Cloning and expression of new receptors belonging to the immunoglobulin superfamily from the marine sponge Geodia cydonium

A cDNA encoding a receptor tyrosine kinase (RTK) was previously cloned and expressed from the marine sponge (Porifera) Geodia cydonium. In addition to the two intracellular regions characteristic for RTKs, two immunoglobulin (Ig)-like domains are found in the extracellular part of the sponge RTK. In the present study it is shown that no further Ig-like domain is present in the upstream region of the cDNA as well as of the gene hitherto known from the sponge RTK. Two different full-length cDNAs have been isolated and cha racterized in the present study, which possess two Ig-like domains, one tra nsmembrane segment, and only a short intracellular part, without a TK domai n. The two deduced polypeptides were preliminarily termed sponge adhesion m olecules (SAM). The longer form of the SAM, GCSAML, encodes a deduced aa se quence, GCSAML, which comprises in the open reading frame 505 amino acids ( aa) and has a calculated M-r of 53911. The short form, GCSAMS, has 313 aa r esidues and an M-r of 33987. The two Ig-like domains in GCSAML and GCSAMS a re highly similar to the corresponding Ig-like domains in the RTKs from G. cydonium; the substitutions on both the aa and nt level are restricted to a few sites. Phylogenetic analyses revealed that the Ig-like domain 1 is sim ilar to the human Ig lambda chain variable region, while the Ig-like domain 2 is related more closely to the human Ig heavy chain variable region. Tra nsplantation experiments (autografting) were performed to demonstrate that the level of expression of the two new genes, GCSAML and GCSAMS, is upregul ated during the self/self fusion process. Immunohistochemical analyses usin g antibodies raised against the two Ig-like domains demonstrate a strong ex pression in the fusion zone between graft and host. This finding has been s upported by northern blotting experiments that revealed that especially GCS AML is strongly upregulated after autografting (up to 12-fold); the express ion of GCSAMS reaches a value of 5-fold if compared with the controls. The results presented here demonstrate that the expression of the new molecules described, comprising two Ig-like domains, is upregulated during the proce ss of autograft fusion.