A Human Protein with Sequence Similarity to Drosophila Mastermind Coordinates the Nuclear Form of Notch and a CSL Protein To Build a Transcriptional Activator Complex on Target Promoters

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Abstract
Mastermind (Mam) has been implicated as an important positive regulator of the Notch signaling pathway by genetic studies usingDrosophila melanogaster. Here we describe a biochemical mechanism of action of Mam within the Notch signaling pathway. Expression of a human sequence related to Drosophila Mam (hMam-1) in mammalian cells augments induction of Hairy Enhancer of split (HES) promoters by Notch signaling. hMam-1 stabilizes and participates in the DNA binding complex of the intracellular domain of human Notch1 and a CSL protein. Truncated versions of hMam-1 that can maintain an association with the complex behave in a dominant negative fashion and depress transactivation. Furthermore,Drosophila Mam forms a similar complex with the intracellular domain of Drosophila Notch andDrosophila CSL protein during activation of Enhancer of split, the Drosophila counterpart ofHES. These results indicate that Mam is an essential component of the transcriptional apparatus of Notch signaling.