DIFFERENTIAL INTERACTIONS OF ID PROTEINS WITH BASIC-HELIX-LOOP-HELIX TRANSCRIPTION FACTORS

Dimerization of three Id proteins (Idl, Id2, and Id3) with the four cl ass A E proteins (E12, E47, E2-2, and HEB) and two groups of class B p roteins, the myogenic regulatory factors (MRFs: MyoD, myogenin, Myf-5 and MRF4/Myf-6), and the hematopoietic factors (Scl/Tal-1, Tal-2, and Lyl-1) were tested in a quantitative yeast 2-hybrid assay, All three I ds bound with high affinity to E proteins, but a much broader range of interactions was observed between Ids and the class B factors, Idl an d Id2 interacted strongly with MyoD;and Myf-5 and weakly with myogenin and MRF4/Myf-6, whereas Id3 interacted weakly with all four MRFs. Sim ilar specificities were observed in co-immunoprecipitation and mammali an a-hybrid analyses, No interactions were found between the Ids and a ny of the hematopoietic factors, Each Id was able to disrupt the abili ty of E protein-MyoD complexes to transactivate from a muscle creatine kinase reporter construct in vivo, Finally, mutagenesis experiments s howed that the differences between Idl and Id3 binding map to three am ino acids in the first helix and to a small cluster of upstream residu es, The Id proteins thus display a signature range of interactions wit h all of their potential dimerization partners and may play a role in myogenesis which is distinct from that in hematopoiesis.