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

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Abstract
Three cohesion molecules have been discovered in the slime mould Dictyostelium discoideum. 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 expression. The third glycoprotein 126 (gp126) 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. 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. 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 development. Using a specific antibody against gp126, 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.