CBP P300 AS A COFACTOR FOR THE MICROPHTHALMIA TRANSCRIPTION FACTOR/

The Microphthalmia basic-Helix-loop-Helix-Leucine Zipper (bHLH-LZ) tra nscription factor (Mi) plays a crucial role in the genesis of melanocy tes; mice deficient for a functional (Microphthalmia) gene product lac k all pigment cells, We show here that the Mi activation domain reside s N-terminal to the DNA-binding domain and that as little as 18 amino acids are sufficient to mediate transcription activation, The minimal activation region of Mi is highly conserved in the related transcripti on factor TFE3 and is predicted to adopt an amphipathic alphahelical c onformation, This region of Mi is also highly conserved with a region of E1A known to be essential for binding the CBP/p300 transcription co factor, Consistent with these observations, the Mi activation domain c an interact in vitro with CBP specifically through a region of CBP req uired for complex formation with E1A, P/CAF and c-Fos, and anti p300 a ntibodies can co-immunoprecipitate Mi from both melanocyte and melanom a cell lines, In addition, co-transfection of a vector expressing CBP2 (aas 1621-1891) fused to the VP16 activation domain potentiated the a bility of Mi to activate transcription, confirming the significance of the CBP-Mi interaction observed in vitro. These data suggest that tra nscription activation by Mi is achieved at least in part by recruitmen t of CBP, The parallels between transcription regulation by Microphtha lmia in melanocytes and MyoD in muscle cells are discussed.