Contact sites in aggregating cells of Polysphondylium pallidum

Aggregating cells of the cellular slime mold Polysphondylium pallidum are completely dissociated by univalent antibody fragments (Fab) directed against membrane antigens. The blocking effect on cell adhesion is species specific: Fab against P. pallidum has little effect on cells of Dictyostelium discoideum, and vice versa. Suspended cells of these species agglutinate together, but within the agglutinates they sort out into separate areas.Absorption of the Fab with growth phase cells removes only part of its blocking activity. This indicates the expression of a new class of target sites of adhesion blocking Fab during cell differentiation from the growth phase to the aggregation competent stage. Another class of target sites is already present on the surface of growth phase cells. In both developmental stages cell adhesion is largely resistant to EDTA.The major target sites of adhesion blocking Fab appear to differ from carbohydrate-binding proteins known as pallidin. Removal of the adhesion blocking activity by absorption of Fab with intact cells does not deplete for anti-pallidin Fab. Cell adhesion is only weakly affected by Fab specific for pallidin I and II.