Partial characterization of a novel 'GGA' factor which binds to the osteonectin promoter in bovine bone cells

Osteonectin (On)/SPARC (secreted protein, acidic and rich in cysteine) is a highly conserved extracellular matrix protein found in bone and other tissues throughout vertebrate evolution. In previous studies, approximately 500 bp of DNA 5' to the transcription start point (tsp) and a part of exon 1, including homopurine (Rn)/homopyrimidine (Yn)-rich sequences (the 'GGA box' and its complements), was demonstrated to be important in upregulation of On gene expression in a cell-specific manner. The purpose of this study was to decipher the transcriptional regulation of On through its cis- and trans-acting elements. DNase I footprinting analysis indicated protein binding which may be related to the transcriptional factors, AP2, SP1 and a novel 'GGA' factor which binds to the 3' end of the promoter (-286 to +43 to the tsp). Comparisons of footprinting between nuclear extracts of bone (On-expressing) cells and Madin and Darby bovine kidney (MDBK) (nonexpressing) cells indicate that 'GGA' factor binding to a purine GGGGA/GGA-rich sequence is cell-type specific and therefore may be involved in the cell-specific expression of this gene. From ultraviolet (UV)-crosslinking experiments, this 'GGA' factor was demonstrated to be a single 40-kDa protein.