Phosphorylation of SU(VAR)3-9 by the chromosomal kinase JIL-1

The histone methyltransferase SU(VAR)3-9 plays an important role in
the formation of heterochromatin within the eukaryotic nucleus.
Several studies have shown that the formation of condensed
chromatin is highly regulated during development, suggesting that
SU(VAR)3-9's activity is regulated as well. However, no mechanism
by which this may be achieved has been reported so far. As we and
others had shown previously that the N-terminus of SU(VAR)3-9 plays
an important role for its activity, we purified interaction
partners from Drosophila embryo nuclear extract using as bait a GST
fusion protein containing the SU(VAR)3-9 N-terminus. Among several
other proteins known to bind Su(VAR)3-9 we isolated the chromosomal
kinase JIL-1 as a strong interactor. We show that SU(VAR)3-9 is a
substrate for JIL-1 in vitro as well as in vivo and map the site of
phosphorylation. These findings may provide a molecular explanation
for the observed genetic interaction between SU(VAR)3-9 and JIL-1.