Regulation and function of deubiquitinating enzyme USP19

泛素（Ub）对蛋白质的修饰是细胞生物学几乎所有方面的关键调控过程。底物蛋白可以在一个位点（单泛素化）或多个位点（多泛素化）上被单个泛素修饰。或者，几轮泛素化可以发生在泛素本身，导致多泛素链的形成。泛素的7种赖氨酸中的任何一种或氨基端都可以用于聚合泛素（Peng et al., 2003），因此可以形成大量不同连接的多泛素信号。Ub信号是可逆的，因为泛素可以通过去泛素化酶从底物中去除。不同的Ub信号在细胞中被无数的受体识别，这些受体携带不同的泛素结合基元，识别单或多泛素化底物（Hicke et al., 2005）。

Protein modification by ubiquitin (Ub) is a critical regulatory process for virtually all aspects of cell biology. Substrate proteins can be modified with single ubiquitin on one (monoubiquitylation) or multiple sites (multi-ubiquitylation). Alternatively, several rounds of ubiquitination can occur on ubiquitin itself, leading to the formation of a polyubiquitin chain. Any of the seven lysines, or the amino terminus, of ubiquitin can be used to polymerize ubiquitin (Peng et al., 2003), so there are a huge number of differently linked polyubiquitin signals that can be formed. Ub signals are reversible as ubiquitin can be removed from substrates by deubiquitinating enzymes. The diverse Ub signals are recognized in cells by a myriad of receptors that carry distinct ubiquitin binding motifs recognizing mono- or polyubiquitinated substrates (Hicke et al., 2005). The mechanisms that regulate deubiquitinases in the cells are unclear. Here we characterize 34 human DUBs including 25 USP, 4 OTU, 1 Josephin and 4 UCHL subfamily members. We show that many of these enzymes are reversibly inactivated when oxidized by reactive oxygen species (ROS) in vitro and in the cell. Oxidation occurs preferentially on the catalytic cysteine, abrogating the isopeptide-cleaving activity without affecting these enzymes affinity to ubiquitin. Sensitivity to oxidative inhibition is associated with the activation of the DUBs wherein the active site cysteine is converted to a deprotonated state prone to oxidation. We further demonstrate that this redox-dependent regulation is essential for mono-ubiquitination of PCNA to occur in response to oxidative DNA damage, which initiates a DNA damage tolerance program. These findings establish a novel mechanism of DUB regulation that may be integrated with other redox-dependent signaling circuits to govern cellular adaptation to oxidative stress, a process intimately linked to aging and cancer. We recently characterized the function and regulation of USP19. We identify Hsp90 as a specific partner that binds the catalytic domain of USP19 to promote substrate association. Intriguingly, although overexpressed USP19 interacts with Derlin-1 and other ERAD machinery factors in the membrane, endogenous USP19 is mostly in the cytosol where it binds Hsp90. Accordingly, we detect neither interaction of endogenous USP19 with Derlin-1 nor significant effect on ERAD by USP19 depletion. The USP19 transmembrane domain appears to be partially stabilized in the cytosol by an interaction with its own catalytic domain, resulting in auto-inhibition of its deubiquitinating activity. These results clarify the role of USP19 in ERAD and suggest a novel DUB regulation that involves chaperone association and membrane integration. We also discover a new cellular process that is regulated by USP19. Specifically, we report a pathway termed Misfolding-Associated Protein Secretion (MAPS), which uses the endoplasmic reticulum (ER)-associated deubiquitinase USP19 to preferentially export aberrant cytosolic proteins. Intriguingly, the catalytic domain of USP19 possesses an unprecedented chaperone activity, allowing recruitment of misfolded proteins to the ER surface for deubiquitination. Deubiquitinated cargos are encapsulated into ER-associated late endosomes and secreted to cell exterior. USP19 deficient cells cannot efficiently secrete unwanted proteins and grow more slowly than wild-type cells upon exposure to a proteasome inhibitor. Together, our findings delineate a protein quality control (PQC) pathway, which unlike degradation-based PQC mechanisms, promotes protein homeostasis by exporting misfolded proteins through an unconventional protein secretion process.