Dual role for Dictyostelium contact site B in phagocytosis and developmental size regulation

Abstract

Three cohesion molecules have been discovered in the slime mould Dictyostelium discoideum^1–4. Two of these molecules are involved in morphogenesis at the aggregation stage of the life cycle and thereafter, and may also provide an essential trigger for late gene expression⁵. The third glycoprotein 126 (gpl26) or contact site B, is present on axenically grown vegetative amoebae and persists to the aggregation stage where it is involved in the side-to-side cohesion of cells in aggregation streams^1,4. It is puzzling that vegetative amoebae should possess a cohesion molecule because their solitary habit does not necessitate cohesion. However, they do need to adhere to the substratum and adhesion of bacteria to the cells is a prerequisite for phagocytosis. Vogel et al.⁶ have proposed that the same receptor is involved in phagocytosis and cohesion. It has also been suggested that contact site B-mediated cohesion is a trigger for development⁷. Using a specific antibody against gpl26, we now show that contact site B is a phagocytosis receptor. Furthermore, contact site B is involved in regulating the size of aggregates formed during morphogenesis; it also seems to be involved in cell–substratum adhesion but is not a developmental trigger.