A homeotic mutation in the trithorax SET domain impedes histone binding

Abstract

Trithorax (TRX) is a Drosophila SET domain protein that is required for the correct expression of homeotic genes. Here, we show that the TRX SET domain efficiently binds to core histones and nucleosomes. The primary target for the SET domain is histone H3 and binding requires the N-terminal histone tails. The previously described trx Z11 mutation changes a strictly conserved glycine in the SET domain to serine and causes homeotic transformations in the fly. We found that this mutation selectively interferes with histone binding, suggesting that histones represent a critical target during developmental gene regulation by TRX. The Polycomb group (PcG) of repressors and trithorax group (trxG) of activators target chromatin in order to "freeze" a mitotically stable pattern of gene expression and determined cell fate (Pirrotta 1998;Lyko and Paro 1999;Mahmoudi and Verrijzer 2001). The founding member of the trxG, the Drosophila trx gene, is required throughout development and controls the expression of several developmental regulators, including the homeotic genes (Ingham and Whittle 1980;Ingham 1985;Breen 1999). trx is related to the human Mixed Lineage Leukemia (MLL) gene, which is involved in translocations associated with the majority of cases of infant leukemias (Waring and Cleary 1997). TRX and MLL are part of a highly conserved regulatory network that is required for the correct expression of the homeotic selector genes and determination of segment identity in both mammals and Drosophila. They are very large proteins that contain structural motifs common to chromatin-associated factors such as PHD fingers and a C-terminal SET domain ( Fig. 1A; Mazo et al. 1990;Stassen et al. 1995).The SET domain is a highly conserved 130-150 amino acids motif initially recognized as a common element in chromatin regulators with opposing activities: the suppressor of position affect variegation Su(var)3-9, the PcG protein Enhancer of Zeste [E(z)], and TRX (Jenuwein et al. 1998). The SET domain has been implicated in a multitude of different protein-protein interactions and functions. The SET domains of MLL, yeast Set1p, and E(z) bind to myotubularin-related dual-specificity phosphatases and anti-phosphatases that modulate growth control (Cui et al. 1998). The TRX and MLL SET domains bind to the SNF5 component of the ATP-dependent remodeler SWI/SNF (Rozenblatt-Rosen et al. 1998) and mediate self-association (Rozovskaia et al. 2000). Furthermore, the SET domain of yeast Set1p binds the Mec3p checkpoint protein and has been implicated in DNA repair and telomere function (Corda et al. 1999). The Set1p SET domain alone suffices to mediate telomeric silencing, suggesting that it forms a functional unit (Nislow et al. 1997). Recently, it was shown that SUV39H1, the mammalian homolog of Su(var)3-9, selectively methylates lysine 9 of histone H3 (Rea et al. 2000;Jenuwein 2001). This modification creates a binding site for HP1 and thus can contribute to the propagation of a heterochromatin domain (Bannister et al. 2001;Lachner et al. 2001;...

MLL associates specifically with a subset of transcriptionally active target genes

“…We found that MLL preferentially methylates histone tails that are already acetylated (15). In addition, the trx SET domain binds more avidly to acetylated nucleosomes (40). Another possibility is that the WDR5 protein, a component of the MLL complex, targets MLL to loci dimethylated at histone H3 Lys-4 (29).…”

Histone chaperone ASF1 cooperates with the Brahma chromatin-remodelling machinery

“…GST pull-down experiments with Drosophila core histones were performed as described (Katsani et al 2001).…”

ARABIDOPSIS TRITHORAX1 Dynamically RegulatesFLOWERING LOCUS CActivation via Histone 3 Lysine 4 Trimethylation

“…ATX1 is predicted to contain a SET domain (AlvarezVenegas et al, 2003) that has both histone binding and HMT activity (Rea et al, 2000;Katsani et al, 2001). In vitro assays demonstrated that H3K4 is a substrate for ATX1's HMT activity, while mutant isoforms of ATX1 lacking part of the SET domain have no activity (Alvarez-Venegas et al, 2003).…”