Cloning and expression of human HBP1, a high mobility group protein that enhances myeloperoxidase (MPO) promoter activity

Factors which regulate transcription in immature myeloid cells are of great current interest for the light they may shed upon myeloid differentiation. In the course of screening for transcription factors which interact with t he human myeloperoxidase (MPO) promoter we, for the first time, identified and cloned the cDNA and genomic DNA for human HBP1 (HMG-Box containing prot ein 1), a member of the high mobility group of non-histone chromosomal prot eins. HBP1 cDNA was initially cloned from rat brain in 1994, but its presen ce in human cells or in myeloid tissue had not been described previously. T he sequence of human HBP1 cDNA shows 84% overall homology with the rat HBP1 cDNA sequence. We have subsequently cloned the gene, which is present as a single copy, 25 kbp in length. Northern blotting reveals a single 2.6 kb m RNA transcript which is expressed at higher levels in human myeloid and B l ymphoid cell lines than in T cell lines tested and is present in several no n-myeloid human cell lines. Comparison of the mRNA and genomic sequences re veals the gene to contain 10 exons and 9 introns. The sequence of human HBP 1 mRNA contains a single open reading frame, which codes for a protein 514 amino acids in length. The amino acid sequence specified by the coding regi on shows 95% homology with the rat HBP1 protein. The human protein sequence exhibits a putative DNA-binding domain similar to that seen in rat HBP1 an d shows homology with the activation and repressor domains previously demon strated in the rat protein. We have expressed human HBP1 protein both in vi tro and in prokaryotic and eukaryotic cells. The expressed fusion protein b inds to a sequence in a functionally important region within the basal huma n MPO promoter. In transient co-transfection experiments HBP1 enhances MPO promoter activity. Human HBP1 appears to be a novel transcription factor wh ich is likely to play an important role in regulating transcription in deve loping myeloid cells.