Sp1 and Sp3 physically interact and co-operate with GABP for the activation of the utrophin promoter

The utrophin gene codes for a large cytoskeletal protein closely related to dystrophin which, in the absence of dystrophin, can functionally substitut e it. Utrophin is transcribed by two independently regulated promoters abou t 50 kb apart. The upstream promoter is TATA-less and contains a functional GABP binding site which, in muscle, restricts the promoter activity to pos t-synaptic nuclei. Transient transfections analysis of mutant promoters in rhabdomyosarcoma cells showed that the upstream promoter contains three fun ctional GC elements that are recognised by Spl and Sp3 factors in vitro. Co -transfections of the promoter with Spl, Sp3 and GABP factors in Drosophila SL2 Schneider cells, which lack of endogenous Sp factors, demonstrated tha t both Spl and Sp3 are positive regulators of the utrophin promoter and tha t they activate transcription synergistically with GABP. Consistent with th is result, we observed physical interaction of both Sp factors with the GAB P alpha subunit in vitro. Functional domain interaction analysis of Spl and Sp3 revealed that both factors interact with GABP alpha through their DNA binding zinc finger domain. The modulation and correct interaction between Spl, Sp3 and GABP in muscle cells may be critical for the regulation of the utrophin promoter, and provide new targets for therapies of Duchenne muscu lar dystrophy. (C) 2001 Academic Press.