L3MBTL1, a Histone-Methylation-Dependent Chromatin Lock

Abstract

Distinct histone lysine methylation marks are involved in transcriptional repression linked to the formation and maintenance of facultative heterochromatin, although the underlying mechanisms remain unclear. We demonstrate that the malignant-brain-tumor (MBT) protein L3MBTL1 is in a complex with core histones, histone H1b, HP1gamma, and Rb. The MBT domain is structurally related to protein domains that directly bind methylated histone residues. Consistent with this, we found that the L3MBTL1 MBT domains compact nucleosomal arrays dependent on mono- and dimethylation of histone H4 lysine 20 and of histone H1b lysine 26. The MBT domains bind at least two nucleosomes simultaneously, linking repression of transcription to recognition of different histone marks by L3MBTL1. Consistently, L3MBTL1 was found to negatively regulate the expression of a subset of genes regulated by E2F, a factor that interacts with Rb.

Histone H4 lysine 20 monomethylation promotes transcriptional repression by L3MBTL1

“…The presence of all three repeats in our studies, versus their use of the MBT2 and MBT3 repeats alone in a different peptide screening approach, may account for the observed differences. The lack of binding of 3xMBT to methylated lysines in histone H3 (K4, K9 or K27), in a pull-down assay in our previous report (Trojer et al, 2007), could reflect the use of histone peptides that were shorter in length (o10 amino acids) and less basic than the 15 mers used in this study. However, it is worth noting that a shorter peptide length did not seem to affect the binding of 3xMBT to H4K20me1, and may reflect the preservation of the basic amino-acid composition.…”

“…We conclude that 3xMBT contains a single binding site for the recognition of mono-(and di) methylated H3, H4 (and H1b (Trojer et al, 2007)) histone peptides.…”

“…Several recent studies have identified the methyl-lysine binding properties of the MBT domains in L3MBTL1 and D-Sfmbt; however, the specificity of such binding has not been well addressed Klymenko et al, 2006). We have shown that histone lysine methylation promotes 3xMBT-mediated chromatin compaction in vitro (Trojer et al, 2007), and now provide evidence that the cell cycle-regulated chromatin association of L3MBTL1 occurs as the H4K20me1 mark accumulates, and that the repressive properties of L3MBTL1 are enhanced by the H4K20 monomethylase PR-SET7.…”

“…We have recently shown that L3MBTL1 is bound to the cyclin E promoter (Trojer et al, 2007). As D-l(3)mbt interacts with a repressive E2F (Lewis et al, 2004), and as we find binding of L3MBTL1 to several E2Fs (Gurvich NG et al, unpublished data), we performed chromatin immunoprecipitation assays to determine whether L3MBTL1 and H4K20me1 colocalize to the E2F binding sites in the cyclin E promoter.…”

“…We have shown that L3MBTL1 functions as a transcriptional repressor and that L3MBTL1 is complexed with Rb and can bind to E2F target genes such as c-myc and cyclin E (Trojer et al, 2007). Furthermore, changes in L3MBTL1 expression have been shown to alter normal mitotic progression (Koga et al, 1999;Yohn et al, 2003).…”