SpKrl: a direct target of beta-catenin regulation required for endoderm differentiation in sea urchin embryos

Localization of nuclear beta -catenin initiates specification of vegetal fa tes in sea urchin embryos, We have identified SpKrl, a gene that is activat ed upon nuclear entry of beta -catenin, SpKrl is upregulated when nuclear b eta -catenin activity is increased with LiCl and downregulated in embryos i njected with molecules that inhibit beta -catenin nuclear function. LiCl-me diated SpKrl activation is independent of protein synthesis, indicating tha t SpKrl is a direct target of beta -catenin and TCF. Embryos in which SpKrl translation is inhibited with morpholino antisense oligonucleotides lack e ndoderm, Conversely, SpKrl mRNA injection rescues some vegetal structures i n beta -catenin-deficient embryos, SpKrl negatively regulates expression of the animalizing transcription factor, SpSoxB1, We propose that SpKrl funct ions in patterning the vegetal domain by suppressing animal regulatory acti vities.