F-box/LRR-repeat protein 5 is a protein that in humans is encoded by the FBXL5 gene.
This gene encodes a member of the F-box protein family which is characterized by an approximately 40 amino acid motif, the F-box. The F-box proteins constitute one of the four subunits of ubiquitin protein ligase complex called SCFs (SKP1-cullin-F-box), which function in phosphorylation-dependent ubiquitination. The F-box proteins are divided into 3 classes: Fbws containing WD-40 domains, Fbls containing leucine-rich repeats, and Fbxs containing either different protein-protein interaction modules or no recognizable motifs. The protein encoded by this gene belongs to the Fbls class and, in addition to an F-box, contains several tandem leucine-rich repeats. Alternative splicing of this gene generates 2 transcript variants.
FBXL5 is an iron and oxygen sensor. It promotes IRP2 ubiquitination and then its degradation in an iron- and oxygen-dependent manner. The cryo-EM structure of the FBXL5-IRP2 complex revealed an unexpected 2Fe2S cluster embedded in the leucine-rich repeats domain of the F-box protein in close vicinity of the protein-protein interaction interface. FBXL5, therefore, is an iron-sulfur protein. FBXL5 can only engage IRP2 when its 2Fe2S cluster is in the oxidized state, which explains how oxygen tension dictates IRP2 stability.
