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The coat protein complex II (COPII) mediates forward trafficking of protein and lipid cargoes from the endoplasmic reticulum. COPII is an ancient and essential pathway in all eukaryotes and COPII dysfunction underlies a range of human diseases. Despite this broad significance, major aspects of COPII trafficking remain incompletely understood. For example, while the biochemical features of COPII vesicle formation are relatively well characterized, much less is known about how the COPII system dynamically adjusts its activity to changing physiologic cues or stresses. Recently, post-transcriptional mechanisms have emerged as a major mode of COPII regulation. Here, we review the current literature on how post-transcriptional events, and especially post-translational modifications, govern the COPII pathway.

Intracellular transport undergoes remodeling upon cell differentiation, which involves cell type-specific regulators. Bone morphogenetic protein 2-inducible kinase (BMP2K) has been potentially implicated in endocytosis and cell differentiation but its molecular functions remained unknown. We discovered that its longer (L) and shorter (S) splicing variants regulate erythroid differentiation in a manner unexplainable by their involvement in AP-2 adaptor phosphorylation and endocytosis. However, both variants interacted with SEC16A whose silencing in K562 erythroid leukemia cells affected generation of COPII assemblies and induced autophagic degradation. Variant-specific depletion approach showed that BMP2K isoforms constitute a BMP2K-L/S regulatory system. Therein, L promotes while S restricts recruitment of SEC31A to SEC24B-containing COPII structures forming at SEC16A-positive ER exit sites. Finally, we found L to promote and S to restrict autophagic degradation. Hence, we propose that BMP2K-L favors SEC16A-dependent intracellular processes important for erythroid maturation, such as COPII trafficking and autophagy, in a manner inhibited by BMP2K-S.

Membrane and secretory proteins are essential for almost every aspect of cellular function. These proteins are incorporated into ER-derived carriers and transported to the Golgi before being sorted for delivery to their final destination. Although ER-to-Golgi trafficking is highly conserved among eukaryotes, several layers of complexity have been added to meet the increased demands of complex cell types in metazoans. The specialized morphology of neurons and the necessity for precise spatiotemporal control over membrane and secretory protein localization and function make them particularly vulnerable to defects in trafficking. This review summarizes the general mechanisms involved in ER-to-Golgi trafficking and highlights mutations in genes affecting this process, which are associated with neurological diseases in humans.

COPII mediates Endoplasmic Reticulum to Golgi trafficking of thousands of cargoes. Five essential proteins assemble into a two-layer architecture, with the inner layer thought to regulate coat assembly and cargo recruitment, and the outer coat forming cages assumed to scaffold membrane curvature. Here we visualise the complete, membrane-assembled COPII coat by cryo-electron tomography and subtomogram averaging, revealing the full network of interactions within and between coat layers. We demonstrate the physiological importance of these interactions using genetic and biochemical approaches. Mutagenesis reveals that the inner coat alone can provide membrane remodelling function, with organisational input from the outer coat. These functional roles for the inner and outer coats significantly move away from the current paradigm, which posits membrane curvature derives primarily from the outer coat. We suggest these interactions collectively contribute to coat organisation and membrane curvature, providing a structural framework to understand regulatory mechanisms of COPII trafficking and secretion., Cytosolic coat proteins capture secretory cargo and sculpt membrane carriers for intracellular transport, such as COPII which mediates Endoplasmic Reticulum to Golgi trafficking of thousands of cargoes. Here authors visualise the complete, membrane-assembled COPII coat by cryo-electron tomography and subtomogram averaging, revealing the full network of interactions within and between coat layers.

Farhan discusses Scharaw et al.’s study about how the COPII machinery is used to replenish EGFR at the cell surface., Cell surface levels of epidermal growth factor receptors (EGFRs) are thought to be controlled mainly by endocytic trafficking, with biosynthetic EGFR trafficking presumed to be a constitutive and unregulated process. However, Scharaw et al. (2016. J. Cell Biol. http://dx.doi.org/10.1083/jcb.201601090) demonstrate a role for inducible COPII trafficking in controlling EGFR surface levels.

The 2 major cellular lipid species, sterols and fatty acids, have many roles in physiology. Not only are they essential structural components of eukaryotic cell membranes, but they are also used as sources for cellular energy, serve as precursors for biosynthesis of key cellular macromolecules, are strategically used as post-translational modifications of many proteins, and function as signaling molecules for a wide variety of cellular tasks. Lipid availability is tightly associated with cell growth and cell cycle progression; therefore, regulatory mechanisms have evolved to balance lipid levels with their diverse cellular roles. Because of their intimate association with cellular growth, it is not surprising that small perturbations in the regulatory pathways that modulate lipid levels contribute to the development and progression of a myriad of metabolic diseases. Thus, organisms have evolved sensitive regulatory mechanisms that operate at the transcriptional and post-transcriptional levels to maintain lipid homeostasis. A key transcriptional pathway for regulating lipid metabolism that is conserved throughout the eukarya is mediated by the sterol regulatory element-binding proteins (SREBPs).1 In mammals there are 3 SREBP proteins expressed from 2 unlinked structural genes. The individual SREBP isoforms are regulated by different cellular cues, and all 3 have evolved to mediate slightly different roles in lipid metabolism. Selective SREBP knockout studies, combined with unbiased genome-wide analyses have also provided evidence of a broader role for SREBPs in physiology and metabolism.1 SREBPs are synthesized with 2 closely spaced hydrophobic helices in the middle of the coding sequence that anchor the transcription factor precursor in a hairpin configuration in the ER membrane.1,2 This extra-nuclear, inactive SREBP forms a complex with another ER membrane protein called SREBP cleavage activating protein (SCAP). When cholesterol levels are replete, SCAP interacts with cholesterol in the ER membrane, and in this form SCAP also binds to the ER retention factor Insig. In contrast, as sterol levels decline, the SCAP–cholesterol interaction is dis-favored, SCAP undergoes a conformational change and dissociates from Insig. This dissociation unmasks a small peptide signal within SCAP containing the sequence MELADL, which interacts with Sec24 of the COPII trafficking system. COP II escorts the SCAP–SREBP complex to the Golgi apparatus, where 2 proteases cleave the precursor SREBP, releasing the N-terminal portion, which represents the mature SREBP transcription factor. The soluble SREBP rapidly enters the nucleus and activates expression of SREBP target genes. Nuclear SREBP levels are regulated through an ubiquitin-dependent degradation pathway utilizing Fbxw7 as the requisite ubiquitin–E3 ligase-targeting factor. In addition to low-sterol conditions, it is likely that additional cellular cues alter the SCAP–SREBP itinerary to modulate SREBP activity. The precision and sensitivity of the SCAP–SREBP trafficking process requires a dynamic interplay between all of the interacting components, and changes in their relative concentrations can tip the balance and significantly alter the final levels of nuclear SREBPs. In a new study, we revealed that expression of a conserved microRNA (miR) island from mouse chromosome 6 is directly activated by SREBP-2.2 A single primary RNA transcript from this region is processed to yield 3 separate mature miRs: miR-96, miR-182, and miR-183. The mature 22–24 nucleotide miRs are incorporated into the RISC complex, which is then targeted to specific mRNAs by base-pair complementarity between the miRs and their target mRNAs, resulting in reduced expression of the corresponding protein. We showed that miR-96 targets the Insig2 and miR-182 targets Fbxw7. Since both of these proteins negatively influence the level of nuclear SREBPs, their inhibition by these miR siblings constitutes a feedback pathway to regulate SREBP activity. Because the 2 miRs are encoded by the same primary transcript that is regulated by SREBP-2, this system has the hallmark features of bacterial operons that are involved in regulating prokaryotic metabolism.2 Fbxw7 not only targets nuclear SREBPs for proteasomal degradation, but it also targets several oncogenic and cell cycle-regulatory proteins for degradation.3 Interestingly, miR-182 is overexpressed in several cancer models, where it may influence oncogene activity along with modulating SREBP function.4 In fact, SREBPs activate key genes of cell cycle regulation,5,6 and SREBPs are associated with the elevated rates of lipid synthesis required for growth of tumor cells in vitro and in tumor models in vivo.7 Another microRNA, miR-33, is produced from an intron of the SREBP-2 gene, and because SREBP-2 gene expression is auto-regulated, miR-33 is another SREBP-responsive micro-RNA.8 Interestingly, Fernandez-Hernando and colleagues reported that miR-33 inhibits the expression of the CDK6 and cyclin D1, thereby reducing cell proliferation and cell cycle progression.5 Taken together with the results from our new study, these observations portend a more intricate connection between SREBPs, cell cycle regulation, and tumor metabolism.

Intracellular cargo trafficking involves dramatic changes in membrane shape, the mechanics of which remain poorly understood. We focus on Sar1, the key regulator of the coat protein complex II (COPII) family that ferries newly synthesized proteins from the endoplasmic reticulum (ER), and the only member of the COPII coat that interacts directly with the ER lipid bilayer membrane. To investigate whether Sar1 has a role beyond merely localizing the other COPII proteins, we directly measure the force involved in membrane deformation as a function of its concentration, using optically trapped microspheres to pull tethers from in vitro lipid membranes whose composition and large surface area mimic the composition and geometry of the ER. We find that Sar1 lowers the rigidity (bending modulus) of lipid membranes to nearly zero in a concentration-dependent manner. Moreover, Sar1 lacking its N-terminal amphipathic helix induces negative (concave) spontaneous membrane curvature. These results reveal a paradigm-altering insight into COPII trafficking: Sar1 actively alters the material properties of the membranes it binds to, lowering the energetic cost of curvature generation.View Large Image | View Hi-Res Image | Download PowerPoint Slide

The trafficking of cargo in cells involves dramatic changes in membrane shape and topology. Though trafficking is widely studied and the identities and biochemical interactions of the responsible proteins are well mapped, remarkably little is known about the mechanics involved. We focus on Sar1, the key regulator of the coat protein complex II (COPII) family that ferries newly synthesized proteins from the ER to the Golgi. Sar1 is the only member of the COPII coat that interacts directly with the ER lipid bilayer membrane. It has an amphipathic N-terminal helix; when Sar1 is GTP-bound, the helix is exposed and the hydrophobic hemi-cylinder can insert into the bilayer. To investigate whether Sar1 has a physical role beyond merely localizing the other COPII proteins, we directly measure the force involved in membrane deformation as a function of its presence or absence, using optical traps and membrane-bound microspheres to pull tethers from lipid membranes. The lipid composition and large available surface area mimic the composition and geometry of the ER. Measurements of tether forces and radii allow extraction of the membrane bending modulus, the material parameter that dictates the energy required for deformation. We find that the bending modulus measured in the presence of Sar1 with a non-hydrolyzable GTP analogue, at concentrations sufficient for dense membrane coverage, is half that measured without Sar1 or with Sar1-GDP. These results reveal a paradigm-altering insight into COPII trafficking: Sar1 actively alters the material properties of the membranes it binds to, lowering the energetic cost of curvature generation.