Molecular cloning of a novel human I-mfa domain-containing protein that differently regulates human T-cell leukemia virus type I and HIV-1 expression

Regulation of viral genome expression is the result of complex cooperation between viral proteins and host cell factors. We report here the characteri zation of a novel cellular factor sharing homology with the specific cystei ne-rich C-terminal domain of the basic helix-loop-helix repressor protein I -mfa, The synthesis of this new factor, called HIC for Human I-mfa domain-C ontaining protein, is controlled at the translational level by two differen t codons, an ATG and an upstream non-ATG translational initiator, allowing the production of two protein isoforms, p32 and p40, respectively. We show that the HIC protein isoforms present different subcellular localizations, p32 being mainly distributed throughout the cytoplasm, whereas p40 is targe ted to the nucleolus. Moreover, in trying to understand the function of RIG , we have found that both isoforms stimulate in T-cells the expression of a luciferase reporter gene driven by the human T cell leukemia virus type I- long terminal repeat in the presence of the viral transactivator Tax. We de monstrate by mutagenesis that the I-mfa-like domain of HIC is involved in t his regulation. Finally, we also show that HIC is able to down-regulate the luciferase expression from the human immunodeficiency virus type 1-long te rminal repeat induced by the viral transactivator Tat, From these results, we propose that HIC and I-mfa represent two members of a new family of prot eins regulating gene expression and characterized by a particular cysteine- rich C-terminal domain.