The FLI-1 transcription factor is a short-lived phosphoprotein in T cells

The FLI-1 transcription factor is a member of the ETS gene family, most closely related to ERG. In this study, the FLI-1 protein products were characterized using a specific monoclonal antibody previously developed against bacterially expressed protein. In the human T-cell line Jurkat, both isoforms of FLI-1, p51 and p48, are phosphorylated, primarily on serine residues. FLI-1 phosphorylation is increased by a Ca(2+)-mediated process, and inhibitor studies indicate that protein phosphatase 2A, at least in part, controls FLI-1 phosphorylation level. FLI-1 phosphorylation is not stimulated by phorbal 12-myristate 13-acetate (PMA), a protein kinase C activator, and in this it differs from ERG protein phosphorylation. The p51 isoform has a half-life of 105 min, and p48 has a half-life of 80 min; in contrast, the ERG protein is much more stable with a half-life of 21 h. Newly synthesized FLI-1 protein decreased during human T cell activation. Our data suggest that although the FLI-1 and ERG genes are highly homologous, their distinct properties may contribute to their different roles in gene regulation.