XFGF-9 - A NEW FIBROBLAST GROWTH-FACTOR FROM XENOPUS EMBRYOS

We have identified the Xenopus homologue of mammalian FGF-9 (XFGF-9). Sequence comparison between Xenopus and mammals shows that they share 93% identity at the amino acid level, malting FGF-9 the most highly co nserved member within the family, The sequence shows that there is no N-terminal signal sequence but that there is an internal hydrophobic s equence resembling a transmembrane domain, By using an in vitro transl ation system, we demonstrate that XFGF-9 can be glycosylated by micros omes but shows no signal peptide cleavage, This suggests that it can b e secreted using the internal hydrophobic domain to cross the endoplas mic reticulum membrane. Expression studies using RNAse protections and in situ hybridization show that XFGF-9 is expressed both maternally a nd zygotically. The maternal mRNA is detected at a higher level than o ther forms (XFGF-2 and eFGF), mainly in the animal hemisphere. A propo rtion of the maternal transcript persists until the early gastrula sta ge when it is joined by zygotic expression around the blastopore regio n, and thereafter the mRNA content shows some increase during further development, Zygotic XFGF-9 is expressed uniformly along the dorsal ax is, as well as in the head region. We have expressed recombinant XFGF- 9 protein in bacteria, and show that it has a mesoderm-inducing activi ty in the animal cap assay, with a similar specific activity to other fibroblast growth factor (FGFs), We have injected a synthetic mRNA int o eggs, and show that it has both mesoderm-inducing activity in animal caps and also a posteriorizing activity in whole embryos, The levels of biological activity shown by the XFGF-9 mRNA injections compared to XFGF-2 and eFGF show that there is at least some extracellular functi on, This supports the biochemical results, suggesting that the protein has at least some capacity to be secreted. (C) 1996 Wiley-Liss, Inc.