Many protists possess a specialised osmoregulatory organelle called the contractile vacuole complex (CVC). The CVC functions as a ‘bladder’ for unicellular organisms by filling with excess water transferred from the cytoplasm and contracting to expel the water through a pore in the plasma membrane. Many aspects of CVC function are not understood, such as how the CVC is replicated during mitosis and what factors drive contraction. Aaron Turkewitz and colleagues here (Cheng et al., 2023) endogenously label four CVC-associated genes with fluorescent tags to enable live imaging of CVC dynamics in the ciliate protist Tetrahymena thermophila and demonstrate the presence of distinct CVC subcompartments displaying different protein compositions. Under conditions of osmotic stress, which induces CVC contraction, the four probes display differential dynamics, linking the composition of compartments, such as the junctional pore and spongiome, a tubulovesicular network surrounding the vacuole, to their differing functions. By monitoring the CVC during contraction and mitosis, they describe how the CVC directionally collapses toward the membrane into a lens shape, which is refilled via membrane blebbing, and suggest that new CVCs might be seeded in dividing Tetrahymena by membrane tubules that transport CVC proteins along cortical cytoskeletal meridians. This study delivers a wealth of new information about the CVC and will enable further detailed functional analyses of this understudied organelle.
