Polycomb group (PcG) proteins form large multimeric complexes (PcG bodies) which are involved in the stable repression of gene expression. The human PcG protein, Pc2, has been shown to recruit the transcriptional corepressor, CtBP, to PcG bodies. We show that CtBP is sumoylated at a single lysine. In vitro, CtBP sumoylation minimally requires the SUMO E1 and E2 (Ubc9) and SUMO-1. However, Pc2 dramatically enhances CtBP sumoylation. In vivo, this is likely due to the ability of Pc2 to recruit both CtBP and Ubc9 to PcG bodies, thereby bringing together substrate and E2, and stimulating the transfer of SUMO to CtBP. These results demonstrate that Pc2 is a SUMO E3, and suggest that PcG bodies may be sumoylation centers.

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TG-interacting factor (TGIF) is

SUMMARYTgif1 and Tgif2 are transcriptional co-repressors that limit the response to TGFb signaling and play a role in regulating retinoic-acidmediated gene expression. Mutations in human TGIF1 are associated with holoprosencephaly, but it is unclear whether this is a result of deregulation of TGFb/Nodal signaling, or of effects on other pathways. Surprisingly, mutation of Tgif1 in mice results in only relatively mild developmental phenotypes in most strain backgrounds. Here, we show that loss-of-function mutations in both Tgif1 and Tgif2 result in a failure of gastrulation. By conditionally deleting Tgif1 in the epiblast, we demonstrate that a single wildtype allele of Tgif1 in the extra-embryonic tissue allows the double null embryos to gastrulate and begin organogenesis, suggesting that extra-embryonic Tgif function is required for patterning the epiblast. Genetically reducing the dose of Nodal in embryos lacking all Tgif function results in partial rescue of the gastrulation defects. Conditional double null embryos have defects in leftright asymmetry, which are also alleviated by reducing the dose of Nodal. Together, these data show that Tgif function is required for gastrulation, and provide the first clear evidence that Tgifs limit the transcriptional response to Nodal signaling during early embryogenesis.

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