The Enhancer of Trithorax and Polycomb Corto Interacts with Cyclin G in Drosophila

Abstract

Background Polycomb (PcG) and trithorax (trxG) genes encode proteins involved in the maintenance of gene expression patterns, notably Hox genes, throughout development. PcG proteins are required for long-term gene repression whereas TrxG proteins are positive regulators that counteract PcG action. PcG and TrxG proteins form large complexes that bind chromatin at overlapping sites called Polycomb and Trithorax Response Elements (PRE/TRE). A third class of proteins, so-called “Enhancers of Trithorax and Polycomb” (ETP), interacts with either complexes, behaving sometimes as repressors and sometimes as activators. The role of ETP proteins is largely unknown.Methodology/Principal FindingsIn a two-hybrid screen, we identified Cyclin G (CycG) as a partner of the Drosophila ETP Corto. Inactivation of CycG by RNA interference highlights its essential role during development. We show here that Corto and CycG directly interact and bind to each other in embryos and S2 cells. Moreover, CycG is targeted to polytene chromosomes where it co-localizes at multiple sites with Corto and with the PcG factor Polyhomeotic (PH). We observed that corto is involved in maintaining Abd-B repression outside its normal expression domain in embryos. This could be achieved by association between Corto and CycG since both proteins bind the regulatory element iab-7 PRE and the promoter of the Abd-B gene.Conclusions/SignificanceOur results suggest that CycG could regulate the activity of Corto at chromatin and thus be involved in changing Corto from an Enhancer of TrxG into an Enhancer of PcG.

Cyclin G is involved in meiotic recombination repair inDrosophila melanogaster

“…Egg chambers mutant for cycG exhibited dorsoventral patterning defects and failed to accumulate a wildtype level of Grk protein during oogenesis. CycG has been found to associate with the chromatin factor Corto and to control the transcription of the Abdominal B gene (Salvaing et al, 2008a;Salvaing et al, 2008b). Here, however, grk mRNA levels were unaltered in the cycG mutant, indicating that CycG does not act as a transcriptional regulator of grk.…”

“…S1). CycG is ubiquitously expressed during embryonic and larval development (Salvaing et al, 2008a). However, during oogenesis cycG mRNA was strongly enriched in the presumptive oocyte already from vitellarial stage 1 on, and was expressed at low levels in region 2 pro-oocytes (Fig.…”

“…Since CycG has been implicated in the transcriptional regulation of Abdominal B (Salvaing et al, 2008a), we wondered, whether it might affect transcription of grk mRNA as well. We did not observe conspicuous changes in the accumulation of grk mRNA in cycG mutant egg chambers, although the amount of Grk protein is reduced (Fig.…”

“…100 ovary pairs of the respective genotypes were homogenised in RIPA I buffer [50 mM Tris-HCl pH 7.5, 150 mM NaCl, 1% Triton X-100, 0.1% SDS, protease and phosphatase inhibitor cocktail (Roche; Basel, Switzerland)] and incubated for 15 min on ice. After preincubation with immobilized protein AAgarose beads (Roche; Basel, Switzerland) the cleared lysate was incubated for several hours at 4˚C in RIPA II (10 mM Tris pH 7.5 including protease and phosphatase inhibitor cocktail (Roche; Basel, Switzerland)) with either guinea pig anti-CycG (Salvaing et al, 2008a) or rat anti-HA bound to Protein A-Agarose beads (Roche; Basel, Switzerland). After several washes with RIPA II buffer at 4˚C samples were run on a 10% SDS-PAGE followed by western blotting.…”

“…CycG had been identified as a cofactor of the chromatin factor Corto that is involved in the transcriptional regulation of the homeotic gene Abdominal B (Salvaing et al, 2008a;Salvaing et al, 2008b). More recent data, relying mainly on gain of function and RNAi based studies, point to a role of CycG in the regulation of cell proliferation and growth (Faradji et al, 2011).…”