A virtuous cycle operated by ERp44 and ERGIC-53 guarantees proteostasis in the early secretory compartment.

The composition of the secretome depends on the combined action of cargo receptors that facilitate protein transport and sequential checkpoints that restrict it to native conformers. Acting after endoplasmic reticulum (ER)-resident chaperones, ERp44 retrieves its clients from downstream compartments. To guarantee efficient quality control, ERp44 should exit the ER as rapidly as its clients, or more. Here, we show that appending ERp44 to different cargo proteins increases their secretion rates. ERp44 binds the cargo receptor ER-Golgi intermediate compartment (ERGIC)-53 in the ER to negotiate preferential loading into COPII vesicles. Silencing ERGIC-53, or competing for its COPII binding with 4-phenylbutyrate, causes secretion of Prdx4, an enzyme that relies on ERp44 for intracellular localization. In more acidic, zinc-rich downstream compartments, ERGIC-53 releases its clients and ERp44, which can bind and retrieve non-native conformers via KDEL receptors. By coupling the transport of cargoes and inspector proteins, cells ensure efficiency and fidelity of secretion.

PINK1 and Parkin: The odd couple.

In 2004, PINK1 was established as a gene linked to early onset of autosomal recessive juvenile Parkinsonism. Since then, tremendous efforts allowed involving the gene product in diverse events but with a strong focus on its partnership with the protein Parkin for the degradation of damaged mitochondria through mitophagy. Yet, it is now clear that the importance of PINK1 encompasses a wider spectrum of intracellular processes. In this minireview, we highlight some of the PINK1 interplays and recent advances, including its growing involvement in immunity and also its emerging place in this era of mitochondria-organelles contact sites.

Redox crosstalk at endoplasmic reticulum (ER) membrane contact sites (MCS) uses toxic waste to deliver messages.

Many cellular redox reactions housed within mitochondria, peroxisomes and the endoplasmic reticulum (ER) generate hydrogen peroxide (H2O2) and other reactive oxygen species (ROS). The contribution of each organelle to the total cellular ROS production is considerable, but varies between cell types and also over time. Redox-regulatory enzymes are thought to assemble at a "redox triangle" formed by mitochondria, peroxisomes and the ER, assembling "redoxosomes" that sense ROS accumulations and redox imbalances. The redoxosome enzymes use ROS, potentially toxic by-products made by some redoxosome members themselves, to transmit inter-compartmental signals via chemical modifications of downstream proteins and lipids. Interestingly, important components of the redoxosome are ER chaperones and oxidoreductases, identifying ER oxidative protein folding as a key ROS producer and controller of the tri-organellar membrane contact sites (MCS) formed at the redox triangle. At these MCS, ROS accumulations could directly facilitate inter-organellar signal transmission, using ROS transporters. In addition, ROS influence the flux of Ca2+ ions, since many Ca2+ handling proteins, including inositol 1,4,5 trisphosphate receptors (IP3Rs), SERCA pumps or regulators of the mitochondrial Ca2+ uniporter (MCU) are redox-sensitive. Fine-tuning of these redox and ion signaling pathways might be difficult in older organisms, suggesting a dysfunctional redox triangle may accompany the aging process.

Regulation of Calcium Fluxes by GPX8, a Type-II Transmembrane Peroxidase Enriched at the Mitochondria-Associated Endoplasmic Reticulum Membrane.

Glutathione peroxidases (GPXs) are enzymes that are present in almost all organisms with the primary function of limiting peroxide accumulation. In mammals, two of the eight members (GPX7 and GPX8) reside in the endoplasmic reticulum (ER). A peculiar feature of GPX8 is the concomitant presence of a conserved N-terminal transmembrane domain (TMD) and a C-terminal KDEL-like motif for ER localization. AIMS: Investigating whether and how GPX8 impacts Ca2+ homeostasis and signaling. RESULTS: We show that GPX8 is enriched in mitochondria-associated membranes and regulates Ca2+ storage and fluxes. Its levels correlate with [Ca2+]ER, and cytosolic and mitochondrial Ca2+ fluxes. GPX7, which lacks a TMD, does not share these properties. Deleting or replacing the GPX8 TMD with an unrelated N-terminal membrane integration sequence abolishes all effects on Ca2+ fluxes, whereas appending the GPX8 TMD to GPX7 transfers the Ca2+-regulating properties. Innovation and Conclusion: The notion that the TMD of GPX8, in addition to its enzymatic activity, is essential for regulating Ca2+ dynamics reveals a novel level of integration between redox-related proteins and Ca2+ signaling/homeostasis. Antioxid. Redox Signal. 27, 583-595.

A dynamic study of protein secretion and aggregation in the secretory pathway.

Precise coordination of protein biogenesis, traffic and homeostasis within the early secretory compartment (ESC) is key for cell physiology. As a consequence, disturbances in these processes underlie many genetic and chronic diseases. Dynamic imaging methods are needed to follow the fate of cargo proteins and their interactions with resident enzymes and folding assistants. Here we applied the Halotag labelling system to study the behavior of proteins with different fates and roles in ESC: a chaperone, an ERAD substrate and an aggregation-prone molecule. Exploiting the Halo property of binding covalently ligands labelled with different fluorochromes, we developed and performed non-radioactive pulse and chase assays to follow sequential waves of proteins in ESC, discriminating between young and old molecules at the single cell level. In this way, we could monitor secretion and degradation of ER proteins in living cells. We can also follow the biogenesis, growth, accumulation and movements of protein aggregates in the ESC. Our data show that protein deposits within ESC grow by sequential apposition of molecules up to a given size, after which novel seeds are detected. The possibility of using ligands with distinct optical and physical properties offers a novel possibility to dynamically follow the fate of proteins in the ESC.