Purification and properties of LR1, an inducible DNA binding protein from mammalian B lymphocytes.

LR1 is a lipopolysaccharide-inducible B cell-specific DNA binding activity, with sites in the immunoglobulin heavy chain enhancer and switch regions. We describe the purification of this 106-kDa protein from nuclear extracts of PD31 murine pre-B cells, using three chromatographic steps. The DNA binding activity of LR1 is dependent upon phosphorylation, and we show that LR1 is post-translationally modified by N-acetylglucosamine addition. The importance of regulatory modification to LR1 DNA binding activity is apparent from the chromatographic pattern on the anion exchange resin Mono Q: while the LR1 polypeptide elutes over a broad salt range from the column, DNA binding activity is confined to a narrow peak eluting at the highest salt.