. Ca2+-release channels, type inositol 1,4,5-trisphosphate receptors (InsP(3)R), may serve for fine-tuning of local cytosolic Ca2+ concentration and mediate numerous membrane-to-membrane interactions within the tubular spongiome meshwork. Such activity is suggested by the occurrence of organelle-specific soluble N-ethylmaleimide sensitive factor attachment protein receptor (SNARE) and Ras-related in brain (Rab) proteins, which may regulate functional requirements. For tubulation, F-Bin-amphiphysin-Rvs (F-BAR) proteins are available. In addition, there is indirect evidence for the occurrence of H+/Ca2+ exchangers (to sequester Ca2+) and mechanosensitive Ca2+-channels (for signaling the filling sate). The periodic activity of the CVC may be regulated by the mechanosensitive Ca2+-channels. Such channels are known to colocalize with and to be functionally supported by stomatins, which were recently detected in the CVC. A Kif18-related kinesin motor protein might control the length of radial arms. Two additional InsP(3)-related channels and several SNAREs are associated with the pore. De novo organelle biogenesis occurs under epigenetic control during mitotic activity and may involve the assembly of gamma-tubulin, centrin, calmodulin and a never in mitosis A-type (NIMA) kinase - components also engaged in mitotic processes.