The repressor which binds the -75 GATA motif of the GPB promoter contains Ku70 as the DNA binding subunit

Glycophorin B (GPB) is an abundant cell surface glycoprotein which is only expressed in human erythroid cells. Previous functional analysis demonstrated that the repression of the GPB promoter is determined by the binding of a ubiquitous factor which recognizes a GATA motif centered at position -75. In erythroid cells this ubiquitous factor is displaced by the binding of the erythroid-specific factor hGATA1. Here, we have identified the Ku70 protein as a candidate GPB repressor DNA binding subunit through the screening of a human HeLa expression library using the -75 GATA sequence as bait (one-hybrid method). Electrophoretic mobility shift assays demonstrated that the ubiquitous factor that binds the -75 GATA sequence was the Ku70-Ku80 (Ku) heterodimer. Co-transfection experiments demonstrated that overexpression of Ku70 in the K562 erythroleukeamic cell line resulted in transcriptional repression of the chloramphenicol acetyltransferase reporter gene when placed under the control of the wild-type GPB promoter. Conversely, no repression was observed when a mutation that abolished the binding of Ku was introduced in the GPB promoter construct. Altogether, these results indicate that Ku binds in vivo to the -75 WGATAR motif and is involved in negative regulation of the GPB promoter. These findings suggest that, besides its role in many functions, Ku is also involved in transcriptional regulation of erythroid genes.