SUMOylation is a covalent post-translational modification, whereby SUMO is attached by an iso-peptide linkage to lysine(s) residue in the target protein ( Bayer et al., 1998 Johnson, 2004 Mukhopadhyay and Riezman, 2007). Furthermore, small ubiquitin-like modifier (SUMO) protein modification of transcription factors has been associated with repression ( Geiss-Friedlander and Melchior, 2007 Gill, 2005 Hay, 2005 Yang and Sharrocks, 2004). Another pathway involves the inactivation of transcription mediated by protein–protein interactions, which prevents the assembly of RNA polymerase or general transcription factors required to establish the preinitiation complex at transcription start sites ( Breiling et al., 2001 Inostroza et al., 1992 Meisterernst and Roeder, 1991 Um et al., 1995). One pathway involves the recruitment of chromatin regulators, including chromatin remodeling complex, that cause localized histone deacetylation that ultimately results in transcriptional repression of specific genes ( Hassig et al., 1997 Kadosh and Struhl, 1998 Nan et al., 1998 Rundlett et al., 1998 Xue et al., 1998 Zhang et al., 1997). Transcriptional repression is mediated through several pathways ( Cowell, 1994 Johnson, 1995). Our understanding of the molecular network that establishes heterochromatin structure and triggers transcriptional repression remains far from complete. Heterochromatic regions are typically composed of repetitive sequences present at centromeres and telomeres, are usually late replicating and transcriptionally silent ( Buhler and Gasser, 2009 Fodor et al., 2010 Schoeftner and Blasco, 2009 Vermaak and Malik, 2009). Recent evidence has also indicated that HP1 proteins participate in transcriptional repression both in heterochromatin and euchromatin ( Hediger and Gasser, 2006 Kwon and Workman, 2008). Thus, HP1 proteins are implicated in transcriptional repression by establishing specialized, higher-order chromatin structures ( Eissenberg and Elgin, 2000 Kellum, 2003a Kellum, 2003b Kwon and Workman, 2008 Maison and Almouzni, 2004 Nielsen et al., 2001 Stewart et al., 2005). The heterochromatin protein HP1 binds to histone H3 trimethylated at lysine 9 (H3me3K9) and contributes to spreading of heterochromatin, resulting in silencing of gene expression at those sites ( Bannister et al., 2001 Jacobs and Khorasanizadeh, 2002 Lachner et al., 2001 Nielsen et al., 2002). Although gene activation is generally associated with euchromatic sites with increased histone acetylation, gene inactivation at heterochromatin is marked with methylation of histone H3 at lysine 9 and hypoacetylation of histones (for reviews, see Dillon and Festenstein, 2002 Hubner and Spector, 2010 Richards and Elgin, 2002). Taken together, our data demonstrate the role of a novel heterochromatin-associated protein in transcriptional repression.Īccurate control of gene expression is crucial for cell survival and is clearly dependent on the chromatin status ( Narlikar et al., 2002). Finally, overexpression of BEND3 causes premature chromatin condensation and extensive heterochromatinization, resulting in cell cycle arrest. We further demonstrate that BEND3 is SUMOylated and that such modifications are essential for its role in transcriptional repression. Furthermore, tethering BEND3 inhibits transcription from the locus, indicating that BEND3 is involved in transcriptional repression through its interaction with histone deacetylases and Sall4, a transcription repressor. Using an in vivo gene locus, we have been able to demonstrate that BEND3 associates with the locus only when it is heterochromatic and dissociates upon activation of transcription.
BEND3 colocalizes with HP1 and H3 trimethylated at K9 at heterochromatic regions in mammalian cells. We have identified BEND3, a quadruple BEN domain-containing protein that is highly conserved amongst vertebrates. Specific chromatin-interacting proteins play vital roles in the maintenance of chromatin structure.
In eukaryotes, higher order chromatin structure governs crucial cellular processes including DNA replication, transcription and post-transcriptional gene regulation.
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