Cell cycle-dependent regulation of the Skp2 promoter by GA-binding protein

Skp2 is the F-box protein component of an SCF-type ubiquitin ligase that interacts specifically with p27^Kip1 and thereby promotes its ubiquitylation and degradation. The abundance of Skp2 mRNA oscillates in a cell cycle-dependent manner, being maximal in S and G₂ phases. The regulation of Skp2 transcription was investigated by cloning the promoter region of the mouse gene and determination of its activity in a luciferase reporter assay. Deletion analysis identified a minimal ∼0.3-kb promoter region with marked transcriptional activity and a 105-bp essential sequence within this region. Electrophoretic mobility shift assays indicated the presence in nuclear extracts of proteins that bind to this sequence. Site-directed mutagenesis revealed that the core binding motif, CACTTCCG, which is similar to that of GA-binding protein (GABP), is essential for Skp2 transcription. "Supershift" analysis indicated that the protein-probe complexes detected by electrophoretic mobility shift assays contain GABP. Endogenous GABP bound to Skp2 promoter element in a cell cycle-dependent manner. Furthermore, overexpression of GABPβ increased Skp2 promoter activity, and suppression of GABPα or GABPβ by a small interfering RNA resulted in the reduction of Skp2 promoter activity. These data suggest that the cell cycle-dependent binding of GABP to the Skp2 promoter plays an important role in the regulation of Skp2 expression and cell cycle progression from G₁ to S phase.