Regulation Of Apoptosis In The Xenopus Embryo By Bix3

Members of the Bix family of homeobox-containing genes are expressed in the vegetal hemisphere of the Xenopus embryo at the early gastrula stage. Misexpression of at least some of the family members causes activation of mesoderm- and endoderm-specific genes and it is known that some of the proteins, including Bix2 and Bix3, interact with Smad proteins via a motif that is also present in the related protein Mixer. In this paper we study the function of Bix3. Misexpression of Bix3, similar to misexpression of other members of the Bix family, causes the activation of a range of mesendodermal genes, but the spectrum of genes induced by Bix3 differs from that induced by Bix1. More significantly, we find that overexpression of Bix3 also causes apoptosis, as does depletion of Bix3 by use of antisense morpholino oligonucleotides. The ability of Bix3 to causes apoptosis is not associated with its ability to activate transcription and nor with its possession of a Smad interaction motif. Rather, Bix3 lacks a C-terminal motif, which, in Bix1, acts in cis to inhibit apoptosis. Mutation of this sequence in Bix1 causes the protein to acquire apoptosis-inducing activity.