Constitutive overexpression of the contact site A glycoprotein enables growth-phase cells of Dictyostelium discoideum to aggregate.

The contact site A (csA) glycoprotein is a developmentally regulated cell adhesion molecule which mediates EDTA-stable cell contacts during the aggregation stage of Dictyostelium discoideum. A transformation vector was constructed which allows overexpression of the csA protein during the growth phase. In that stage the csA protein is normally not expressed; in the transformants it was transported to the cell surface and carried all modifications investigated, including a phospholipid anchor and two types of oligosaccharide chain. csA expression enabled the normal non-aggregative growth-phase cells to form EDTA-stable contacts in suspension and to assemble into three-dimensional aggregates when moving on a substratum. After prolonged cultivation of csA overexpressing transformants in nutrient medium the developmental program was found to be turned on, as it normally occurs only in starving cells. During later development of transformed cells, the csA glycoprotein remained present on the cell surface, while it is down-regulated in the wild type. It was detected in both the prestalk and prespore regions of the multicellular slugs made from transformed cells.