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The multi-domain protein Np95 connects DNA methylation and histone modification
21.12.2009
DNA methylation and histone modifications play
a central role in the epigenetic regulation of gene
expression and cell differentiation. Recently, Np95
(also known as UHRF1 or ICBP90) has been found
to interact with Dnmt1 and to bind hemimethylated
DNA, indicating together with genetic studies a
central role in the maintenance of DNA methylation.
Using in vitro binding assays we observed a
weak preference of Np95 and its SRA (SET- and
Ring-associated) domain for hemimethylated CpG
sites. However, the binding kinetics of Np95 in
living cells was not affected by the complete loss
of genomic methylation. Investigating further links
with heterochromatin, we could show that Np95
preferentially binds histone H3 N-terminal tails
with trimethylated (H3K9me3) but not acetylated
lysine 9 via a tandem Tudor domain. This domain
contains three highly conserved aromatic amino
acids that form an aromatic cage similar to the
one binding H3K9me3 in the chromodomain of
HP1ß. Mutations targeting the aromatic cage of
the Np95 tandem Tudor domain (Y188A and Y191A)
abolished specific H3 histone tail binding. These
multiple interactions of the multi-domain protein
Np95 with hemimethylated DNA and repressive
histone marks as well as with DNA and histone
methyltransferases integrate the two major
epigenetic silencing pathways.
