ACCUMULATION IN MARINE SPONGE GRAFTS OF THE MESSENGER-RNA ENCODING THE MAIN PROTEINS OF THE CELL-ADHESION SYSTEM

Specific cell adhesion in the marine sponge Microciona prolifera is me diated by an extracellular aggregation factor complex, whose main prot ein component, termed MAFp3, is highly polymorphic. We have now identi fied MAFp4, an similar to 400-kDa protein, from the aggregation factor that is translated from the same mRNA as MAFp3. The existence of mult iple potential sites for N-glycosylation and calcium binding suggests a direct involvement of MAFp4 in the species-specific aggregation of s ponge cells. The deduced partial polypeptide consists of a 16-fold rei terated motif that shows significant similarity to a repeat in an endo glucanase from the symbiontic bacterium Azorhizobium caulinodans and t o the intracellular loop of mammalian Na+-Ca2+ exchangers. Restriction fragment length polymorphism analysis indicated that the genomic vari ability of MAFp4 is high and comparable to that of MAFp3. Their combin ed polymorphism correlates with allogeneic responses studied in a popu lation of 23 sponge individuals. Peptide mass fingerprinting of trypti c digests of the polymorphic MAFp3 bands observed on polyacrylamide ge ls after chemical deglycosylation of the Microciona aggregation factor revealed that the variability detected on Southern blots at least par tially reflects the individual variability of aggregation factor prote in components. Polyclonal antibodies raised against MAFp3 strongly cro ssreacted with a 68-kDa protein localized in sponge cell membranes. Im munohistochemical use of the anti-MAFp3 antibodies strongly stained a cell layer along the line of contact in allogeneic grafts. We show tha t the transcription level of the MAFp3/MAFp4 mRNA in sponge allo- and isografts is clearly increased in comparison with non-grafted tissue. These data are discussed with respect to a possible evolutionary relat ionship between cell adhesion and histocompatibility systems.