TRANSCRIPTIONAL DIVERSITY AT THE DUPLICATED HUMAN CD8-BETA LOCI

In order to understand the structural organization of the human CD8beta locus, genomic clones containing CD8beta sequences were isolated and analyzed. Physical linkage of these clones with the CD8alpha locus using pulsed-field electrophoresis revealed a duplication of the CD8beta locus. CD8B-1 lies 35 kb upstream from the CD8alpha locus and contains eight exons, including four alternatively spliced cytoplasmic exons.The CD8B-2 gene contains six exons and is at present unlinked to CD8B-1. Analysis of sequences upstream to the leader exon of the CD8B-1 and CD8B-2 genes revealed a GC-rich promoter which lacks canonical ''CCAAT'' and ''TATA'' motifs, but which has sites for multiple transcriptional activators and three additional elements which are conserved in the murine CD8beta promoter. Seven unique CD8beta cDNA isoforms were isolated and characterized, which derive from alternative splicing of the transmembrane and/or cytoplasmic exons. Three cDNA are membrane spanning, while the remaining four isoforms lack a transmembrane region and are potentially secreted. These transcripts are differentially expressed in the thymus and in the periphery. Transfection experiments in murine fibroblasts confirmed that the membrane CD8beta isoforms could be expressed as heterodimers with the CD8alpha chain. The regulated expression of multiple CD8beta cytoplasmic isoforms and their potential role in T lymphocyte signal transduction is discussed.