Assembly and function of a TCR enhancer complex is dependent on LEF-l-induced DNA bending and multiple protein-protein interactions

In this study we examine the molecular basis for the synergistic regulation of the minimal TCRa enhancer by multiple proteins. We find that reconstitution of TCRc~ enhancer function in nonlymphoid cells requires expression of the lymphoid-specific proteins LEF-1, Ets-1 and PEBP2a (CBFc~I, and a specific arrangement of their binding sites in the enhancer. We show that Ets-1 cooperates with PEBP2a to bind adjacent sites at one end of the enhancer, forming a ternary complex that is unstable by itself. Stable occupancy of the Ets-1and PEBP2oL-binding sites in a DNase I protection assay was found to depend on both a specific helical phasing relationship with a nonadjacent ATF/CREB-binding site at the other end of the enhancer and on LEF-1. The HMG domain of LEF-1 was found previously to bend the DNA helix in the center of the TCRot enhancer. We now show that the HMG domain of the distantly related SRY protein, which also bends DNA, can partially replace LEF-1 in stimulating enhancer function in transfection assays. Taken together with the observation that Ets-1 and members of the ATF/CREB family have the potential to associate in vitro, these data suggest that LEF-1 can coordinate the assembly of a specific higher-order enhancer complex by facilitating interactions between proteins bound at nonadjacent sites.