Id2 binds to the b1 and b2 segments in p204. (A) Schematic diagrams of GST-204 fusion proteins used to map the sites of Id2 binding on p204. The numbers refer to amino acid residues in p204. N, N terminal; a1, a2, b1, and b2, segments encoded by single exons. The a1 and a2 segments constitute the a segment, and the b1 and b2 segments constitute the b segment. The strengths of the binding of Id2 to the various GST-204 segments, as shown in panel B, are indicated. (B) Binding of Id2 by p204 and its segments. Glutathione-Sepharose beads carrying GST, GST-204, or its segments, as indicated, were incubated with extracts prepared from C2C12 myoblasts, and the bound Id2 was detected by immunoblotting with anti-Id2 antibodies. The Id2 band is indicated. (C) Expression of free GST or of GST linked to p204 or its segments. Samples (0.5 μg) of affinity-purified GST, GST-204, or its segments, as indicated, were examined by SDS-PAGE and Coomassie blue staining. The positions of size markers are indicated. For further details, see Materials and Methods.