In situ localization of the specific cell-binding fragment on the sponge aggregation factor

Abstract

Electron microscopical studies revealed that the functionally intact (native) form of the aggregation factor (AF) from the sponge Geodia cydonium has a ball-like structure. A monoclonal antibody (McAb 5132-1311), directed against the 47-kDa cell-binding domain of the AF, was used for the in situ detection of this 47-kDa fragment. After detection of the antigen-antibody reaction with protein A-colloidal gold, three or four molecules of the 47-kDa protein were observed per one AF particle. The 47-kDa protein can be detached from the particle after sodium dodecyl sulfate and Nonidet treatment, releasing the “sunburst” core structure of the AF. The 47-kDa protein, separated from the core AF by gel filtration, has been identified by an immunoblotting procedure. From these experiments we conclude that the 47-kDa cell-binding domain is only present on native AF. It is concluded that the 47-kDa protein binds to the aggregation receptor on the plasma membrane in a polyvalent manner.