TFE3 CONTAINS 2 ACTIVATION DOMAINS, ONE ACIDIC AND THE OTHER PROLINE-RICH, THAT SYNERGISTICALLY ACTIVATE TRANSCRIPTION

TFE3 is a basic-helix-loop-helix-zipper (bHLHZIP) domain-containing pr otein that binds mu E3 sites in regulatory elements in the immunoglobu lin heavy chain gene. The protein is a transcriptional activator that is expressed in vivo as two alternately spliced isoforms with differen t activating properties: TFE3L contains an N-terminal acidic activatio n domain; TFE3S lacks this activation domain and is a dominant negativ e inhibitor of TFE3L. We show that TFE3L and TFE3S contain a second, C -terminal activation domain rich in proline residues. This pro-rich ac tivation domain has activity in a Gal4 fusion assay comparable to the N-terminal acidic activation domain present in TFE3L. The TFE3 pro-ric h activation domain contains regions of strong homology with the relat ed proteins microphthalmia and TFEB, suggesting that these regions are important for function. Using two different assays, we show that the N- and C-terminal activation domains of TFE3 act synergistically. This synergism explains in part the ability of TFE3S to act as a dominant negative. Our domain analysis of TFE3 is incorporated into a general s tructural model for the TFE3 protein that predicts that the activation domains of TFE3 will be widely separated in space.