PHOSPHORYLATION OF PHOSPHOLIPASE C-GAMMA-1 AND ITS ASSOCIATION WITH THE FGF RECEPTOR IS DEVELOPMENTALLY-REGULATED AND OCCURS DURING MESODERM INDUCTION IN XENOPUS-LAEVIS

We have examined phosphorylation of phospholipase C gamma 1 (PLC gamma 1) and its association with FGFRI during mesoderm induction in animal pole explants and during early development in Xenopus embryos. In exp lants, PLC gamma 1 became associated with FGFRI during mesoderm induct ion by FGF or by vegetal cells, the source of the natural inducer. Bot h PLC gamma 1 and FGFR1 were phosphorylated on tyrosine, indicating th at both proteins were activated. Phosphorylation of these two proteins occurred very early during the induction process (within 0.5 hr), pro viding evidence that a member of the FGF family is a component of the vegetal inducing signal. PLC gamma 1 was also associated with FGFR1 in Xenopus blastulae and this association was specific to presumptive me soderm cells. Examination of the PLC gamma 1 phosphorylation pattern d uring early Xenopus development and its association with FGFR1 reveale d that maximum phosphorylation and association of these two proteins o ccurred during early- to mid-blastula stages, concurrent with mesoderm induction in vive. This spatiotemporal pattern PLC gamma 1-FGFR1 asso ciation and phosphorylation suggests that PLC gamma 1 is involved in i ntracellular signaling during mesoderm induction in Xenopus. Seven add itional phosphotyrosyl bands were coimmunoprecipitated with either PLC gamma 1 or FGFR1 from Xenopus blastulae; these bands may represent ad ditional components of an FGFR1 signaling complex. One of these phosph otyrosyl bands was identified as NCK. In addition, growth factor recep tor-binding protein, and son-of-sevenless two upstream regulators of R AS signaling, were co-immunoprecipitated with FGFR1. (C) 1994 Academic Press, Inc.