Calcium-dependent self-aggregation of toposome, a sea urchin cell adhesion molecule.

Sea urchin embryos can be easily dissociated into single cells by exposure to Ca2+ and Mg2+ -free seawater. When transferred back to normal seawater, isolated cells spontaneously form aggregates capable of development. Here, the Ca2+ -dependent self-aggregation of toposome, a 22S glycoprotein complex which mediates cell-cell adhesion in sea urchin embryos, has been investigated using the purified molecule. Results show that the 22S complex is completely converted to 15S particles by sedimentation on sucrose isokinetic gradients in the presence of EDTA. Reconstitution of the 22S complex is achieved by readdition of Ca2+. We propose that the 15S particle constitutes the toposome functional unit on the cell surface.