A single KH domain in Bicaudal-C links mRNA binding and translational repression functions to maternal development.

Bicaudal-C (Bicc1) is a conserved RNA binding protein that represses the translation of selected mRNAs to control development. In Xenopus embryos Bicc1 binds and represses specific maternal mRNAs to control anterior-posterior cell fates. However, it is not known how Bicc1 binds its RNA targets or how binding affects Bicc1-dependent embryogenesis. Focusing on the KH domains, we analyzed Bicc1 mutants for their ability to bind RNA substrates in vivo and in vitro Analyses of these Bicc1 mutants demonstrated that a single KH domain, KH2 was critical for RNA binding in vivo and in vitro , while the KH1 and KH3 domains contributed minimally. The Bicc1 mutants were also assayed for their ability to repress translation, and results mirrored the RNA binding data, with KH2 being the only domain essential for repression. Finally, maternal knock-down and rescue experiments indicated that the KH domains were essential for Bicc1's regulation of embryogenesis. These data advance our understanding of how Bicc1 selects target mRNAs and provide the first direct evidence that Bicc1's RNA binding functions are essential for both Bicc1-dependent translational repression and maternal vertebrate development.