Molecular evolution of the metazoan extracellular matrix: Cloning and expression of structural proteins from the demosponges Suberites domuncula and Geodia cydonium

One crucial event during evolution to multicellularity was the development of either direct cell-cell contact or indirect interaction via extracellula r matrix (ECM) molecules. The identification of those polypeptides provides conclusive data on the phylogenetic relationship of metazoan phyla and hel ps us to understand the position of the Metazoa among the other kingdoms. R ecently it became evident that the ECM of sponges is amazingly complex; it is composed of fibrous molecules, e.g., collagen, and their corresponding r eceptors, which are highly similar to those existing in other metazoan phyl a. While these data already support the view of monophyly of Metazoa, addit ional studies are required to understand whether these molecules, which are similar in their primary sequence, also have the same function throughout the metazoan kingdom. In the present study we identified the ligand for one of the autopomorphic characters of Metazoa, the single-transmembrane recep tor protein with the receptor tyrosine kinase (RTK) from G. cydonium, as an example: the putative mucus-like protein from G. cydonium. This protein wa s upregulated during autograft fusion in the homologous system with kinetic s similar to those of the RTK. Additionally, a cDNA was isolated from S. do muncula whose deduced polypeptide displays a high sequence similarity to de rmatopontin, an ECM molecule found exclusively in Metazoa. Furthermore. it is documented that expression of the fibrous ECM molecule collagen is regul ated by the characteristic metazoan morphogens myotrophin and endothelial m onocyte-activating polypeptide. These data indicate that the ECM of sponges is not an unstructured ground substance but provides the basis for integra ted cell communication.