Alternative ESC and ESC-Like Subunits of a Polycomb Group Histone Methyltransferase Complex Are Differentially Deployed during Drosophila Development

Abstract

The Extra sex combs (ESC) protein is a Polycomb group (PcG) repressor that is a key noncatalytic subunit in the ESC-Enhancer of zeste [E(Z)] histone methyltransferase complex. Survival of esc homozygotes to adulthood based solely on maternal product and peak ESC expression during embryonic stages indicate that ESC is most critical during early development. In contrast, two other PcG repressors in the same complex, E(Z) and Suppressor of zeste-12 [SU(Z)12], are required throughout development for viability and Hox gene repression. Here we describe a novel fly PcG repressor, called ESC-Like (ESCL), whose biochemical, molecular, and genetic properties can explain the long-standing paradox of ESC dispensability during postembryonic times. Developmental Western blots show that ESCL, which is 60% identical to ESC, is expressed with peak abundance during postembryonic stages. Recombinant complexes containing ESCL in place of ESC can methylate histone H3 with activity levels, and lysine specificity for K27, similar to that of the ESC-containing complex. Coimmunoprecipitations show that ESCL associates with E(Z) in postembryonic cells and chromatin immunoprecipitations show that ESCL tracks closely with E(Z) on Ubx regulatory DNA in wing discs. Furthermore, reduced escl⁺ dosage enhances esc loss-of-function phenotypes and double RNA interference knockdown of ESC/ESCL in wing disc-derived cells causes Ubx derepression. These results suggest that ESCL and ESC have similar functions in E(Z) methyltransferase complexes but are differentially deployed as development proceeds.

We thank Nicole Francis and Dan Mallin for advice and help with the baculovirus system. We also thank Nicole Francis for providing H3/H4 tetramers used as the substrate in some of the HMTase assays. We are grateful to Jürg Müller for sharing information and helpful discussions about ESCL. We thank Martin Milner and Deborah Cottam for the gift of MCW12 cells and for advice concerning their culture. We appreciate the input and help provided by Carrie Ketel and Sarah Malmquist in generating several of the constructs used in this work.

This work was supported by National Institutes of Health (NIH) grants GM49850 to J.A.S. and GM046567 to R.S.J. N.J. was supported in part by NIH training grant HD07480.