PU.1 is a component of a multiprotein complex which binds an essential site in the murine immunoglobulin lambda 2-4 enhancer.

B-cell-specific enhancers have been identified in the immunoglobulin lambda locus 3' of each constant-region cluster. These enhancers contain two distinct domains, lambda A and lambda B, which are essential for enhancer function. lambda B contains a near-consensus binding site for the Ets family of transcription factors. In this study, we have identified a B-cell-specific protein complex which binds the lambda B motif of the lambda 2-4 enhancer in vitro and appears necessary for the activity of the enhancer in vivo, since mutations in lambda B which prevent this interaction also eliminate enhancer function. This complex contains PU.1, a member of the Ets family, and a transcriptional activator whose expression is restricted to cells of the hematopoietic system with the exception of T lymphocytes. In addition, it contains a factor which binds specifically to a region adjacent to the PU.1 binding site. This factor cannot bind lambda B autonomously but appears to require interaction with the PU.1 protein to stabilize its association with the DNA. This complex may be identical or related to the PU.1/NF-EM5 complex which interacts with a homologous DNA element in the immunoglobulin kappa 3' enhancer.