Nuclear punctate distribution of ALL-1 is conferred by distinct elements at the N terminus of the protein

The ALL-I gene positioned at 11q23 is directly involved in human acute leukemia either through a variety of chromosome translocations or by partial tandem duplications. ALL-I is the human homologue of Drosophila trithorax which plays a critical role in maintaining proper spatial and temporal expression of the Antennapedia-bithorax homeotic genes determining the fruit fly's body pattern. Utilizing specific antibodies, we found that the ALL-I protein distributes in cultured cells in a nuclear punctate pattern. Several chimeric ALL-I proteins encoded by products of the chromosome translocations and expressed in transfected cells showed similar speckles. Dissection of the ALL-I protein identified within its approximate to 1,100 N-terminal residues three polypeptides directing nuclear localization and at least two main domains conferring distribution in dots. The latter spanned two short sequences conserved with TRITHORAX. Enforced nuclear expression of other domains of ALL-I, such as the PHD (zinc) fingers and the SET motif, resulted in uniform nonpunctate patterns. This indicates that positioning of the ALL-I protein in subnuclear structures is mediated via interactions of ALL-I N-terminal elements. We suggest that the speckles represent protein complexes which contain multiple copies of the ALL-I protein and are positioned at ALL-I target sites on the chromatin. Therefore, the role of the N-terminal portion of ALL-I is to direct the protein to its target genes.