FUNCTIONALLY DISTINCT ISOFORMS OF THE CRE-BP DNA-BINDING PROTEIN MEDIATE ACTIVITY OF A T-CELL-SPECIFIC ENHANCER

Expression of the CD3-delta gene of the T-cell receptor (TCR) complex is regulated by a T-cell-specific enhancer. A highly conserved 40-bp motif (element delta-A) within the CD3-delta enhancer is responsible for mediating its activity and specificity. Element delta-A exhibits sequence similarities to the cyclic AMP response element (CRE) but does not respond to changes in the level of cyclic AMP. Using the delta-A element as a probe, we have isolated three cDNA clones encoding three distinct protein isoforms, products of differential splicing and alternate promoter usage of the CRE-BP gene. These isoforms share the DNA binding and dimerization domains at the C terminus of the protein but differ at their N termini. In transfection assays, their activities as transcription regulators differ: CRE-BP2 is a potent activator, CRE-BP3 is a weak activator, and CRE-BP1 is transcriptionally inert. Mutations in the basic region of the CRE-BP1 protein which abrogate its ability to bind DNA render this protein a dominant repressor of the delta-A enhancer. Antibodies to the CRE-BP protein interact specifically with the ubiquitous and predominantly T-cell-restricted nuclear complexes that bind to the delta-A element and suggest the presence of this protein in homo- and heterodimeric complexes. Since the delta-A motif is also present in the enhancer and promoter of the TCR alpha and beta-genes, the CRE-BP isoforms may mediate expression of other members of the CD3/TCR complex during T-cell development.