Tiziana Anelli

Protein quality control in the early secretory pathway

Eukaryotic cells are able to discriminate between native and non‐native polypeptides, selectively transporting the former to their final destinations. Secretory proteins are scrutinized at the endoplasmic reticulum (ER)–Golgi interface. Recent findings reveal novel features of the underlying molecular mechanisms, with several chaperone networks cooperating in assisting the maturation of complex proteins and being selectively induced to match changing synthetic demands. ‘Public’ and ‘private’ chaperones, some of which enriched in specializes subregions, operate for most or selected substrates, respectively. Moreover, sequential checkpoints are distributed along the early secretory pathway, allowing efficiency and fidelity in protein secretion.

ERp44, a novel endoplasmic reticulum folding assistant of the thioredoxin family

In human cells, Ero1‐Lα and ‐Lβ (hEROs) regulate oxidative protein folding by selectively oxidizing protein disulfide isomerase. Specific protein–protein interactions are probably crucial for regulating the formation, isomerization and reduction of disulfide bonds in the endoplasmic reticulum (ER). To identify molecules involved in ER redox control, we searched for proteins interacting with Ero1‐Lα. Here, we characterize a novel ER resident protein (ERp44), which contains a thioredoxin domain with a CRFS motif and is induced during ER stress. ERp44 forms mixed disulfides with both hEROs and cargo folding intermediates. Whilst the interaction with transport‐competent Ig‐K chains is transient, ERp44 binds more stably with J chains, which are retained in the ER and eventually degraded by proteasomes. ERp44 does not bind a short‐lived ribophorin mutant lacking cysteines. Its overexpression alters the equilibrium of the different Ero1‐Lα redox isoforms, suggesting that ERp44 may be involved in the control of oxidative protein folding.

Thiol-mediated protein retention in the endoplasmic reticulum: the role of ERp44

Formation of disulfide bonds, an essential step for the maturation and exit of secretory proteins from the endoplasmic reticulum (ER), is controlled by specific ER‐resident enzymes. A pivotal element in this process is Ero1α, an oxidoreductin that lacks known ER retention motifs. Here we show that ERp44 mediates Ero1α ER localization through the formation of reversible mixed disulfides. ERp44 also prevents the secretion of an unassembled cargo protein with unpaired cysteines. We conclude that ERp44 is a key element in thiol‐mediated retention. It might also favour the maturation of disulfide‐linked oligomeric proteins and their quality control.

Sequential steps and checkpoints in the early exocytic compartment during secretory IgM biogenesis

The biogenesis of secretory IgM occurs stepwise under stringent quality control, formation of μ2L2 preceding polymerization. How is efficiency of IgM secretion coupled to fidelity? We show here that ERp44, a soluble protein involved in thiol‐mediated retention, interacts with ERGIC‐53. Binding to this hexameric lectin contributes to ERp44 localization in the ER‐golgi intermediate compartment. ERp44 and ERGIC‐53 increase during B‐lymphocyte differentiation, concomitantly with the onset of IgM polymerization. Both preferentially bind μ2L2 and higher order intermediates. Their overexpression or silencing in non‐lymphoid cells promotes or decreases secretion of IgM polymers, respectively. In IgM‐secreting B‐lymphoma cells, μ chains interact first with BiP and later with ERp44 and ERGIC‐53. Our findings suggest that ERGIC‐53 provides a platform that receives μ2L2 subunits from the BiP‐dependent checkpoint, assisting polymerization. In this process, ERp44 couples thiol‐dependent assembly and quality control.

Ero1α regulates Ca(2+) fluxes at the endoplasmic reticulum-mitochondria interface (MAM)

AIMS The endoplasmic reticulum (ER) is involved in many functions, including protein folding, redox homeostasis, and Ca(2+) storage and signaling. To perform these multiple tasks, the ER is composed of distinct, specialized subregions, amongst which mitochondrial-associated ER membranes (MAM) emerge as key signaling hubs. How these multiple functions are integrated with one another in living cells remains unclear. RESULTS Here we show that Ero1α, a key controller of oxidative folding and ER redox homeostasis, is enriched in MAM and regulates Ca(2+) fluxes. Downregulation of Ero1α by RNA interference inhibits mitochondrial Ca(2+) fluxes and modifies the activity of mitochondrial Ca(2+) uniporters. The overexpression of redox active Ero1α increases passive Ca(2+) efflux from the ER, lowering [Ca(2+)](ER) and mitochondrial Ca(2+) fluxes in response to IP3 agonists. INNOVATION The unexpected observation that Ca(2+) fluxes are affected by either increasing or decreasing the levels of Ero1α reveals a pivotal role for this oxidase in the early secretory compartment and implies a strict control of its amounts. CONCLUSIONS Taken together, our results indicate that the levels, subcellular localization, and activity of Ero1α coordinately regulate Ca(2+) and redox homeostasis and signaling in the early secretory compartment.

Crystal structure of human ERp44 shows a dynamic functional modulation by its carboxy-terminal tail

ERp44 mediates thiol‐dependent retention in the early secretory pathway, forming mixed disulphides with substrate proteins through its conserved CRFS motif. Here, we present its crystal structure at a resolution of 2.6 Å. Three thioredoxin domains—a, b and b′—are arranged in a clover‐like structure. A flexible carboxy‐terminal tail turns back to the b′ and a domains, shielding a hydrophobic pocket in domain b′ and a hydrophobic patch around the CRFS motif in domain a. Mutational and functional studies indicate that the C‐terminal tail gates the CRFS area and the adjacent hydrophobic pocket, dynamically regulating protein quality control.

ER storage diseases: a role for ERGIC-53 in controlling the formation and shape of Russell bodies.

Owing to the impossibility of reaching the Golgi for secretion or the cytosol for degradation, mutant Ig-μ chains that lack the first constant domain (μΔCH1) accumulate as detergent-insoluble aggregates in dilated endoplasmic reticulum cisternae, called Russell bodies. The presence of similar structures hallmarks many ER storage diseases, but their pathogenic role(s) remain obscure. Exploiting inducible cellular systems, we show here that Russell bodies form when the synthesis of μΔCH1 exceeds the degradation capacity. Condensation occurs in different sub-cellular locations, depending on the interacting molecules present in the host cell: if Ig light chains are co-expressed, detergent-insoluble μΔCH1-light chain oligomers accumulate in large ribosome-coated structures (rough Russell bodies). In absence of light chains, instead, aggregation occurs in smooth tubular vesicles and is controlled by N-glycan-dependent interactions with ER-Golgi intermediate compartment 53 (ERGIC-53). In cells containing smooth Russell bodies, ERGIC-53 co-localizes with μΔCH1 aggregates in a Ca2+-dependent fashion. Our findings identify a novel ERGIC-53 substrate, and indicate that interactions with light chains or ERGIC-53 seed μΔCH1 condensation in different stations of the early secretory pathway.

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

Summary To 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.

Pathogenesis of ER Storage Disorders: Modulating Russell Body Biogenesis by Altering Proximal and Distal Quality Control

In many protein storage diseases, detergent‐insoluble proteins accumulate in the early secretory compartment (ESC). Protein condensation reflects imbalances between entry into (synthesis/translocation) and exit from (secretion/degradation) ESC, and can be also a consequence of altered quality control (QC) mechanisms. Here we exploit the inducible formation of Russell bodies (RB), dilated ESC cisternae containing mutant Ig‐µ chains, as a model to mechanistically dissect protein condensation. Depending on the presence or absence of Ig‐L chains, mutant Ig‐µ chains lacking their first constant domain (Ch1) accumulate in rough or smooth RB (rRB and sRB), dilations of the endoplasmic reticulum (ER) and ER‐Golgi intermediate compartment (ERGIC), respectively, reflecting the proximal and distal QC stations in the stepwise biogenesis of polymeric IgM. Either weakening ERp44‐dependent distal QC or facilitating ER‐associated degradation (ERAD) inhibits RB formation. Overexpression of PDI or ERp44 inhibits µΔCh1 secretion. However, PDI inhibits while ERp44 promotes µΔCh1 condensation. Both Ero1α silencing and overexpression prevent RB formation, demonstrating a strict redox dependency of the phenomenon. Altogether, our findings identify key controllers of protein condensation along the ESC as potential targets to handle certain storage disorders.

Physiology and pathology of proteostasis in the early secretory compartment

The endoplasmic reticulum (ER), the port of entry for proteins into the secretory pathway, is a multifunctional organelle emerging as a central integrator of numerous signalling pathways. The mechanisms that control proteostasis are integral part of this signalling network, providing cues for morphological and functional cell remodelling, proliferation, inflammation and cell death. The complexity of ER responses is exploited during physiological and pathological tissue development, cell differentiation and lifespan control. This essay outlines some of the mechanisms that link proteostasis within the early secretory compartment to signalling in development and disease.