Structure and elevator mechanism of the mammalian sodium/proton exchanger NHE9

Na+ /H+ exchangers (NHEs) are ancient membrane-bound nanomachines that work to regulate intracellular pH, sodium levels and cell volume. NHE activities contribute to the control of the cell cycle, cell proliferation, cell migration and vesicle trafficking. NHE dysfunction has been linked to many diseases, and they are targets of pharmaceutical drugs. Despite their fundamental importance to cell homeostasis and human physiology, structural information for the mammalian NHE was lacking. Here, we report the cryogenic electron microscopy structure of NHE isoform 9 (SLC9A9) from Equus caballus at 3.2 Å resolution, an endosomal isoform highly expressed in the brain and associated with autism spectrum (ASD) and attention deficit hyperactivity (ADHD) disorders. Despite low sequence identity, the NHE9 architecture and ion-binding site are remarkably similar to distantly related bacterial Na+ /H+  antiporters with 13 transmembrane segments. Collectively, we reveal the conserved architecture of the NHE ion-binding site, their elevator-like structural transitions, the functional implications of autism disease mutations and the role of phosphoinositide lipids to promote homodimerization that, together, have important physiological ramifications.

LRRK2 to the rescue of damaged endomembranes

Mutations in several genes encoding for lysosomal proteins are involved in Parkinson's disease (PD). In this issue, Herbst et al (2020) show that PD-related leucine-rich repeat kinase 2 (LRRK2) is activated in response to pathogen or membranolytic drug-induced damage of phagolysosomes and lysosomes in macrophages, and regulates endolysosomal homeostasis by controlling the balance between membrane repair and degradation.

Lipoic acid and autophagy: new insights into stem cell aging

The gastrointestinal tract undergoes homeostatic self-renewal to replace aged and damaged epithelial cells. This process, sustained by intestinal stem cells (ISCs), can operate accurately for many years but gradually declines with age. Although stem cell aging has been intensively explored, the mechanisms remain poorly understood. In this issue of EMBO Reports, Du et al report that alpha-lipoic acid (ALA) sustains an active endocytosis-autophagy network that effectively reverses age-dependent ISC hyperplasia in Drosophila (Du et al, 2020). This work suggests a new strategy for treating aging-associated gastrointestinal diseases.

A snapshot of membrane protein insertion

The cytoplasm is the main place for protein translation from where nascent proteins are transported to their working areas, including the inside, outside, and membrane of the cell. The majority of newly synthesized membrane proteins is co-translationally inserted into the membrane by the evolutionary conserved Sec translocon. In this issue of EMBO Reports, Kater et al [1] use single-particle cryo-electron microscopy to visualize a high-resolution structure of the E. coli SecYEG translocon:ribosome-nascent chain complex in a lipid environment constituted by nanodiscs. This snapshot represents an early intermediate state in membrane protein insertion and provides important information for understanding the molecular mechanism of membrane protein biogenesis.

EGAD! There is an ERAD doppelganger in the Golgi

Disposal of membrane proteins in the late secretory pathway is thought to be exclusively facilitated by ESCRT-dependent lysosomal degradation. In this issue of The EMBO Journal, Schmidt et al define a previously uncharacterized endosome and Golgi-associated degradation (EGAD) pathway. This pathway, which has remarkable similarities to ERAD in the endoplasmic reticulum, operates in post-ER organelles via the proteasome and contributes to lipid homeostasis in eukaryotic cells.

To Fis or not to Fuse? This is the question!

Mitochondrial shaping proteins are categorized as fusion and fission factors based on their opposing actions on the architecture of mitochondria. Mammalian Fis1 fragments mitochondria into small units and was initially considered a major mitochondrial fission factor, with this major role being challenged. Yu et al (2019 ) define that Fis1 induces mitochondrial fragmentation by blocking mitochondrial fusion. This study highlights the limitation of categorizing mitochondrial dynamics proteins based on architecture measurements, given the general tendency to associate fragmentation with activation of fission.

Tuberculosis's cargoman: bacteria load RNA into host extracellular vesicles

Tuberculosis remains one of the deadliest infectious diseases worldwide. Mycobacterium tuberculosis (M.tb) has developed various mechanisms to manipulate the human host, in particular by disrupting the host phagosome and the immune response. It is becoming evident that secreted extracellular vesicles (EVs) are involved in the dynamic crosstalk between M.tb and the host cells. These vesicles shuttle different cargo components, such as RNA , lipids, and proteins, between cells. In this issue of EMBO Reports , Cheng and Schorey 1 describe a previously unknown EV‐mediated process, regulating M.tb RNA loading into EVs and their internalization by naïve macrophages. They identify the mycobacterial Sec2 secretion system as involved in RNA loading into EVs and show that secreted vesicles contain bacterial RNA that not only promotes IFN ‐β production upon entry into target cells, but also leads to M.tb clearance via the activation of the host's RIG ‐I/MAVS signaling pathway. Importantly, combined treatment with secreted EVs and antibiotics decreases bacterial load in a mouse model, improving lung pathology compared to treatment with antibiotics alone.

Nup62-mediated nuclear import of p63 in squamous cell carcinoma

Nuclear pore complexes (NPC s) are multiprotein channels that bridge the nucleus with the cytoplasm and regulate all nucleo‐cytoplasmic traffic. NPC s are built by the repetition of ~30 different proteins known as nucleoporins (Nups). Accumulating evidence has revealed a diversity in NPC composition that is critical for cell‐specific functionality and fate determination. A new report by Hazawa et al 1 now identifies the central transport channel nucleoporin Nup62 as a novel regulator of cell proliferation and differentiation in squamous cell carcinoma (SCC ), via modulation of p63 nucleo‐cytoplasmic transport. These findings provide further evidence on how alterations in NPC composition might be utilized to determine cell fate.

Mitochondrial permeability transition, F1 FO -ATPase and calcium: an enigmatic triangle

In response to calcium mitochondria can undergo a permeability transition-a sudden increase of the permeability of the inner mitochondrial membrane to ions and solutes resulting in cell death. Salvatore Nesci discusses recent advances in the identification of the structure that forms the mitochondria permeability transition pore.

The N- and C-terminal ends of RPGR can bind to PDE6δ

This study shows that the prenylated C‐terminus of RPGR can bind to PDE6δ with high affinity, suggesting two distinct binding sites of the RPGR/PDE6δ complex. The serine residue at the −3 position relative to the prenylated cysteine seems to play a key role in defining the selectivity of PDE6δ towards ciliary prenylated cargo.