The superfamily of Bin/Amphipysin/Rvs (BAR)-domain proteins is characterised by their ability to sculpt membranes and, thus, have important roles in trafficking and cell migration. They comprise three subfamilies: the BAR- and F-BAR-domain proteins generate positive membrane curvature and bend the plasma membrane inwards; the inverse BAR domain (I-BAR) proteins induce negative membrane curvature, which promotes cell protrusions. There are five mammalian I-BAR homologues that have been well studied and proposed to directly link membrane deformation with actin polymerisation during migration and cell morphogenesis. However, because the I-BAR homologues are likely to have some functional redundancy, the exact function of I-BAR proteins is unclear. The genome of Dictyostelium discoideum only encodes a single I-BAR protein, IBARa, and thus presents an ideal system to elucidate I-BAR protein function, as demonstrated by Jan Faix and colleagues in this work (p. 1279). The authors first solved the crystal structure of the N-terminal fragment of IBARa, revealing an all-α-helical structure that self-associates into a zeppelin-shaped antiparallel dimer that is consistent with the shape they observe in solution when using X-ray scattering. The authors then move on to analyse the localisation of IBARa and show that, apart from localisation to filopodia tips and phagosomes, the protein is predominantly enriched at the contractile vacuole (CV), the osmoregulatory organelle of protists. Consistently, IBARa-knockout mutants have a higher number of CVs and show defects in growth, osmoregulation and phagocytosis. Importantly, the mutants are also impaired in cell migration and cytokinesis, indicating that IBARa has an important role in a number of processes that require a dynamic rearrangement of cellular membranes.