Sponge aggregation factor: In situ localization by fluorescent monoclonal antibody techniques

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Abstract
The aggregation factor (AF) from sponges mediates a heterophilic interaction of homologous cells. Applying electron microscopical means, we succeeded only very rarely in identifying the 90 S AF particle in tissue sections from Geodia cydonium. By means of a fluorescent antibody technique, we have now localized the cell binding domain of the AF in situ. Previous studies in this laboratory have led to the identification of the 47‐kDa cell binding protein of the AF, using the monoclonal antibody (mab) 5D2‐D11 [Gramzow M, Bachmann M, Zahn RK, Uhlenbruck G, Dorn A, Müller WEG, J Cell Biol, 102:1344–1349, 1986]. This mab and mab 7D5, directed against a 92‐kDa protein in the AF complex, were chosen for the fluorescent studies. By using mab 5D2‐D11, the plasma membranes of cells from different regions in the sponge could be brightly stained. However, mab 7D5 reacted only very weakly with the sponge surfaces. By applying the immuno‐blotting technique it was furthermore demonstrated that the cell binding protein is present both in the associated form with AF complex and in a free state. Moreover, it was established that the 47‐kDa binding protein is not present in (1) homologous glycoconjugates, (2) lectin, or (3) collagen; these components are known to be involved in cell‐matrix interaction.