The in vivo role of annexin VII (synexin): characterization of an annexin VII-deficient Dictyostelium mutant indicates an involvement in Ca(2+)-regulated processes.

Dictyostelium discoideum cells harbor two annexin VII isoforms of 47 and 51 kDa which are present throughout development. In immunofluorescence and cell fractionation studies annexin VII was found in the cytoplasm and on the plasma membrane. In gene disruption mutants lacking both annexin VII isoforms growth, pinocytosis, phagocytosis, chemotaxis and motility were not significantly impaired under routine laboratory conditions, and the cells were able to complete the developmental cycle on bacterial plates. On non-nutrient agar plates development was delayed by three to four hours and a significant number of aggregates was no longer able to form fruiting bodies. Exocytosis as determined by measuring extracellular cAMP phosphodiesterase, alpha-fucosidase and alpha-mannosidase activity was unaltered, the total amounts of these enzymes were however lower in the mutant than in the wild type. The mutant cells were markedly impaired when they were exposed to low Ca2+ concentrations by adding EGTA to the nutrient medium. Under these conditions growth, motility and chemotaxis were severely affected. The Ca2+ concentrations were similar in mutant and wild-type cells both under normal and Ca2+ limiting conditions; however, the distribution was altered under low Ca2+ conditions in SYN-cells. The data suggest that annexin VII is not required for membrane fusion events but rather contributes to proper Ca2+ homeostasis in the cell.