FBXO24 targets SLC25A26 for K6-linked polyubiquitylation to maintain mitochondrial function during spermiogenesis

Asthenozoospermia is one of the major causes of male infertility, typically resulting from malformed flagella and dysfunctional mitochondria. However, the pathogenic mechanisms underlying asthenozoospermia remain unclear. Here, we show that FBXO24, a F-box protein within the SCF E3 ubiquitin ligase complex, is required for maintaining mitochondrial function and ATP production during spermiogenesis. Using Fbxo24 knockout mice, we demonstrate that the depletion of FBXO24 leads to male infertility due to a malformed sperm head and severe motility defects. The decreased motility resulted from dysfunction of mitochondria that was characterized by disorganized mitochondrial clustering, reduced mitochondrial membrane potential and elevated reactive oxygen species levels. Based on quantitative proteomics, we identified SLC25A26, a mitochondrial S-adenosylmethionine transporter, as a previously unreported substrate for FBXO24. Mechanistically, FBXO24 mediates K6-linked polyubiquitylation of SLC25A26 at lysine residue 31, targeting it for degradation. Elevated SLC25A26 induced mitochondrial fragmentation, suppressed glycolysis and oxidative phosphorylation, and decreased ATP production. All these results suggest that FBXO24 safeguards mitochondrial integrity by controlling SLC25A26 stability, ensuring ATP production for sperm motility. It also suggests that some mutations of FBXO24 might be associated with asthenozoospermia in human.