The E3 Ubiquitin Ligase GREUL1 Anteriorizes Ectoderm during Xenopus Development

We have identified a family of RING finger proteins that are orthologous to Drosophila Goliath (G1, Gol). One of the members, GREUL1 (Goliath Related E3 Ubiquitin Ligase 1), can convert Xenopus ectoderm into XAG-1- and Otx2-expressing cells in the absence of both neural tissue and muscle. This activity, combined with the finding that XGREUL1 is expressed within the cement gland, suggests a role for GREUL1 in the generation of anterior ectoderm. Although GREUL1 is not a direct inducer of neural tissue, it can activate the formation of ectopic neural cells within the epidermis of intact embryos. This suggests that GREUL1 can sensitize ectoderm to neuralizing signals. In this paper, we provide evidence that GREUL1 is an E3 ubiquitin ligase. Using a biochemical assay, we show that GREUL1 catalyzes the addition of polyubiquitin chains. These events are mediated by the RING domain since a mutation in two of the cysteines abolishes ligase activity. Mutation of these cysteines also compromises GREUL1's ability to induce cement gland. Thus, GREUL1's RING domain is necessary for both the ubiquitination of substrates and for the conversion of ectoderm to an anterior fate.