Molecular mechanism of regulation of growth factor receptor sorting at endosomes

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Upon binding of growth factors to their cell surface receptors, the growth factor/receptor complexes are internalized and transported to early endosomes. After sorted at early endosomes, the complexes are further transported to lysosome and degraded. Conjugation of ubiquitin to the receptors serves as a sorting signal for transport to lysosomes. In this study, we examined the role of the deubiquitinating enzyme UBPY and ALG2 in regulation of growth factor receptor sorting at endosomes, and the following results were obtained.(1)Overexpression of UBPY reduced the ubiquitination level of EGF receptor (EGFR), and delayed its degradation in EGF-stimulated cells. Overexpression of Hrs caused the accumulation of endogenous UBPY on exaggerated endosomes. A catalytically inactive UBPY mutant clearly localized on endosomes, where it overlapped with EGFR when cells were stimulated with EGF. Depletion of endogenous UBPY by RNA interference resulted in elevated ubiquitination and accelerated degradation of EGF-activated EGFR. These results suggest that UBPY negatively regulates the rate of EGFR degradation by deubiquitinating EGFR on endosomes.(2)At endosomes, ligand-activated receptors are incorporated into luminal vesicles of endosomes that bud inward from its limiting membrane. Endosomes that contain such luminal vesicles are called the multivesicular body (MVB). We examined the role of ALG2 in the formation of the luminal vesicles. Depletion of endogenous ALG2 by RNA interference resulted in the reduction of the level of phospholipid LBPA, which is specifically localized to the membrane of the luminal vesicles. This reduction was also observed by an electron microscopy analysis. These results suggest thatALG2 plays an important role in the formation of the luminal vesicles in MVB.