Molecular mechanism of endosomal sorting of growth factors and receptors

点击翻译按钮获取中文摘要

Upon stimulation with growth factors, their receptors are rapidly internalized as a complex with bound ligands, and are transported to the endosome. From the endosome, ligand-bound receptors are transported to the lysosome for degradation. In this study, we examined the molecular mechanisms how Hrs and STAM regulate the endosomal sorting of growth factor/receptor complexes, and obtained the following results.1.STAM mutants lacking the Hrs-binding activity were defective in causing the enlargement of early endosomes, accumulating ubiquitinated proteins on this aberrant organelle, and inhibiting the degradation of ligand-activated epidermal growth factor receptors(EGFR). These results indicate that association with Hrs on the early endosome is a prerequisite for STAM function.2.Overexpression of a deubiquitinating enzyme UBPY that is a STAM binding protein, reduced the ubiquitination level of EGFR and delayed its degradation in EGF-stimulated cells. In contrast overexpression of a catalytically-inactive UBPY mutant did not show such effects. These results indicate that UBPY plays an important role through reduction of the ubiquitination level of EGFR in regulation of EGFR sorting.3.ALG-2 was identified as a novel Hrs-binding protein by a yeast two-hybrid screening. Hrs, ALG-2 and an ALG-2 binding protein Alix were bound in a Ca^<2+>-dependent manner. Treatment of cells with a Ca^<2+> ionophore induced colocalization of these proteins to the early endosome. These results suggest that Hrs regulates endosomal sorting by recruiting Alix through ALG-2 to the early endosome in a Ca^<2+>-dependent manner.