GABP forms a heterodimeric complex on the mNE promoter. (A) GABPα and GABPβ were prepared as GST fusion proteins. EMSA was performed with the mNE promoter probe and purified bacterially expressed GABPα (lanes 2 and 4) and GABPβ (lanes 3 and 4). Filled arrow, binding by monomeric GABPα; open arrow, a heterodimeric complex formed by GABPα and GABPβ. (B) A mutation of the ets site (GGAA → TTAA) was incorporated into the mNE promoter probe. EMSA was performed with the wild-type probe (mNE, lanes 1 to 3) or with the mutated probe (mutNE, lanes 4 to 6) and purified GST-GABPα (lanes 2 and 5) or U937 nuclear extract (lanes 3 and 6). Open arrow, binding by GST-GABPα that was disrupted by the ets site mutation (compare lane 2 and lane 5). Filled arrow, binding by GABP from U937 nuclear extract that was disrupted by the mutation (compare lane 3 and lane 6).