Molecular cloning of a tyrosine kinase gene from the marine spongeGeodia cydonium: a new member belonging to the receptor tyrosine kinase class II family

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Abstract
We have isolated and characterized a cDNA from the marine sponge Geodia cydonlum coding for a new member of the tyrosine protein kinase (TK) family. The cDNA encodes a protein of Mr = 68 710, termed GCTK, which is homologous to class II receptor tyrosine kinases (RTKs). GCTK contains conserved amino acids (aa) characteristic of all protein kinases, and the sequences DLATRN and PIRWMATE which are highly specific for TKs. Furthermore, the sequence N-L-Y-x(3)-Y-Y-R Is highly homologous to the sequence D-[LIV]-Y-x(3)-Y-Y-R found only in class II RTKs. The sponge TK, when compared with mammalian class II RTKs, shows maximum 31% homology in the TK domain indicating that this the oldest member of class II RTK started to diverge from the common ancestral protein kinase 650 million years ago. Using GCTK as a probe we identified three mRNA signals ranging from 2μ6 to 0μ6 kb. Kinase activity was localized only in the cell membranes from G. cydonium (Mr = 65 000), and was not detected in the cytosol of this organism. Antibodies raised against a synthetic peptide, corresponding to the aa residues within the catalytic domain of the sponge TK, recognized strongly two proteins of Mr = 65 000; these proteins, present in membrane fractions, also bound to the anti-phosphotyrosine antibody. These data suggest that the TK cloned from the sponge is a membrane-associated 65 kDa protein. Moreover these results demonstrate that RTKs are present from the lowest group of multicellular eukaryotes, sponges, to mammals, and may suggest that RTKs are involved in a signal transduction pathway.