Cell binding fragments from a sponge proteoglycan-like aggregation factor.

The marine sponge Microciona prolifera aggregation factor (MAF) is a 2 X 10(7) dalton proteoglycan. MAF mediates species-specific cell-cell recognition through two functionally different sites: a Ca2+-independent species-specific cell binding site and a Ca2+-dependent MAF-MAF binding site. Dissociation procedures combined with protease treatment were used to produce cell-binding pieces from the large complex. The seven different sized fragments produced were all uronic acid-rich glycoproteins of the apparent molecular weights: 15 X 10(6), 2.5 X 10(5), 1.2 X 10(5), 7 X 10(4), 2.7 X 10(4), 5 X 10(3), and 3.6 X 10(3). Each of the fragments retained species-specific binding to Microciona cells and was also capable of inhibiting MAF-promoted cell aggregation. However, the fragments were unable to bind to MAF-conjugated agarose beads in the presence or absence of CA2+ ions. These three properties are those expected for the cell binding site of MAF. Since the binding affinity decreased linearly with decreasing molecular weight of the fragments, we believe that the cell binding sites in MAF may be highly polyvalent, although to fully support such a concept, a detailed chemical characterization of each of the fragments is needed. A high valency of cell binding sites would overcome a relatively low Ka for the single site and would thereby not only guarantee specificity but also explain the need for the large size of the proteoglycan complex found to mediate species-specific sponge aggregation.