Windei, the Drosophila homolog of mAM/MCAF1, is an essential cofactor of the H3K9 methyl transferase dSETDB1/Eggless in germ line development

The epigenetic regulation of gene expression by the covalent modification of histones is a fundamental mechanism required for the proper differentiation of germ line cells during development. Trimethylation of histone 3 lysine 9 (H3K9me3) leads to chromatin silencing and the formation of heterochromatin by recruitment of heterochromatin protein 1 (HP1). dSETDB1/Eggless (Egg), the ortholog of the human methyltransferase SETDB1, is the only essential H3K9 methyltransferase in Drosophila and is required for H3K9 trimethylation in the female germ line. Here we show that Windei (Wde), the Drosophila homolog of mouse mAM and human MCAF1, is an essential cofactor of Egg required for its nuclear localization and function in female germ line cells. By deletion analysis combined with coimmunoprecipitation, we have identified the protein regions in Wde and Egg that are necessary and sufficient for the interaction between the two proteins. We furthermore identified a region of Egg that gets covalently modified by SUMOylation, which may facilitate the formation of higher order chromatin-modifying complexes. Together with Egg, Wde localizes to euchromatin, is enriched on chromosome 4, and binds to the Painting of fourth (POF) protein. Our data provide the first genetic and phenotypic analysis of a mAM/MCAF1 homolog in a model organism and demonstrate its essential function in the survival of germ line cells.

Author Summary Germ line cells are the only cells in an organism that are able to transmit their genetic material to the next generation by forming eggs or sperm. They do not participate in the formation or function of tissues and organs and therefore show a unique pattern of transcription, with many genes being silenced that are only required for somatic functions. The covalent modification of histones by methylation, acetylation, and other mechanisms is crucial for these global alterations in the transcriptional program. Among the modifications involved in silencing of chromatin regions, methylation of histone 3 lysine 9 (H3K9) is among the most important ones. Methylation of this residue in Drosophila is controlled by three different histone methyl transferases, but only one of these, dSETDB1/Eggless, is essential for viability and fertility of the fly. Here we describe an essential cofactor for dSETDB1/Eggless that is specifically required in germ line cells for their survival. This cofactor, that we called Windei, binds to dSETDB1/Eggless and recruits it to the nucleus. Null mutations in windei show strongly reduced trimethylation of H3K9 in germ line cells, demonstrating that Windei is one of the factors required for controlling chromatin organization in the germ line.