T cell differentiation in the thymus is dependent upon signaling through the TCR and is characterized by the resulting changes in expression patterns of CD4 and CD8 surface coreceptor molecules. Although recent studies have characterized the effects of proximal TCR signaling on T cell differentiation, the downstream integration of these signals remains largely unknown. The growth factor independence-1 (GFI1) and GFI1B transcriptional repressors may regulate cytokine signaling pathways to affect lymphocyte growth and survival. In this study, we show that Gfi1 expression is induced upon induction of the T cell program. Gfi1B expression is low and dynamic during T cell development, but is terminated in mature thymocytes. Transgenic expression of GFI1 and GFI1B in T cells allowed us to determine the functional consequences of constitutive expression. GFI1 potentiates response to TCR stimulation and IL-2, whereas GFI1B-transgenic T cells are defective in T cell activation. Moreover, GFI1B-transgenic thymocytes display reduced expression of the late-activation marker IL-7Rα, and a decrease in CD4−8+ single-positive T cells that can be mitigated by transgenic expression of BCL2 or GFI1. These data show that GFI1 and GFI1B are functionally unique, and implicate a role for GFI1 in the integration of activation and survival signals.

The EBNA-LP protein encoded by the open reading frame in the leader exons of the Epstein-Barr nuclear antigen messages is essential for efficient immortalization of B lymphocytes. Protein-protein interaction studies using affinity precipitation of proteins from [35S]methionine-labeled cell lysates and bacterially expressed maltose binding protein EBNA-LP fusions were performed. A cellular 68/72-kDa doublet protein was detected. This banding pattern was shown to be identical to that obtained in affinity precipitations with fusions of glutathione-S-transferase and Sp1 (a basal transcription factor). For both EBNA-LP and Sp1 the specific interacting cellular proteins have been identified as heat shock proteins (HSP) 72/73. Affinity precipitation of HSP 72/73 with deletion mutants of EBNA-LP maps the interaction domain on EBNA-LP to exon Y2 which is required for immortalization. Immunoprecipitation of EBNA-LP from EBV-positive lymphoblastoid cell lines coprecipitated the HSP 72/73 proteins, indicating that the interaction occurs in vivo as well as in vitro. The association of HSPs with a widening range of nuclear proteins involved in gene expression and proliferation control now includes Sp1 and EBNA-LP and suggests that there is a central role for molecular chaperones in these processes.

EBNA-LP is a viral nuclear oncoprotein implicated in the immortalization of B lymphocytes by Epstein-Barr virus. An analysis of EBNA-LP migration on polyacrylamide gels was performed with protein derived from the X50-7 lymphoblastoid cell line blocked by hydroxyurea or aphidicolin at the G1/S phase of the cell cycle or by nocodazole at the G2/M phase. More slowly migrating species of EBNA-LP were detected in G2/M phase-arrested cell extracts. Release from nocodazole G2/M block or treatment with phosphatase caused the more slowly migrating species of EBNA-LP to disappear. Analyses of 32PO(4)(3-)-labeled EBNA-LP protein immunoprecipitated from the drug-synchronized cells showed that phosphorylated EBNA-LP was present throughout the cell cycle but that phosphorylation increased in G2 and was maximal at G2/M. Phosphoamino acid analysis revealed that all phosphorylation was on serine residues only. The ability of EBNA-LP to be phosphorylated by p34(cdc2) kinase and casein kinase II exclusively on serines implicates these enzymes as being potentially involved in EBNA-LP phosphorylation.