F-Box Protein 15 (FBXO15)

Members of the F-box protein family, such as FBXO15, are characterized by an approximately 40-amino acid F-box motif. Inactivation of Oct3/Oct4 (PO5F1) in ES cells also led to rapid extinction of Fbx15 expression. Reporter gene analysis demonstrated that this ES cell-specific expression required an 18-bp enhancer element located upstream from the transcription initiation site.

The enhancer contains an octamer-like motif and an adjacent Sox-binding motif. Deletion or point mutation of either motif abolished the enhancer activity. The 18-bp fragment became active in mouse fibroblasts when Oct3/Oct4 and Sox2 were coexpressed. Gel mobility shift assay demonstrated cooperative binding of Oct3/Oct4 and Sox2 to the Fbx15 enhancer sequence.