PHOSPHORYLATION OF E47 AS A POTENTIAL DETERMINANT OF B-CELL-SPECIFIC ACTIVITY

The E2A gene encodes two basic helix-loop-helix proteins designated E1 2 and E47. Although these proteins are widely expressed, they are requ ired only for the B-lymphocyte lineage where DNA binding is mediated d istinctively by E47 homodimers. By studying the properties of Delta E4 7, an N-terminal truncation of E47, we provide evidence that phosphory lation may contribute to B-cell-specific DNA binding by E47. Two serin es N terminal to the Delta E37 basic helix-loop-helix domain were foun d to be phosphorylated in a variety of cell types bat were hypophospho rylated in B cells, Phosphorylating these serines in vitro inhibited D NA binding by Delta E47 homodimers but not by Delta E47-containing het erodimers, such as Delta E47:MyoD. These results argue that hypophosph orylation may be a prerequisite for activity of E47 homodimers in B ce lls, suggesting the use of an inductive (nonstochastic) step in early B-cell development.