Identification of a factor that accelerates substrate release from the signal recognition particle.

The eukaryotic signal recognition particle (SRP) cotranslationally recognizes the first hydrophobic segment of nascent secretory and membrane proteins and delivers them to a receptor at the endoplasmic reticulum (ER). How substrates are released from SRP at the ER to subsequently access translocation factors is not well understood. We found that TMEM208 can engage the substrate binding domain of SRP to accelerate release of its bound cargo.

Mechanism of NACHO-mediated assembly of pentameric ligand-gated ion channels.

Pentameric ligand-gated ion channels (pLGICs) are cell surface receptors of crucial importance for animal physiology. This diverse protein family mediates the ionotropic signals triggered by major neurotransmitters and includes γ-aminobutyric acid receptors (GABARs) and acetylcholine receptors (nAChRs). Receptor function is fine-tuned by a myriad of endogenous and pharmacological modulators.

Soluble αβ-tubulins reversibly sequester TTC5 to regulate tubulin mRNA decay.

Microtubules, built from heterodimers of α- and β-tubulins, control cell shape, mediate intracellular transport, and power cell division. The concentration of αβ-tubulins is tightly controlled through a posttranscriptional mechanism involving selective and regulated degradation of tubulin-encoding mRNAs. Degradation is initiated by TTC5, which recognizes tubulin-synthesizing ribosomes and recruits downstream effectors to trigger mRNA deadenylation.

Structural analysis of the dynamic ribosome-translocon complex.

The protein translocon at the endoplasmic reticulum comprises the Sec61 translocation channel and numerous accessory factors that collectively facilitate the biogenesis of secretory and membrane proteins. Here, we leveraged recent advances in cryo-electron microscopy (cryo-EM) and structure prediction to derive insights into several novel configurations of the ribosome-translocon complex. We show how a transmembrane domain (TMD) in a looped configuration passes through the Sec61 lateral gate during membrane insertion; how a nascent chain can bind and constrain the conformation of ribosomal protein uL22; and how the translocon-associated protein (TRAP) complex can adjust its position during different stages of protein biogenesis.

A unifying model for membrane protein biogenesis.

α-Helical integral membrane proteins comprise approximately 25% of the proteome in all organisms. The membrane proteome is highly diverse, varying in the number, topology, spacing and properties of transmembrane domains. This diversity imposes different constraints on the insertion of different regions of a membrane protein into the lipid bilayer.

Getting membrane proteins into shape.

In this issue, Ji et al. show how a multipass membrane protein that initially inserts into the endoplasmic reticulum in a mostly inverted topology is post-translationally dislocated, re-inserted, and folded with the help of ATP13A1, a P-type ATPase.

How to enjoy and thrive in graduate school.

Scientific research is an exploration of the unknown. The process is full of uncertainty, missteps, delightful surprises, painful lessons, and ultimately a measure of insight into nature. In this Science and Society article I suggest a few practical strategies that helped me navigate these challenges at the earliest stages of becoming a cell biologist.

Things you should learn in graduate school.

Becoming an academic scientist is neither linear nor formulaic. Rather, graduate education is mostly an apprenticeship without a discrete curriculum, clear sign posts of progress, or specific training metrics. In this science and society article I offer a few thoughts from my own experience for what a student beginning this journey might want to learn.

EMC rectifies the topology of multipass membrane proteins.

Most eukaryotic multipass membrane proteins are inserted into the membrane of the endoplasmic reticulum. Their transmembrane domains (TMDs) are thought to be inserted co-translationally as they emerge from a membrane-bound ribosome. Here we find that TMDs near the carboxyl terminus of mammalian multipass proteins are inserted post-translationally by the endoplasmic reticulum membrane protein complex (EMC).

Control of mRNA fate by its encoded nascent polypeptide.

Cells tightly regulate mRNA processing, localization, and stability to ensure accurate gene expression in diverse cellular states and conditions. Most of these regulatory steps have traditionally been thought to occur before translation by the action of RNA-binding proteins. Several recent discoveries highlight multiple co-translational mechanisms that modulate mRNA translation, localization, processing, and stability.

Mechanism of orphan subunit recognition during assembly quality control.

Cells contain numerous abundant molecular machines assembled from multiple subunits. Imbalances in subunit production and failed assembly generate orphan subunits that are eliminated by poorly defined pathways. Here, we determined how orphan subunits of the cytosolic chaperonin CCT are recognized.

Mechanism of ribosome-associated mRNA degradation during tubulin autoregulation.

Microtubules play crucial roles in cellular architecture, intracellular transport, and mitosis. The availability of free tubulin subunits affects polymerization dynamics and microtubule function. When cells sense excess free tubulin, they trigger degradation of the encoding mRNAs, which requires recognition of the nascent polypeptide by the tubulin-specific ribosome-binding factor TTC5.

Mechanism of signal-anchor triage during early steps of membrane protein insertion.

Most membrane proteins use their first transmembrane domain, known as a signal anchor (SA), for co-translational targeting to the endoplasmic reticulum (ER) via the signal recognition particle (SRP). The SA then inserts into the membrane using either the Sec61 translocation channel or the ER membrane protein complex (EMC) insertase. How EMC and Sec61 collaborate to ensure SA insertion in the correct topology is not understood.

Mechanism of an intramembrane chaperone for multipass membrane proteins.

Multipass membrane proteins play numerous roles in biology and include receptors, transporters, ion channels and enzymes. How multipass proteins are co-translationally inserted and folded at the endoplasmic reticulum is not well understood. The prevailing model posits that each transmembrane domain (TMD) of a multipass protein successively passes into the lipid bilayer through a front-side lateral gate of the Sec61 protein translocation channel.

Substrate-driven assembly of a translocon for multipass membrane proteins.

Most membrane proteins are synthesized on endoplasmic reticulum (ER)-bound ribosomes docked at the translocon, a heterogeneous ensemble of transmembrane factors operating on the nascent chain. How the translocon coordinates the actions of these factors to accommodate its different substrates is not well understood. Here we define the composition, function and assembly of a translocon specialized for multipass membrane protein biogenesis.

The Biogenesis of Multipass Membrane Proteins.

Multipass membrane proteins contain two or more α-helical transmembrane domains (TMDs) that span the lipid bilayer. They are inserted cotranslationally into the prokaryotic plasma membrane or eukaryotic endoplasmic reticulum membrane. The Sec61 complex (SecY complex in prokaryotes) provides a ribosome docking site, houses a channel across the membrane, and contains a lateral gate that opens toward the lipid bilayer.

Quality Control: Maintaining molecular order and preventing cellular chaos.

We asked experts from different fields-from genome maintenance and proteostasis to organelle degradation via ubiquitin and autophagy-"What does quality control mean to you?" Despite their diverse backgrounds, they converge on and discuss the importance of continuous quality control at all levels, context, communication, timing, decisions on whether to repair or remove, and the significance of dysregulated quality control in disease.

The Function, Structure, and Origins of the ER Membrane Protein Complex.

The endoplasmic reticulum (ER) is the site of membrane protein insertion, folding, and assembly in eukaryotes. Over the past few years, a combination of genetic and biochemical studies have implicated an abundant factor termed the ER membrane protein complex (EMC) in several aspects of membrane protein biogenesis. This large nine-protein complex is built around a deeply conserved core formed by the EMC3-EMC6 subcomplex.

Mechanism of signal sequence handover from NAC to SRP on ribosomes during ER-protein targeting.

The nascent polypeptide-associated complex (NAC) interacts with newly synthesized proteins at the ribosomal tunnel exit and competes with the signal recognition particle (SRP) to prevent mistargeting of cytosolic and mitochondrial polypeptides to the endoplasmic reticulum (ER). How NAC antagonizes SRP and how this is overcome by ER targeting signals are unknown. Here, we found that NAC uses two domains with opposing effects to control SRP access.

A unified evolutionary origin for the ubiquitous protein transporters SecY and YidC.

Background: Protein transporters translocate hydrophilic segments of polypeptide across hydrophobic cell membranes. Two protein transporters are ubiquitous and date back to the last universal common ancestor: SecY and YidC. SecY consists of two pseudosymmetric halves, which together form a membrane-spanning protein-conducting channel.

The mechanisms of integral membrane protein biogenesis.

Roughly one quarter of all genes code for integral membrane proteins that are inserted into the plasma membrane of prokaryotes or the endoplasmic reticulum membrane of eukaryotes. Multiple pathways are used for the targeting and insertion of membrane proteins on the basis of their topological and biophysical characteristics. Multipass membrane proteins span the membrane multiple times and face the additional challenges of intramembrane folding.

Identification of a quality-control factor that monitors failures during proteasome assembly.

In eukaryotic cells, half of all proteins function as subunits within multiprotein complexes. Imbalanced synthesis of subunits leads to unassembled intermediates that must be degraded to minimize cellular toxicity. Here, we found that excess PSMC5, a subunit of the proteasome base, was targeted for degradation by the HERC1 ubiquitin ligase in mammalian cells.

Translation stress and collided ribosomes are co-activators of cGAS.

The cyclic GMP-AMP synthase-stimulator of interferon genes (cGAS-STING) pathway senses cytosolic DNA and induces interferon-stimulated genes (ISGs) to activate the innate immune system. Here, we report the unexpected discovery that cGAS also senses dysfunctional protein production. Purified ribosomes interact directly with cGAS and stimulate its DNA-dependent activity in vitro.

An intramembrane chaperone complex facilitates membrane protein biogenesis.

Integral membrane proteins are encoded by approximately 25% of all protein-coding genes. In eukaryotes, the majority of membrane proteins are inserted, modified and folded at the endoplasmic reticulum (ER). Research over the past several decades has determined how membrane proteins are targeted to the ER and how individual transmembrane domains (TMDs) are inserted into the lipid bilayer.