| Members of the Id family of basic helix-loop-helix (bHLH) proteins include Id1, Id2, Id3 and Id4. They are ubiquitously expressed and dimerize with members of the class A and bHLH proteins. Due to the absence of the basic region, the resulting heterodimers cannot bind DNA. The Id-type proteins thus appear to negatively regulate DNA binding of bHLH proteins. Id1 inhibits DNA binding of E12 and Myo D and inhibits muscle-specific gene expression. Under conditions that facilitate muscle cell differentiation, the Id protein levels fall, which allows E12 and/or E47 to form heterodimers with Myo D and myogenin to activate myogenic differentiation. Expression of each of the Id proteins is strongly dependent on growth factor activation. A reduction of Id mRNA levels by antisense oligonucleotides leads to a delayed re-entry of arrested cells into the cell cycle following growth factor stimulation. |
