SPFGFR, A NEW MEMBER OF THE FIBROBLAST GROWTH-FACTOR RECEPTOR FAMILY,IS DEVELOPMENTALLY-REGULATED DURING EARLY SEA-URCHIN DEVELOPMENT

We describe the cloning of a new fibroblast growth factor receptor, Sp FGFR1, that is differentially regulated at the level of transcript abu ndance during sea urchin embryogenesis. Sequence representing the cons erved tyrosine kinase domain was obtained by reverse transcription-pol ymerase chain reaction using degenerate primers, and the entire open r eading frame was obtained by standard cDNA library screening methods. SpFGFR contains a series of domains characteristic of FGFRs: three imm unoglobulin-like motifs, an acid box, a transmembrane domain, a relati vely long juxtamembrane sequence, a split tyrosine kinase domain, and two conserved intracellular tyrosine residues. Alternative splicing of SpFGFR generates two variants (Ig3L and Ig3S), which differ by insert ion in the center of the Ig3 domain of 34 extra amino acids, encoded b y an additional exon. Transcripts encoding both variants accumulate wh en morphogenesis beans with mesenchyme cell ingression and gastrulatio n. SpFGFR transcripts accumulate in all cell types of the embryo, alth ough in situ hybridization shows that they are somewhat enriched in ce lls of oral ectoderm and endoderm. Transcripts encoding the Ig3S varia nt, whose structure resembles more closely that of vertebrate receptor s, are enriched in endomesoderm, suggesting that the SpFGFR variants c ould play distinct roles in the sea urchin embryo.