BSAP can repress enhancer activity by targeting PU.1 function

PU.1 and BSAP are transcription factors crucial for proper B-cell developme nt. Absence of PU.1 results in loss of B, T, and myeloid cells, while absen ce of BSAP results in an early block in B-cell differentiation. Both of the se proteins bind to the immunoglobulin kappa chain 3' enhancer, which is de velopmentally regulated during B-cell differentiation. We find here that BS AP can repress 3' enhancer activity. This repression can occur in plasmacyt oma lines or in a non-B-cell line in which the enhancer is activated by add ition of the appropriate enhancer binding transcription factors. We show th at the transcription factor PU.1 is a target of the BSAP-mediated repressio n. Although PU.1 and BSAP can physically interact through their respective DNA binding domains, this interaction does not affect DNA binding. When PU. 1 function is assayed in isolation on a multimerized PU.1 binding site, BSA P targets a portion of the PU.1 transactivation domain (residues 7 to 30) f or repression. The BSAP inhibitory domain (residues 358 to 385) is needed f or this repression. Interestingly, the coactivator protein p300 can elimina te this BSAP-mediated repression. We also show that PU.1 can inhibit BSAP t ransactivation and that this repression requires PU.1 amino acids 7 to 30. Transfection of p300 resulted in only a partial reversal of PU.1-mediated r epression of BSAP. When PU.1 function is assayed in the context of the immu noglobulin kappa chain 3' enhancer and associated binding proteins, BSAP re presses PU.1 function by a distinct mechanism. This repression does not req uire the PU.1 transactivation or PEST domains and cannot be reversed by p30 0 expression. The possible roles of BSAP and PU.1 antagonistic activities i n hematopoietic development are discussed.