Abstract
BackgroundMethylation of histone H4 on lysine 20 plays critical roles in chromatin structure and function via mono- (H4K20me1), di- (H4K20me2), and trimethyl (H4K20me3) derivatives. In previous analyses of histone methylation dynamics in mid-gestation mouse embryos, we documented marked changes in H4K20 methylation during cell differentiation. These changes were particularly robust during myogenesis, both in vivo and in cell culture, where we observed a transition from H4K20me1 to H4K20me3. To assess the significance of this change, we used a gain-of-function strategy involving the lysine methyltransferases SUV420H1 and SUV420H2, which catalyze H4K20me2 and H4K20me3. At the same time, we characterized a second isoform of SUV420H1 (designated SUV420H1_i2) and compared the activity of all three SUV420H proteins with regard to localization and H4K20 methylation.Principal FindingsImmunofluorescence revealed that exogenous SUV420H1_i2 was distributed throughout the cell, while a substantial portion of SUV420H1_i1 and SUV420H2 displayed the expected association with constitutive heterochromatin. Moreover, SUV420H1_i2 distribution was unaffected by co-expression of heterochromatin protein-1α, which increased the targeting of SUV420H1_i1 and SUV420H2 to regions of pericentromeric heterochromatin. Consistent with their distributions, SUV420H1_i2 caused an increase in H4K20me3 levels throughout the nucleus, whereas SUV420H1_i1 and SUV420H2 facilitated an increase in pericentric H4K20me3. Striking differences continued when the SUV420H proteins were tested in the C2C12 myogenic model system. Specifically, although SUV420H1_i2 induced precocious appearance of the differentiation marker Myogenin in the presence of mitogens, only SUV420H2 maintained a Myogenin-enriched population over the course of differentiation. Paradoxically, SUV420H1_i1 could not be expressed in C2C12 cells, which suggests it is under post-transcriptional or post-translational control.ConclusionsThese data indicate that SUV420H proteins differ substantially in their localization and activity. Importantly, SUV420H2 can induce a transition from H4K20me1 to H4K20me3 in regions of constitutive heterochromatin that is sufficient to enhance myogenic differentiation, suggesting it can act an as epigenetic ‘switch’ in this process.
Highlights
An essential component of chromatin biology involves the posttranslational modification of the core histones H2A, H2B, H3 and H4
SUV420H2 can induce a transition from H4K20me1 to H4K20me3 in regions of constitutive heterochromatin that is sufficient to enhance myogenic differentiation, suggesting it can act an as epigenetic ‘switch’ in this process
Analyses in mid-gestation mouse embryos reveal a marked elevation of H4K20me3 in muscle and neural lineages and a corresponding decrease in H4K20me1 as cells became post-mitotic [21]. These findings suggest that the balance of histone H4K20 methylation states is important in the transition from cell proliferation to differentiation, which is further supported by the decrease in H4K20me3 that occurs in malignancy [23,24,25]
Summary
An essential component of chromatin biology involves the posttranslational modification of the core histones H2A, H2B, H3 and H4 This can involve the addition of acetyl, methyl, phosphate, ADP-ribosyl, or ubiquitin moieties to over 30 target sites, which is further extended by the placement of multiple methyl groups on lysine and arginine [1,2,3,4]. In the case of lysine methylation, this involves the transfer of up to three methyl groups from S-adenosylmethionine to create mono-, di-, and trimethyl derivatives [5,6] These lysine modifications are limited to histones H3, where they can occur on multiple residues, and H4, which contains a single target site at position 20 (H4K20) [6]. We characterized a second isoform of SUV420H1 (designated SUV420H1_i2) and compared the activity of all three SUV420H proteins with regard to localization and H4K20 methylation
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