J. Ng et al., A Drosophila ESC-E(Z) protein complex is distinct from other polycomb group complexes and contains covalently modified ESC, MOL CELL B, 20(9), 2000, pp. 3069-3078
The extra sex combs (ESC) and Enhancer of zeste [E(Z)] proteins, members of
the Polycomb group (PcG) of transcriptional repressors, interact directly
and are coassociated in fly embryos. We report that these two proteins are
components of a 600-kDa complex in embryos. Using gel filtration and affini
ty chromatography, we show that this complex is biochemically distinct from
previously described complexes containing the PcG proteins Polyhomeotic, P
olycomb, and Sex comb on midleg. In addition, we present evidence that ESC
is phosphorylated in vivo and that this modified ESC is preferentially asso
ciated in the complex with E(Z). Modified ESC accumulates between 2 and 6 h
of embryogenesis, which is the developmental time when esc function is fir
st required. We And that mutations in E(z) reduce the ratio of modified to
unmodified ESC in vivo. We have also generated germ line transformants that
express ESC proteins bearing site-directed mutations that disrupt ESC-E(Z)
binding in vitro. These mutant ESC proteins fail to provide esc function,
show reduced levels of modification in vivo, and are still assembled into c
omplexes. Taken together, these results suggest that ESC phosphorylation no
rmally occurs after assembly into ESC-E(Z) complexes and that it contribute
s to the function or regulation of these complexes. We discuss how biochemi
cally separable ESC-E(Z) and PC-PH complexes might work together to provide
PcG repression.