Phosphorylation of Phospholipase Cγ1 and Its Association with the FGF Receptor is Developmentally Regulated and Occurs during Mesoderm Induction in Xenopus laevis

Abstract

We have examined phosphorylation of phospholipase Cγ1 (PLCγ1) and its association with FGFR1 during mesoderm induction in animal pole explants and during early development in Xenopus embryos. In explants, PLCγ1 became associated with FGFR1 during mesoderm induction by FGF or by vegetal cells, the source of the natural inducer. Both PLCγ1 and FGFR1 were phosphorylated on tyrosine, indicating that both proteins were activated. Phosphorylation of these two proteins occurred very early during the induction process (within 0.5 hr), providing evidence that a member of the FGF family is a component of the vegetal inducing signal. PLCγ1 was also associated with FGFR1 in Xenopus blastulae and this association was specific to presumptive mesoderm cells. Examination of the PLCγ1 phosphorylation pattern during early Xenopus development and its association with FGFR1 revealed that maximum phosphorylation and association of these two proteins occurred during early- to mid-blastula stages, concurrent with mesoderm induction in vivo. This spatiotemporal pattern PLCγ1-FGFR1 association and phosphorylation suggests that PLCγ1 is involved in intracellular signaling during mesoderm induction in Xenopus. Seven additional phosphotyrosyl bands were coimmunoprecipitated with either PLCγ1 or FGFR1 from Xenopus blastulae; these bands may represent additional components of an FGFR1 signaling complex. One of these phosphotyrosyl bands was identified as NCK. In addition, growth factor receptor-binding protein, and son-of-sevenless two upstream regulators of RAS signaling, were co-immunoprecipitated with FGFR1.