Selective utilization of basic helix-loop-helix–leucine zipper proteins at the immunoglobulin heavychain

The mE3 E box within the immunoglobulin heavy-chain (IgH) enhancer binds several proteins of the basic helix-loop-helix–leucine zipper (bHLHzip) class, including TFE3, USF1, and Max. Both TFE3 and USF have been described as transcriptional activators, and so we investigated their possible roles in activating the IgH enhancer in vivo. Although TFE3 activated various enhancer-based reporters, both USF1 and Max effectively inhibited transcription. Inhibition by USF correlated with the lack of a strong activation domain and was the result of the protein neutralizing the mE3 site. The effects of dominant-negative derivatives of TFE3 and USF1 confirmed that TFE3, or a TFE3-like protein, is the primary cellular bHLHzip protein that activates the IgH enhancer. In addition to providing a physiological role for TFE3, our results call into question the traditional view of USF1 as an obligate transcriptional activator. B-cell-specific transcription of the immunoglobin heavy-chain (IgH) locus is directed primarily by an enhancer located within the intron separating the J and C regions of the gene. Transfection experiments and in vitro binding studies have defined numerous sites within an approximately 250-bp region critical for enhancer activity. Among the proteins that bin