A pH-Regulated Quality Control Cycle for Surveillance of Secretory Protein Assembly

Abstract

SummaryTo warrant the quality of the secretory proteome, stringent control systems operate at the endoplasmic reticulum (ER)-Golgi interface, preventing the release of nonnative products. Incompletely assembled oligomeric proteins that are deemed correctly folded must rely on additional quality control mechanisms dedicated to proper assembly. Here we unveil how ERp44 cycles between cisGolgi and ER in a pH-regulated manner, patrolling assembly of disulfide-linked oligomers such as IgM and adiponectin. At neutral, ER-equivalent pH, the ERp44 carboxy-terminal tail occludes the substrate-binding site. At the lower pH of the cisGolgi, conformational rearrangements of this peptide, likely involving protonation of ERp44’s active cysteine, simultaneously unmask the substrate binding site and −RDEL motif, allowing capture of orphan secretory protein subunits and ER retrieval via KDEL receptors. The ERp44 assembly control cycle couples secretion fidelity and efficiency downstream of the calnexin/calreticulin and BiP-dependent quality control cycles.

Regulation and Quality Control of Adiponectin Assembly by Endoplasmic Reticulum Chaperone ERp44

“…This situation is analogous to the results reported in a recent study (37). We also investigated the pH stability of the ERp44-peptide complexes to answer the question of "how the ERp44-cargo complexes dissociate when retrieved to the ER" (39).…”

“…For instance, the Cys 160 /Cys 212 pair has been implicated in the regulation of binding to the IP3R1 channel (38), whereas the biological roles of the Cys 289 /Cys 272 pair and that of residue Cys 63 have not yet been assigned. Cys 29 was shown to be responsible for thiolmediated retention and release of ERp44 substrates such as IgM antibodies and oxidoreductase Ero1-␣ and also suggested to be implicated in a similar process with adiponectin (34,36,37). Binding and release of ERp44 substrates is believed to be pH-dependent occurring in the following manner.…”

“…Unlike other protein-disulfide isomerase family members, which are mainly found in the ER, the majority of ERp44 is localized in the ERGolgi intermediate compartment/cis-Golgi region (31,(33)(34)(35). The x-ray crystal structure of ERp44 (36) and recent findings (37) have revealed new insights into ERp44 action.…”

“…It is believed that at lower pH in cis-Golgi (pH 6.7) Cys 29 becomes protonated, a state that exhibits higher affinity to target Cys residues of ERp44 substrates, and the C-terminal tail rearranges to make the active site accessible. This exposes the RDEL motif at the end of the C-terminal tail, which can facilitate binding of the ERp44-client complex to KDEL receptors (37).…”

Ratiometric Imaging of pH Probes

“…Predominantly generated by ion exchangers, pumps, and channels, proton fluxes across the plasma membrane regulate dynamic changes in cytosolic or intracellular pH (pHi) that contribute to cell proliferation, cytoskeleton remodeling, and glycolytic metabolism (Casey, Grinstein, & Orlowski, 2010;Webb, Chimenti, Jacobson, & Barber, 2011). Proton fluxes across membranes of intracellular organelles generate pH gradients that drive vesicle trafficking and posttranslational modification and sorting of cargo proteins (Marshansky & Futai, 2008;Rivinoja, Kokkonen, Kellokumpu, & Kellokumpu, 2006;Vavassori et al, 2013). Although in normal conditions proton fluxes are homeostatically controlled to maintain pHi and organelle pH within narrow physiological ranges, dysregulated proton fluxes enable many diseases and pathologies, including cancer (Cardone, Casavola, & Reshkin, 2005;Stock & Schwab, 2009;Webb et al, 2011), neurodegenerative disorders (Harguindey, Reshkin, Orive, Arranz, & Anitua, 2007), and a number of myopathies and cardiovascular dysfunctions (VaughanJones, Spitzer, & Swietach, 2009).…”