Human CART1, a paired-class homeodomain protein, activates transcription through palindromic binding sites

Homeodomain proteins play important roles in animal development by controlling the expression of genes involved in determining cell fates. The recently cloned human Cart1 gene encodes a paired-class homeodomain (hCART1), whose rodent homolog is mainly expressed in early chondrocytes and in prechondrocytic mesenchymal cells. To better understand its role as a transcription factor, the author has selected specific hCART1 binding sites from a random pool of oligonucleotides. It is reported here that all sites obtained contain a palindrome consisting of two TAAT sequences separated by three or four base pairs. In electromobility shift assays, recombinant hCART1 proteins bind to a palindromic probe as a multimer, possibly a homodimer. In transient transfection assays, hCART1 activates transcription from reporter plasmids containing hCART1 binding sites in HeLa cells, demonstrating both site-dependence and dosage-dependence. It is also shown here that hCART1 localizes to nucleus. These data indicate that hCART1 is a sequence-specific transcription activator in HeLa cells. In combination with data from previous studies in which hCART1 represses transcription in different cell types and promoters, they suggest that hCART1 may be a transcription modulator with both repression and activation activities.