Dissection of Drosophila MTF-1 reveals a domain for differential target gene activation upon copper overload vs. copper starvation.

Abstract Metal-responsive transcription factor-1 (MTF-1) is a zinc finger protein conserved from mammals to insects. It mediates protection against heavy metal load by activating the expression of metallothionein and other genes. In Drosophila , MTF-1 serves a dual function in that it not only helps to protect against heavy metal load but also induces the expression of Ctr1B , the gene for an intestinal copper importer, upon copper starvation. By dissecting Drosophila MTF-1 function, we have identified determinants for nuclear import and export, and characterized a phosphorylation site mutant (T127A) that differentially affects MTF-1 target genes. Further, by generating a series of fusion proteins with the heterologous DNA binding domain of Gal4 we identified a strong, constitutive activation domain in the central region of MTF-1 (aa 352–540). By contrast, an extended fusion protein that includes MTF-1's C-terminus (aa 352–791) is not active in standard conditions but induced by copper load. The paramount regulatory importance of the C-terminal part, that harbors a cysteine-rich “metallothionein-like” domain, was corroborated by different experiments. Transgenic flies expressing C-terminally truncated MTF-1 variants displayed high constitutive transcription of both, the genes for metallothioneins and the copper importer Ctr1B . The indiscriminate activation of these genes that are normally induced under opposite conditions of copper load and copper starvation manifested itself in a shortened lifespan, crippled wings, and female sterility.