Electron microscopical characterization of sponge aggregation factors

Glycan-to-Glycan Binding: Molecular Recognition through Polyvalent Interactions Mediates Specific Cell Adhesion

Glycan-to-glycan binding was shown by biochemical and biophysical measurements to mediate xenogeneic self-recognition and adhesion in sponges, stage-specific cell compaction in mice embryos, and in vitro tumor cell adhesion in mammals. This intermolecular recognition process is accepted as the new paradigm accompanying high-affinity and low valent protein-to-protein and protein-to-glycan binding in cellular interactions. Glycan structures in sponges have novel species-specific sequences. Their common features are the large size >100 kD, polyvalency >100 repeats of the specific self-binding oligosaccharide, the presence of fucose, and sulfated and/or pyruvylated hexoses. These structural and functional properties, different from glycosaminoglycans, inspired their classification under the glyconectin name. The molecular mechanism underlying homophilic glyconectin-to-glyconectin binding relies on highly polyvalent, strong, and structure-specific interactions of small oligosaccharide motifs, possessing ultra-weak self-binding strength and affinity. Glyconectin localization at the glycocalyx outermost cell surface layer suggests their role in the initial recognition and adhesion event during the complex and multistep process. In mammals, Le^x-to-Le^x homophilic binding is structure-specific and has ultra-weak affinity. Cell adhesion is achieved through highly polyvalent interactions, enabled by clustering of small low valent structure in plasma membranes.

Cell adhesion in sponges: potentiation by a cell surface 68 kDa proteoglycan-binding protein

Constitutive, stable intercellular adhesion is one of the distinguishing properties of metazoans, of which the sponges (Phylum Porifera) are the most primitive representatives. In sponges, intercellular adhesion is mediated by the large proteoglycan-like cell agglutinating molecule ‘aggregation factor’, which binds to cell surfaces via an oligosaccharide moiety. Previous studies indicated that this aggregation factor binds to two proteins associated with the surface of sponge cells. One of these, a 68 kDa peripheral membrane protein, was isolated by affinity chromatography on aggregation factor conjugated to Sepharose. This monomeric 68 kDa glycoprotein plays a key role in sponge cell adhesion since it potently inhibits the binding of aggregation factor to cell surfaces and completely prevents aggregation factor-mediated cell adhesion. The 68 kDa aggregation factor ligand binds with high affinity to both aggregation factor (KD = 2 × 10(−9) M) and cell surfaces (KD = 6 × 10(−8) M) providing evidence that it serves as an intramolecular bridge between the aggregation factor molecule and a cell surface receptor. Therefore, this early metazoan protein may represent one of the earliest extracellular matrix adhesion proteins to have arisen in the course of metazoan evolution.