Species-specific aggregation factor in sponges : VIII. Nature and alteration of cell surface charge.

Isolated cells from the siliceous spongeGeodia cydonium have been studied with respect to their partition behaviour in a two-phase aqueous polymer system. With this method it is possible to determine subtle changes in the cell surface charge. Addition of a homologous aggregation factor to the isolated cells lowers the partition rate, a finding which indicates that after binding of the aggregation factor to the cells their surface charge is reduced. The partition rate of the cells is strongly correlated with their content of membranebound sialic acid.Sixty-nine percent of the total, membrane-bound hexuronic acid is associated with the aggregation receptor; 1.8×10 aggregation receptor molecules are present on the surface of one cell which means that the average surface density amounts to 2.8×10 molecules per μm.Removal of the aggregation receptor molecules from the cell surface results in a decrease of the partition rate in the two-phase system. After charging the receptor-depleted cells with soluble aggregation receptor, the partition behaviour of these cells can be reconstituted.