H3 Lys-79 methyltransferases

Lys-9 methyltransferase, DIMS, is similar in sequence to Clr4 and Su(var)3-9 and has a SET domain flanked by cysteine rich elements [191]. Murine ESET (ERG-associated protein with SET domain) has these domains and methylates free H3 [192]. This murine methyltransferase, which has high sequence similarity to human SETDBl (SET domain, bifurcated 1), interacts with the transcription factor ERG. SETBl, a specific H3 Lys-9 methyltransferase, interacts with KAP-1 copressor, which binds to KRAB domain zinc-finger proteins [193]. SETBl methylates Lys-9 when Lys-4 is methylated but enzymatic activity is inhibited when Ser-10 is phosphorylated or when Lys-14 is acetylated. 

Mammalian G9a is a SET-domain histone methyltransferase that methylated H3 at Lys-9 and Lys-27 in vitro and at Lys-9 in vivo [196]. The consensus sequence for G9a appears to be TKXXARKS. G9a is dilferent from Suv39hl is several ways. G9a nuclear localization is distinct from that of Suv39hl, which locate to heterochromatic foci [197]. Suv39hlj2 double mutant mice lose H3 Lys-9 methylation at pericentromeric heterochromatic regions but broad methylation of chromatin remains. It is the latter that is lost in GPa-deficient cells [196]. G9a, molecular mass about 100 kDa, methylates free H3 and nucleosomal H3 with a preference for the former however, the presence of H1 stimulates the methylation of chromatin substrates. Suv39hl, molecular mass about 650 kDa, methylates free H3 and H3 in nucleosomes with equivalent efficiency, but when HI is present, methylation of chromatin substrates is lessened [198]. 

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Histone methyltransferases may associate or act cooperatively with either HATs or HDACs. In fission yeast the H3 Lys-14 deacetylase Clr3 interacts functionally with H3 Lys-9 methyltransferase Clr4. Clr4 methylates Lys-9 of H3, a process facilitated by Rikl, resulting in the recruitment of Swi6 and heterochromatin assembly [157,221]. In Drosophila SU(VAR)3-9 H3 Lys-9 methyltransferase is in complex with HDACl [222]. Thus, HDACl would deacetylate acetylated Lys-9 allowing methylation by SU(VAR)3-9 at this site to occur. CBP, a potent HAT, is associated with a histone methyltransferase that methylated H3 at Lys-9 and to a lesser extent Lys-4. H3 methylation at Lys-9 did not alter the HAT activity of CBP, and vice versa acetylation of H3 (predominantly Lys-14) did not affect the associated histone methyltransferase activity [223]. 

The mammalian inactive X-chromosomes in females is associated with H3 methylated at Lys-9 but does not have H3 methylated at Lys-4 [165,166]. Deletion of murine Suv39h genes does not affect H3 Lys-9 methylation at the inactive X-chromosome. Thus, different histone methyltransferases are involved in the methylation of H3 Lys-9 in constitutive and facultative heterochromatin [166]. Further, HPl is not associated with the inactive X-chromosome. Methylation of H3 Lys-9 is an early event in X inactivation in mammals [167,168]. The H3 Lys-9 methylation occurs approximately at the same time as H3 Lys-9 hypoacetylation and Lys-4 hypomethylation, and happens before transcriptional inactivation of the X-linked genes. Upstream of the Xist gene, which codes for a strictly nuclear RNA involved in X inactivation, is a constitutive hotspot of H3 methylated at Lys-9. This early event of H3 Lys-9 methylation occurs simultaneously with or immediately following the association of Xist RNA with the X-chromosome. It has been proposed that the hotspot upstream of the Xist gene serves as a nucleation site for the spreading of Xist RNA and methylated Lys-9 H3 [167]. 

In vitro studies with unmodified and modified N-terminal peptides of H3 demonstrated that Lys-14 acetylation did not interfere with methylation at Lys-9 by Suv39hl, while phosphorylation at Ser-10 and acetylation at Lys-9 did (Fig. 7). Further dimethylation of Lys-9 reduced enzymatic activity [186], A Suv39h double null primary mouse embryonic fibroblasts had higher levels of Ser-10 phos-phorylated H3 than wild type cells. These mutant cells had increased numbers of micro- and polynuclei. Oversized nuclei were characteristic of subpopulation of cells. The level of Lys-9 methylated FI3 in wild type cells and Suv39h double null cells was similar, demonstrating that other FI3 methyltransferases were involved [195]. Phosphorylation of Ser-10 by Ipll/aurora was also studied. Acetylation at Lys-14 promoted the activity of the mitotic kinase, while dimethylation, but not acetylation at Lys-9, reduced activity of the kinase [186]. 

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