In the last two decades, histone modifications have been recognized as being pivotal to nearly all DNA templated processes in eukaryotes (for reviews, see Refs. 1
). Eukaryotic transcription is highly influenced by various histone modification events. Besides the well studied marks of histone acetylation, several histone marks have been linked to active transcription: mono-ubiquitination of histone H2BK123 (Lys-120 in vertebrates) and methylation of histones H3K4, H3K36, and H3K79 (for reviews, see Refs. 6
Yeast KMT3, the first enzyme identified as mediating H3K36 methylation (10
), is linked to transcription elongation via its interaction with the Ser-2-phosphorylated C-terminal domain of RNA polymerase II (11
) through its C-terminal SRI domain (15
). KMT3-mediated H3K36 methylation (H3K36me)4
serves as a docking site to recruit a histone deacetylase complex, Rpd3s, which in turn deacetylates the nucleosomes residing at the coding regions to ensure the fidelity of transcription initiation (16
). On the other hand, H3K36 methylation can also recruit the NuA3 histone acetyltransferase complex through its PhD finger domain-containing subunit Nto1 (20
). The means by which H3K36me-mediated recruitment of histone deacetylase and histone acetyltransferase activities is balanced remains an interesting question. However, NuA3 interacts with the Set1 methyltransferase, and NuA3 function depends on both Set1 and its substrate H3K4 (20
). Therefore a possibility is that these histone deacetylase and histone acetyltransferase complexes are differentially recruited depending on the co-presence of H3K4me and H3K36me within the same or neighboring nucleosomes.
In higher eukaryotes, H3K36 methylation is also linked with active transcription (22
). Interestingly, reduced levels of H3K36 methylation lead to subsequent reductions in H4K16 acetylation (23
), which further impacts dosage compensation in Drosophila
). In mammals, methylation of H3K36 is also mediated by the NSD family of enzymes (26
and NSD family HKMTs share high sequence similarity with yeast KMT3 only at the Set domain, but not other regions. In contrast, KMT3a (also known as HYPB or hSet2) is the mammalian orthologue of yeast KMT3, which shares high sequence similarity with yeast KMT3a at the Set domain, as well as at the WW and SRI domains. KMT3a specifically methylates H3K36 (27
). Interestingly, unlike the yeast KMT3, knockdown of KMT3a results in the reduction of H3K36me3 exclusively, without affecting the other H3K36 methylation states (me1 and me2) in mammals (28
) as well as in Drosophila
Here we report the purification of human KMT3a complex and the identification of a novel, higher eukaryotic specific subunit, heterogeneous nuclear ribonucleoprotein L (HnRNP-L). Interestingly, although KMT3a has intrinsic activity in vitro, RNAi against HnRNP-L leads to the reduction of H3K36me3 in vivo, without affecting KMT3a expression levels.
HnRNP-L is a member of a large family of RRM domain-containing RNA-binding proteins. It has been implicated in various RNA-related processes, particularly in mediating selective exon inclusion during alternative splicing (29
) and facilitating polyadenylation (32
). These so-called “post-transcriptional” events are known to be “co-transcriptional” (34
) and occur proximal to the chromatin templates. Our finding that HnRNP-L plays a role in regulating chromatin modification suggests the existence of an elaborative cross-talk between the chromatin template and co-transcriptional pre-mRNA processing.