Two Drosophila Ada2 homologues function in different multiprotein complexes. T. Kusch ¹, S. Guelman ², S.M. Abmayr ², J.L. Workman ¹. 1) Howard Hughes Medical Institute, Biochemistry and Mol. Biology, PennState University, University Park, PA 16802; 2) Biochemistry and Mol. Biology, PennState University, University Park, PA 16802.

   To address the function and structure of Gcn5-containing N-acetyltransferase (GNAT) complexes in higher eukaryotes we have pursued GNAT complexes from Drosophila. We characterized homologues of the Gcn5-associated factors Ada2, Ada3, Spt3 and Tra1, and found that humans, mice and flies possess two distinct Ada2-related genes. With help of biochemical and cell biological studies we demonstrate that the two Drosophila Ada2 homologues are in functionally and structurally distinct multiprotein complexes. One of the Ada2 homologues is associated with SAGA-type complexes, and therefore was named dAda2S. The alternative dAda2A variant is part of dGcn5-containing as well as Gcn5-independent complexes. The latter complexes localize to transcriptionally hyperactive loci suggesting novel functions of Ada2 in transcription. Functional studies revealed that both Ada2 variants are essential for embryogenesis, however, only the loss of dAda2S function causes segmentation defects as well as massive apoptosis at onset of early embryonic differentiation. dSAGA complexes from dAda2S mutants appear to be targeted normally to chromatin, but lack their catalytic subunit dGcn5. Our findings suggest that mammals and insects have two distinct Ada2 variants, which characterize functionally distinct multiprotein complexes involved in transcriptional regulation.