Purified mu EBP-E binds to immunoglobulin enhancers and promoters

We describe the purification to apparent homogeneity of the murine immunoglobulin heavy-chain (IgH) enhancer-binding protein mu EBP-E from murine plasmacytoma cells by ion exchange and affinity chromatography. Glycerol gradient sedimentation, UV cross-linking, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis confirm that mu EBP-E is a 45-kilodalton molecular mass protein. Orthophenanthroline-copper chemical nuclease footprinting with purified protein has identified high-affinity binding sites for mu EBP-E within the IgH enhancer at the previously identified site E and at sites within IgH promoters and in the kappa light-chain enhancer. Equilibrium binding studies indicate that the dissociation constants for mu EBP-E binding to site E within the enhancer and to a binding site within the V1 heavy-chain promoter are quite low, about 2 x 10(-11) M. Comparison of four mu EBP-E recognition sequences detects only limited sequence similarity among binding sites.