THE MAIN PROTEIN OF THE AGGREGATION FACTOR RESPONSIBLE FOR SPECIES-SPECIFIC CELL-ADHESION IN THE MARINE SPONGE MICROCIONA-PROLIFERA IS HIGHLY POLYMORPHIC

Species-specific cell recognition in sponges, the oldest living metazo ans, is based on a proteoglycan-like aggregation factor. We have scree ned individual sponge cDNA libraries, identifying multiple related for ms for the aggregation factor core protein (MAFp3). Northern blots sho w the presence in several human tissues of transcripts strongly bindin g a MAFp3-specific probe. The open reading frame for MAFp3 is not inte rrupted in the 5' direction, revealing variable protein sequences that contain numerous introns equally spaced. We have studied tissue histo compatibility within a sponge population, finding 100% correlation bet ween rejection behavior and the individual-specific restriction fragme nt length polymorphism pattern using aggregation factor-related probes . PCR amplifications with specific primers showed that at least some o f the MAFp3 forms are allelic and distribute in the population used. A pronounced polymorphism is also observed when analyzing purified aggr egation factor in polyacrylamide gels. Protease digestion of the polym orphic glycosaminoglycan-containing bands indicates that glycans are a lso responsible for the variability. The data presented reveal a high polymorphism of aggregation factor components, which matches the eleva ted sponge alloincompatibility, suggesting an involvement of the cell adhesion system in sponge allogeneic reactions.