THE OLIGOMERIC INTEGRITY OF TOPOSOME IS ESSENTIAL FOR ITS MORPHOGENETIC FUNCTION

Sea urchin embryos are uniquely suitable for the study of morphogenetic cell interactions. Efforts to identify the molecules responsible for morphogenetic cell adhesion led to the isolation of a 22S glycoprotein complex from Paracentrotus lividus sea urchin embryo, that has been called toposome. The biological activity of toposome in mediating cellular adhesion has been fully documented. Its function in determining positional guidance during the development of the sea urchin embryo has been proposed. Here studies on the molecular structure of toposome are reported showing that, under non-reducing conditions, it is resolved in sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) in a major band with an apparent molecular weight of 260 kDa, a doublet of 180-160 kDa and a lower band of 80 kDa. Digestion with EndoH endoglycosidase reduced the molecular sizes of the bands of 10%, 20% and 40%, respectively. In order to establish if the oligomeric integrity of toposome was essential for its function, the biological activity of each subunit on cells dissociated from sea urchin blastula embryos was tested. The resulting swimming embryoids were lacking skeleton, while reaggregating cells supplemented with native toposome developed into pluteus-like structures with skeletal elements