A Stat5-overlapping site is critical for the IgJ enhancer activity in the plasma cells and bound by a ubiquitous protein.

Although the IgJ enhancer chromatin is induced open by an IL-2/Stat5 signaling during terminal B cell differentiation, the opened chromatin of IgJ enhancer is then maintained in the absence of IL-2/Stat5 signaling. Nevertheless, the sequence overlapping the Stat5 site was shown still to be essential for the function of IgJ enhancer in the plasma cells. An in vivo footprint was identified over the Stat5-overlapping site, indicating that the site should be bound by a certain other protein than Stat5. In EMSA using the Stat5-overlapping sequence as a probe, its specific binding protein was identified. The specific binding protein corresponded neither to any of other Stat family proteins, nor to any of potential candidate proteins as tested in EMSA using their corresponding oligo DNA competitors and antibodies. Although its identity remains to be found by its purification, the protein binding specifically to the Stat5-overlapping site was shown to be expressed rather ubiquitously in B and non-B cells, and its molecular weight appeared to be below 52 kDa as determined in the UV-crosslinking-coupled SDS-PAGE.