Ubiquitin-Specific Protease 43 Promotes Degradation of Phenylalanine Hydroxylase in Mammalian Cells

Phenylalanine hydroxylase (PAH) is a primary enzyme in phenylalanine metabolism, dysfunction of which leads to the metabolic disorder phenylketonuria (PKU). Several missense mutations in the PAH gene are associated with PKU due to toxic accumulation of phenylalanine in the blood and urine. The appropriate PAH protein level in cells is regulated by E3 ligases and deubiquitinating enzymes (DUBs). In this study, we identified USP43 as a negative regulator of PAH protein stability. We demonstrate that overexpression of USP43 decreases the PAH protein level, while depletion of USP43 increases PAH protein expression. Our study indicates that USP43 interacts with and promotes polyubiquitination of PAH, which subsequently decreases the PAH protein half-life. However, USP43 depletion stabilizes and extends the PAH protein half-life by preventing its degradation. Finally, we demonstrate that the depletion of USP43 enhances PAH metabolic activity, underscoring its therapeutic value for PKU.