Regulation of myelin basic protein gene transcription by Sp1 and Pur alpha: Evidence for association of Sp1 and Pur alpha in brain

Direct interaction between transcription factors may provide a mechanism fo r the regulatory function of these proteins on transcription of the respons ive genes. These interactions may be facilitated if the target DNA sequence s for the participant regulatory proteins are overlapped or positioned in c lose proximity to each other within the promoter of the responsive genes. I n earlier studies, we identified a cellular protein, named Pur alpha, which upon binding to the MB1 regulatory DNA sequence of the myelin basic protei n (MBP) gene, stimulates its transcription in central nervous system (CNS) cells. Here, we provide evidence for binding of the ubiquitous DNA binding transcription factor, Sp1, to the MB1 DNA motif at the region that partiall y overlaps with the Furor binding site. We demonstrate that binding of Furo r to its target sequence is enhanced by inclusion of Sp1 in the binding rea ction. Under this condition, binding of Sp1 to the MB1 regulatory sequence remained fairly unchanged, and no evidence for the formation of Pur alpha:M B1:Sp1 was observed. This observation suggests that transient interaction o f Furor and Sp1 may result in stable association of Furor and the MB1 eleme nt. In support of this notion, results from immunoprecipitation/Western blo t studies have established association of Furor and Sp1 in nuclear extracts from mouse brain. Of interest, Furor appears to bind to the phosphorylated form of Sp1 which is developmentally regulated and that coincides with the periods when MBP gene expression is at its maximum level. Results from cot ransfection studies revealed that ectopic expression of Furor and Sp1 syner gistically stimulates MBP promoter activity in CNS cells. The importance of these findings in stage-specific expression of MBP during brain developmen t is discussed. J. Cell. Physiol. 181:160-168, 1999. (C) 1999 Wiley-Liss, I nc.