Regulation of Endocytic Sorting by ESCRT–DUB-Mediated Deubiquitination

Abstract

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.

Ubiquitin-Dependent Sorting in Endocytosis

“…Interestingly, the activity of Doa4 appears to be regulated in response to Ub levels indicating that disposal of Ub via the MVB pathway may be used as a regulatory device to alter the activity of the entire ubiquitin/ proteasome system (Kimura et al 2009). The best candidate mammalian Doa4 ortholog is Usp8/UBPY, which can also bind ESCRT-III; however, its role in rescuing Ub from lysosomal degradation has not been specifically determined (Wright et al 2011). Indeed, USP8/ UBPY as well as a similarly positioned DUb, AMSH, can bind both ESCRT-0 and ESCRT-III and a host of experiments looking at different cargoes suggest they can play a role in deubiquitinating cargo before MVB sorting to promote cargo recycling (McCullough et al 2004;Mizuno et al 2005;Bowers et al 2006;Berlin et al 2010;Zhou et al 2013), cargo deubiquitination after sorting as a way to release them from the ESCRT apparatus and promote sorting into MVBs (Alwan and van Leeuwen 2007;Balut et al 2011), or as a way of deubiquitinating ESCRT components to rescue them from degradation or inactive conformations (Row et al 2007;Sierra et al 2010).…”

“…Artificial ubiquitination of ESCRTs can inactivate them. Moreover, ESCRTs-0 and -I associate with a variety of Ub ligases and DUbs and their depletion and overexpression hint at a variety of regulatory roles apart from merely modifying Ub cargo (Wright et al 2011). Nevertheless, preventing ubiquitination of ESCRT-0 and ESCRT-I does not block their ability to execute MVB sorting indicating that the role of ESCRTubiquitination may be for an ancillary function (such as controlling their levels via targeting to the proteasome) or providing an additional layer of regulation (Shields and Piper 2011).…”

Identification of Ubiquitin System Factors in Growth Hormone Receptor Transport

“…Furthermore, the ability of Usp8 to bind both ESCRT-I and ESCRT-III suggests a dual function for Usp8 in receptor sorting. Indeed, at the plasma membrane Usp8 can inhibit degradation of receptors by removing the ubiquitin tag that directs them to the multivesicular bodies, while at the multivesicular bodies Usp8 can promote degradation of receptors by allowing incorporation of receptors into the ILVs (Wright et al, 2011). We demonstrate here that depletion of Usp8 increases the level of intracellular GH receptor indicative of a defect in sorting towards the lysosome, most likely at the level of ESCRT-III.…”

Roles for Trafficking and O-Linked Glycosylation in the Turnover of Model Cell Surface Proteins

“…It is generally thought that ubiquitin tagging of cytoplasmic domains either at the cell surface or on endosomes targets them to late endosomes/lysosomes for degradation. Deubiquitinating activities may permit them to avoid ultimate destruction and recycle back to the cell surface (4,45). For proteins that lack cytoplasmic sequences (e.g.…”