huASH1 protein, a putative transcription factor encoded by a human homologue of the Drosophila ash1 gene, localizes to both nuclei and cell-cell tight junctions

During animal development, regions of the embryo become committed to positi on-specific: identities, which are determined by spatially restricted expre ssion of Hox/homeotic genes. This expression pattern is initially establish ed by the activity of the segmentation genes and is subsequently maintained during the proliferative stage through the action of transcription factors encoded by the trithorax (trx) and Polycomb (Pc) groups of genes. trithora x (trx) and ash1 (absent, small, or homeotic 1) are members of the Drosophi la trx group. Their products are associated with chromosomes and are believ ed to activate transcription of target genes through chromatin remodeling. Recently, we reported molecular studies indicating that TRX and ASH1 protei ns act in concert to bind simultaneously to response elements located at cl ose proximity within the same set of target genes. Extension of these and o ther studies to mammalian systems required identification and cloning of th e mammalian homologue of ash1 (the mammalian homologue of trx, ALL-1, was p reviously cloned). We have identified a human expressed sequence tag (EST) clone with similarity to the SET domain of Drosophila ASH1, and used it to clone the human gene. huASH1 resides at chromosomal band 1q21. The gene is expressed in multiple tissues as an approximate to 10.5-kb transcript and e ncodes a protein of 2962 residues. The protein contains a SET domain, a PHD finger, four AT hooks, and a region with homology to the bromodomain. The last region is not present in Drosophila ASH1, and as such might confer to the human protein a unique additional function, Using several anti-huASH1 A b for immunostaining of cultured cells, we found that the protein is distri buted in intranuclear speckles, and unexpectedly also in intercellular junc tions. Double-immunofluorescence labeling of huASH1 and several junctional proteins localized the huASH1 protein into tight junctions. The significanc e of huASH1 dual location is discussed. In particular, we consider the poss ibility that translocation of the protein between the junctional membrane a nd the nucleus may be involved in adhesion-mediated signaling.