Novel redistribution of the Ca²⁺-dependent cell adhesion molecule DdCAD-1 during development of Dictyostelium discoideum

Dictyostelium discoideum cells express DdCAD-1, a Ca²⁺-dependent cell-cell adhesion molecule soon after the initiation of development. DdCAD-1 is a soluble protein which shares a significant degree of sequence similarity with E-cadherin. Laser scanning confocal microscopy of the subcellular localization of DdCAD-1 has revealed a nonrandom pattern of DdCAD-1 distribution. DdCAD-1 is present mostly as diffusely stained material in the cytoplasm during the initial stage of development. However, a drastic redistribution takes place before the onset of cell aggregation, when DdCAD-1 becomes localized predominantly at the cell periphery and an enrichment of DdCAD-1 occurs on membrane ruffles. A high concentration of DdCAD-1 also becomes associated with lamellipodia and filopodia, which often appear to participate in cell contact formation. Although DdCAD-1 is present in high concentrations in contact regions during early development, it disappears rapidly from these areas during cell aggregation. This redistribution is accompanied by an accumulation of the Ca²⁺-independent cell adhesion molecule gp80 in contact regions. During chemotactic migration, DdCAD-1 is present primarily on cells at the tip and on the outer margin of cell streams. In contrast, gp80 is concentrated in contact regions among cells within well developed streams. This dynamic redistribution suggests a unique role for DdCAD-1 in the recruitment of cells into streams and in the formation of initial contacts, but it may not be required to maintain stable contacts in the presence of gp80.