Id2 is a primary partner for the E2-2 basic helix-loop-helix transcription factor in the human placenta.

We screened a term placental cDNA library by the yeast two-hybrid approach with Id2, a negative regulator of basic helix-loop-helix (bHLH) factors. Of the clones obtained, approximately one-third were the E2-2 bHLH transcription factor. Id2 and E2-2 were shown to interact in direct two-hybrid assays in yeast cells, as well as immunoprecipitation assays in mammalian cells. Immunohistochemical analysis demonstrated co-localization of both Id2 and E2-2 in placental trophoblasts. Co-transfection of JEG-3 cells with E2-2 and Id2, and a luciferase reporter construct under the control of the human chorionic gonadotropin alpha-subunit promoter revealed that E2-2 had a negative effect on CGalpha-subunit transcription, which could be relieved by overexpression of Id2. The library was in turn rescreened with E2-2, and Id2 and Id1 were essentially the only clones obtained. We conclude that Id2 is a primary binding partner for the bHLH transcription factor E2-2 in the human placenta.