IQGAP3 is essential for cell proliferation and motility during zebrafish embryonic development

IQGAPs are scaffolding proteins that regulate actin assembly, exocyst function, cell motility, morphogenesis, adhesion and division. Vertebrates express 3 family members: IQGAP1, IQGAP2, and IQGAP3. IQGAP1 is known to stimulate nucleation of branched actin filaments through N-WASP and the Arp2/3 complex following direct binding to cytoplasmic tails of ligand-activated growth factor receptors, including EGFR, VEGFR2 and FGFR1. By contrast, little is known about functions of IQGAP2 or IQGAP3. Using in situ hybridization on whole mount zebrafish (Danio rerio) embryos, we show that IQGAP1 and IQGAP2 are associated with discrete tissues and organs, while IQGAP3 is mainly expressed in proliferative cells throughout embryonic and larval development. Morpholino knockdowns of IQGAP1 and IQGAP2 have little effect on embryo morphology while loss of function of IQGAP3 affects both cell proliferation and cell motility. IQGAP3 morphant phenotypes are similar to those resulting from overexpression of dominant negative forms of Ras or of Fibroblast Growth Factor Receptor 1 (FGFR1), suggesting that IQGAP3 plays a role in FGFR1-Ras-ERK signaling. In support of this hypothesis, dominant negative forms of FGFR1 or Ras could be rescued by co-injection of zebrafish IQGAP3 mRNA, strongly suggesting that IQGAP3 acts as a downstream regulator of the FGFR1-Ras signaling pathway.