Endocytosis of cell surface receptors mediates cellular homeostasis by coordinating receptor distribution with downstream signal transduction and attenuation. Post-translational modification with ubiquitin of these receptors, as well as the proteins that comprise the endocytic machinery, modulates cargo progression along the endocytic pathway. The interplay between ubiquitination states of cargo and sorting proteins drives trafficking outcomes by directing endocytosed material toward either lysosomal degradation or recycling. Deubiquitination by specific proteinases creates a reversible system that promotes spatial and temporal organization of endosomal sorting complexes required for transport (ESCRTs) and supports regulated cargo trafficking. Two dubiquitinating enzymes--ubiquitin-specific protease 8 (USP8/Ubpy) and associated molecule with the SH3 domain of STAM (AMSH)--interact with ESCRT components to modulate the ubiquitination status of receptors and relevant sorting proteins. In doing so, these ESCRT-DUBs control receptor fate and sorting complex function through a variety of mechanisms described herein.

Reversible ubiquitination is essential for the endocytic sorting and down-regulation of G protein-coupled receptors, such as the chemokine receptor CXCR4. The deubiquitinating enzyme AMSH has been implicated in the endocytic sorting of both G protein-coupled receptors and receptor-tyrosine kinases. Herein, we examine the role of AMSH in the regulation of CXCR4 stability and trafficking and characterize protein-protein interactions critical for this function. Loss of AMSH catalytic activity or depletion by RNAi results in increased steady-state levels of CXCR4 under basal conditions. Analysis of truncation and point mutation of AMSH reveal the importance of an RXXK motif for CXCR4 degradation. The RXXK motif of AMSH interacts with the SH3 domains of the STAM and Grb2 families of adaptor proteins with high affinity. Cells expressing a catalytically inactive mutant of AMSH show basal hyperubiquitination, but not increased degradation, of the ESCRT-0 components STAM1 and Hrs. This is dependent on the RXXK motif of AMSH. Ubiquitination of endocytic machinery modulates their activity, suggesting that AMSH may directly regulate endocytic adaptor protein function. This is reflected in CXCR4 trafficking and provides a mechanism by which AMSH specifies the fate of endocytosed receptors. Taken together, these studies implicate AMSH as a key modulator of receptor fate determination through its action on components of the endocytic machinery.The sorting and trafficking of cell surface receptors through endosomal compartments is a highly regulated process that is essential for maintaining cellular homeostasis and generating adaptive and coordinated responses to external stimuli. To avoid prolonged receptor activation and signaling, receptor-ligand complexes are endocytosed and either recycled back to the plasma membrane or sorted to lysosomes for degradation (1-4). This process is mediated by reversible ubiquitination. The modification of a target protein with ubiquitin moieties is extensively utilized to regulate the assembly of endosomal machinery as well as being a sorting signal for transmembrane proteins within the endosomal system (5-7). The crucial role for ubiquitination in the trafficking of endocytosed receptors has been documented for receptor-tyrosine kinases and a number of G protein-coupled receptors including the chemokine receptor CXCR4 (8 -10). CXCR4 has been extensively studied as a co-receptor for the entry of T-trophic human immunodeficiency virus into CD4ϩ T-cells (11,12). Along with its cognate ligand, Stromal cell-derived factor-1␣ (SDF-1/ CXCL12), CXCR4 plays important roles in hematopoiesis, development, and organization of the immune system and stem cell homing (13-15). CXCR4 deregulation is associated with various pathologies, including human immunodeficiency virus infection, cardiovascular disease, and neurodegenerative diseases and is associated with metastatic disease in a number of cancers (16 -20). Mutations that result in truncation of the C-terminal intracellular region of CXCR4 cause WHIM sy...

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Stimulation of mammalian cells frequently initiates phospholipase D-catalysedhydrolysis of phosphatidylcholine in the plasma membrane to yield phosphatidic acid (PA) a novel lipid messenger. PA plays a regulatory role in important cellular processes such as secretion, cellular shape change and movement. A number of studies have highlighted that PLD-based signalling also plays a pro-mitogenic and pro-survival role in cells and therefore anti-apoptotic. We show that human PLD1b and PLD2a contain functional caspase-3 cleavage sites and identify the critical aspartate residues within PLD1b that affect its activation by phorbol esters and attenuate phosphatidylcholine hydrolysis during apoptosis.