PAX-5 (BSAP) RECRUITS ETS PROTOONCOGENE FAMILY PROTEINS TO FORM FUNCTIONAL TERNARY COMPLEXES ON A B-CELL-SPECIFIC PROMOTER

The paired box transcription factor Pax-5 (B-cell-specific activator p rotein) is a key regulator of lineage-specific gene expression and dif ferentiation in B-lymphocytes. We show that Pax-5 functions as a cell type-specific docking protein that facilitates binding of the early B- cell-specific mb-1 promoter by proteins of the Ets proto-oncogene fami ly. Transcriptional activity of the mb-1 promoter in pre-B-cells is cr itically dependent on binding sites for Pax-5:Ets complexes. Ternary c omplex assembly requires only the Pas-5 paired box and ETS DNA-binding domains. Mutation of a single base pair in the ternary complex bindin g site allows for independent binding by Ets proteins but, conversely, inhibits the binding of Pax-5 by itself. Strikingly, the mutation rev erses the pattern of complex assembly: Ets proteins recruit Pax-5 to b ind the mutated sequence. Recruitment of Net acid Elk-1, but not SAP1a , by Pax-5 defines a functional difference between closely related Ets proteins. Replacement of a valine (V68) in the ETS domain of SAP1a by aspartic acid (as found in c-Ets-1, Elk-1, and Net) enhanced ternary complex formation by more than 60-fold. Together, these observations d efine novel transcription factor interactions that regulate gene expre ssion in B cells.