CELL-ADHESION IN SPONGES - POTENTIATION BY A CELL-SURFACE 68 KDA PROTEOGLYCAN-BINDING PROTEIN

Constitutive, stable intercellular adhesion is one of the distinguishi ng properties of metazoans, of which the sponges (Phylum Porifera) are the most primitive representatives. In sponges, intercellular adhesio n is mediated by the large proteoglycan-like cell agglutinating molecu le 'aggregation factor', which binds to cell surfaces via an oligosacc haride moiety. Previous studies indicated that this aggregation factor binds to two proteins associated with the surface of sponge cells. On e of these, a 68 kDa peripheral membrane protein, was isolated by affi nity chromatography on aggregation factor conjugated to Sepharose. Thi s monomeric 68 KDa glycoprotein plays a key role in sponge cell adhesi on since it potently inhibits the binding of aggregation factor to cel l surfaces and completely prevents aggregation factor-mediated cell ad hesion. The 68 kDa aggregation factor ligand binds with high affinity to both aggregation factor (K-D = 2x10(-9) M) and cell surfaces (K-D = 6x10(-8) M) providing evidence that it serves as an intramolecular br idge between the aggregation factor molecule and a cell surface recept or. Therefore, this early metazoan protein may represent one of the ea rliest extracellular matrix adhesion proteins to have arisen in the co urse of metazoan evolution.