Mutational analysis of p50E4F suggests that DNA binding activity is mediated through an alternative structure in a zinc finger domain that is regulated by phosphorylation.

p50E4F is a cellular transcription factor whose DNA binding activity is stimulated in a phosphorylation-dependent manner by products of the adenovirus E1A oncogene. Although p50E4F does not contain a bZIP DNA binding motif, it binds a tandemly repeated palindromic sequence in the adenovirus E4 promoter that is recognized by a large number of bZIP proteins, but with much greater stability. Analysis of deletions in the p50E4F sequence identified the regions that are responsible for its unique DNA binding properties. Sequence-specific DNA binding and factor dimerization were localized to a C-terminal region containing two C2H2and one CCHC zinc finger motifs; the phosphorylation site critical for DNA binding activity was also localized to this domain. The high stability of p50E4F binding also required residues within the first 83 amino acids of the N-terminus. Analysis of single and double amino acid substitutions in the C-terminal zinc finger domain demonstrated that while the second C2H2zinc finger was required for DNA binding activity, the putative structures of the first C2H2and the CCHC zinc fingers were not. Instead, residues from these other zinc finger motifs appeared to participate in an alternative structure that mediates DNA binding activity and is regulated by phosphorylation.

Stat5a/b contribute to interleukin 7-induced B-cell precursor expansion, but abl- and bcr/abl-induced transformation are independent of stat5.

The cytokines interleukin 7 (IL-7) and interleukin 4 (IL-4) regulate lymphoid differentiation and function and activate the transcription factor Stat5. Using mice deficient for the 2 highly related transcription factors, Stat5a and Stat5b (Stat5a/b(-/-)), we investigated the role of Stat5 for B-cell differentiation, expansion, and function. Peripheral blood B cells of Stat5-deficient mice are significantly reduced, but no proliferation defects in response to various mitogenic stimuli are found. Also, IgM and IgG1 antibody production and immunoglobulin class switching are not affected. Pre- and pro-B cells of Stat5-deficient animals were found to have reduced responses to IL-7. Pro- and pre-B cells are the target cells of the abl oncogene and numerous studies have suggested that Stat5a/b is essential for transformation by derivatives of the Abelson (abl) gene. To assess the role of Stat5a/b in transformation, we have evaluated the ability of various abl derivatives to transform cells from Stat5a/b-deficient mice in vitro or in vivo. We demonstrate that the absence of Stat5a/b is not essential for the induction of lymphoid or myeloid tumors in vivo or on the ability to transform bone marrow cells in vitro.