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An early and critical event in immortalization of human B cells by Epstein-Barr virus (EBV) is induction of CD23 expression. CD23 is constitutively expressed in all EBV-immortalized B cells and its expression is tightly linked with immortalization. We have previously shown that activation of CD23 by EBV occurs at the transcriptional level and is mediated, in part, by EBV-responsive enhancer elements in the region of the type a promoter. We have localized one EBV-responsive enhancer (designated EBVRE) to a 37 bp sequence in intron 1 of type a CD23 that contains a GC-rich sequence that binds nuclear protein(s) from EBV-positive but not EBV-negative cells with sequence specificity. This EBVRE-binding activity was enhanced by protein phosphorylation and did not react with antibodies to the ubiquitous GC box transcription factor, Sp1. We have now shown by protein purification with peptide sequencing and immunological reactivity that p70/p80 Ku autoantigen [the DNA-binding component of DNA-dependent protein kinase (DNA-PK)] binds to this EBVRE with high affinity and sequence specificity. Although Ku autoantigen is ubiquitously expressed, an EBV-specific DNA-protein complex that contains Ku was elicited from EBV-positive but not EBV-negative nuclear extracts. Furthermore, the formation of this EBV-specific DNA-Ku complex was dramatically enhanced by protein phosphorylation. Thus, we have identified EBVRE-binding activity that contains the Ku autoantigen, is DNA sequence specific and is present in EBV-positive but not EBV-negative nuclear extracts. The possible functional significance of the Ku autoantigen-EBVRE interaction is discussed in light of the role of DNA-PK in phosphorylation and activation of several transcription factors. We suggest that phosphorylation of the EBV-specific EBVRE-binding activity by DNA-PK may modulate its activity as a transcription factor.

international immunology

Regulation of the human IgE receptor (Fc epsilonRII/CD23) by Epstein- Barr virus (EBV): Ku autoantigen binds specifically to an EBV- responsive enhancer of CD23