The LSD1-CoREST histone demethylase complex is required to repress neuronal genes in nonneuronal tissues. Here we show that sumoylation of Braf35, one of the subunits of the complex, is required to maintain full repression of neuron-specific genes and for occupancy of the LSD1-CoREST complex at its gene targets. Interestingly, expression of Braf35 was sufficient to prevent neuronal differentiation induced by bHLH neurogenic transcription factors in P19 cells and in neuronal progenitors of the chicken embryo neural tube. Sumoylation of Braf35 is required for this antineurogenic activity. We also show that iBraf, a paralogue of Braf35, forms heterodimers with Braf35. Braf35-iBraf heterodimerization impairs Braf35 interaction with the LSD1-CoREST complex and inhibits Braf35 sumoylation. Consistent with these results, iBraf prevents the antineurogenic activity of Braf35 in vivo. Our data uncover a mechanism of regulation of the LSD1-CoREST complex and provide a molecular explanation for the antagonism between Braf35 and iBraf in neuronal differentiation.C ell differentiation involves large modifications of gene expression that require extensive changes of chromatin epigenetic marks (1). Epigenetic marks are DNA or histone posttranslational modifications that are inherited through cell division and that inform about the transcriptional state of loci. Among histone modifications, histone lysine methylation is of particular interest in development for the broad range of processes in which it is involved, including maintenance of stem cell pluripotency, germ-line determination, cell differentiation, control of HOX genes expression, and so forth (2, 3). Histone lysine methylation was considered a stable posttranslational modification until the discovery of histone demethylases. LSD1/KDM1 (lysine-specific demethylase 1) was the first demethylase identified and catalyzes demethylation of both di-and monomethylated lysine 4 (K4) or lysine 9 (K9) of histone H3 (H3K4me2/1 or H3K9me2/1) (4, 5). The lysine specificity of LSD1 seems to depend on its molecular partners. Thus, when LSD1 is associated with CoREST in the LSD1-CoREST corepressor complex (also called BHC, BRAF-histone deacetylase complex), the preferred substrate is H3K4me2/1, consistent with the fact that methylation of H3K4 is a mark of transcriptionaly active genes. In addition to LSD1 and CoREST, the LSD1-CoREST complex also contains HDAC1-2, BHC80, and BRAF35 (also called HMG20B) (6-9). BRAF35 contains a high-mobility group (HMG) domain and a coiled-coil domain, but its function within the complex is not well-understood (7, 10). Several functions of the LSD1-CoREST complex in differentiation and development have been reported (2). One of the best-characterized functions of the complex is its role in repression of neuronal genes in nonneuronal tissues and neuronal progenitors through its interaction with repressor factor REST (RE1 silencing transcription factor) (11,12). iBRAF (inhibitor of BRAF35, also called HMG20A) is a close paralogue of BRAF35 (13). As BRAF35...