DdAlix, an Alix/AIP1 homolog in Dictyostelium discoideum, is required for multicellular development under low Ca2+ conditions

Apoptosis-linked gene 2 (ALG-2) interacting protein X (Alix), also called AIP1, is a widely conserved protein in eukaryotes. Alix and its homologs are involved in various phenomena such as apoptosis, regulation of cell adhesion, protein sorting, adaptation to stress conditions, and budding of human immunodeficiency virus (HIV). To investigate the role of Alix in development, we identified an Alix homolog in the cellular slime mold Dictyostelium discoideum and disrupted the gene by homologous recombination. The growth of DdAlix deletion mutant (alx-) cells was significantly impaired in the presence of 5 mM Li+. On an agar plate, alx- cells underwent normal development and formed fruiting bodies indistinguishable from those formed by wild-type cells. However, alx- cells could not form fruiting bodies in the presence of 5 mM Li+. Similar results were obtained when cells were developed in the presence of 3,4,5-trimethoxybenzoic acid 8-(diethylamino)octyl ester (TMB-8), which is an antagonist of intracellular Ca2+ store. Furthermore, when the extracellular free Ca2+ was reduced to 10 nM, the ability of alx- cells, but not that of wild-type cells, to form fruiting bodies was impaired. The results indicate that DdAlix is essential for development under low Ca2+ conditions and suggest that DdAlix is involved in Ca2+ signaling during development.