contributed equally to this work STAT factors act as signal transducers of cytokine receptors and transcriptionally activate speci®c target genes. The recently discovered protein PIAS3 binds directly to STAT3 and blocks transcriptional activation. Here, we present experimental evidence implementing the zinc ®nger protein G®-1 as a new regulatory factor in STAT3-mediated signal transduction. The interaction between the two proteins ®rst became evident in a yeast two-hybrid screen but was also seen in coprecipitation experiments from eukaryotic cells. Moreover, we found that both G®-1 and PIAS3 colocalize in a characteristic nuclear dot structure. While PIAS3 exerts a profound inhibitory effect on STAT3-mediated transcription of target promoters, G®-1 can overcome the PIAS3 block and signi®cantly enhances STAT3-mediated transcriptional activation. In primary T cells, G®-1 was able to amplify IL-6-dependent T-cell activation. As G®-1 is a known, dominant proto-oncogene, our ®ndings bear particular importance for the recently described ability of STAT3 to transform cells malignantly and offer an explanation of the oncogenic potential of G®-1 in T lymphocytes.

After rearrangement of the T-cell receptor (TCR)β-locus, early CD4 -/CD8 -double negative (DN) thymic T-cells undergo a process termed 'β-selection' that allows the preferential expansion of cells with a functional TCR β-chain. This process leads to the formation of a rapidly cycling subset of DN cells that subsequently develop into CD4 ϩ /CD8 ϩ double positive (DP) cells. Using transgenic mice that constitutively express the zinc finger protein Gfi-1 and the serine/threonine kinase Pim-1, we found that the levels of both proteins are important for the correct development of DP cells from DN precursors at the stage where 'β-selection' occurs. Analysis of the CD25 ϩ /CD44 -,lo DN subpopulation from these animals revealed that Gfi-1 inhibits and Pim-1 promotes the development of larger β-selected cycling cells ('L subset') from smaller resting cells ('E subset') within this subpopulation. We conclude from our data that both proteins, Pim-1 and Gfi-1, participate in the regulation of β-selection-associated pre-T-cell differentiation in opposite directions and that the ratio of both proteins is important for pre-T-cells to pass the 'E' to 'L' transition correctly during β-selection.

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