Identification of phosphorylation sites in the C-terminal region of charged multivesicular body protein 1A (CHMP1A)

CHMP1B is a target of USP8/UBPY regulated by ubiquitin during endocytosis

Integration and down-regulation of cell growth and differentiation signals rely on plasma membrane receptor endocytosis and sorting towards either recycling vesicles or degradative lysosomes via multivesicular bodies (MVB). In this process, the endosomal sorting complex-III required for transport (ESCRT-III) controls membrane deformation and scission triggering intraluminal vesicle (ILV) formation at early endosomes. Here, we show that the ESCRT-III member CHMP1B can be ubiquitinated within a flexible loop known to undergo conformational changes during polymerization. We demonstrate further that CHMP1B is deubiquitinated by the ubiquitin specific protease USP8 (syn. UBPY) and found fully devoid of ubiquitin in a ~500 kDa large complex that also contains its ESCRT-III partner IST1. Moreover, EGF stimulation induces the rapid and transient accumulation of ubiquitinated forms of CHMP1B on cell membranes. Accordingly, CHMP1B ubiquitination is necessary for CHMP1B function in both EGF receptor trafficking in human cells and wing development in Drosophila. Based on these observations, we propose that CHMP1B is dynamically regulated by ubiquitination in response to EGF and that USP8 triggers CHMP1B deubiquitination possibly favoring its subsequent assembly into a membrane-associated ESCRT-III polymer.

In multicellular organisms, the interpretation and transmission of cell growth and differentiation signals strongly rely on plasma membrane receptors. Once activated by their ligands, these receptors activate downstream signaling cascades and are rapidly internalized into intracellular vesicles that fuse inside the cell to form the endosomal compartment. From there, the receptors are sorted towards either recycling vesicles or degradative lysosomes via multivesicular bodies. Receptors sorting therefore plays a crucial role in the integration and regulation of intracellular signals during development and numerous physio-pathological processes. It requires extensive membrane remodeling and scission events at the level of the endosomal compartment by so-called ESCRT proteins, including CHMP1B. In this study, we provide evidence for dynamic regulation of CHMP1B function and subcellular localization by ubiquitin linkage. We also show the contribution of the ubiquitin specific protease USP8 in this regulation, which is a known actor of intracellular trafficking and Cushing’s disease.

ESCRT Requirements for Murine Leukemia Virus Release

The Murine Leukemia Virus (MLV) is a gammaretrovirus that hijack host components of the endosomal sorting complex required for transport (ESCRT) for budding. To determine the minimal requirements for ESCRT factors in MLV viral and viral-like particles (VLP) release, an siRNA knockdown screen of ESCRT(-associated) proteins was performed in MLV-producing human cells. We found that MLV VLPs and virions primarily engage the ESCRT-I factor Tsg101 and marginally the ESCRT-associated adaptors Nedd4-1 and Alix to enter the ESCRT pathway. Conversely, the inactivation of ESCRT-II had no impact on VLP and virion egress. By analyzing the effects of individual ESCRT-III knockdowns, VLP and virion release was profoundly inhibited in CHMP2A- and CHMP4B-knockdown cells. In contrast, neither the CHMP2B and CHMP4A isoforms nor CHMP3, CHMP5, and CHMP6 were found to be essential. In case of CHMP1, we unexpectedly observed that the CHMP1A isoform was specifically required for virus budding, but dispensable for VLP release. Hence, MLV utilizes only a subset of ESCRT factors, and viral and viral-like particles differ in ESCRT-III factor requirements.