The Mediator complex protein Med31 is required for embryonic growth and cell proliferation during mammalian development.

During development, the mammalian embryo must integrate signals to control growth and proliferation. A failure in the ability to respond to mitogenic stimuli can cause embryonic growth restriction. We have identified a mouse mutant, l11Jus15, from a mutagenesis screen that exhibits growth defects and late-gestation lethality. Here we demonstrate that this phenotype results from a mutation in the Mediator complex gene Med31, which causes degradation of Med31 protein. The Med31 mutant phenotype is not similar to other Mediator complex mouse mutants, and target genes of other Mediator proteins are expressed normally in Med31 mutants, suggesting that Med31 has distinct target genes required for mammalian development. Med31 mutant embryos have fewer proliferating cells than controls, especially in regions that expand rapidly during development such as the forelimb buds. Likewise, embryonic fibroblast cells cultured from mutant embryos have a severe proliferation defect, as well as reduced levels of the cell cycle protein Cdc2. Med31 mutants have normal limb bud patterning but defective or delayed chondrogenesis due to a lack of Sox9 and Col2a1 expression. As the Mediator complex is a transcriptional co-activator, our results suggest that Med31 functions to promote the transcription of genes required for embryonic growth and cell proliferation.