The overgrown hematopoietic organs-31 tumor suppressor gene of Drosophila encodes an Importin-like protein accumulating in the nucleus at the onset of mitosis

The tumor suppressor gene overgrown hematopoietic organs-31 (oho31) of Drosophila encodes a protein with extensive homology to the Importin protein of Xenopus (50% identity), the related yeast SRP1 protein, and the mammalian hSRP1 and RCH1 proteins. A strong reduction in the expression of oho31 by a P element inserted in the 5' untranslated region of the oho31 transcript or a complete inactivation of oho31 by imprecise P element excision leads to malignant development of the hematopoietic organs and the genital disc, as shown by their growth autonomy in transplantation assays. We have cloned the oho31 gene of Drosophila melanogaster and determined its nucleotide sequence. The gene encodes a phosphoprotein of 522 amino acids made of three domains: a central hydrophobic domain of eight repeats of 42-44 amino acids each, displaying similarity to the arm motif found in junctional and nucleopore complex proteins, and flanked by two hydrophilic NH2- and COOH-terminal domains. Immunostaining revealed that the OHO31 protein is supplied maternally and rapidly degraded during the first 13 nuclear divisions. Thereafter, the OHO31 protein is predominantly expressed, albeit at reduced levels, in proliferating tissues. During the interphase of early embryonic cell cycles, the OHO31 protein is present in the cytoplasm and massively accumulates in the nucleus at the onset of mitosis in late interphase and prophase. The nuclear import of OHO31 is, however, less pronounced during later developmental stages. These results suggest that, similar to Importin, OHO31 may act as a cytosolic factor in nuclear transport. Moreover, the cell cycle-dependent accumulation of OHO31 in the nucleus indicates that this protein may be required for critical nuclear reactions occurring at the onset of mitosis.