Membrane protein retrieval from the Golgi apparatus to the endoplasmic reticulum (ER): characterization of the RER1 gene product as a component involved in ER localization of Sec12p

A comparative assessment of reference genes in mouse brown adipocyte differentiation and thermogenesis in vitro

Adipogenic differentiation and thermogenesis in brown adipose tissue (BAT) undergo dynamic processes, altering phenotypes and gene expressions. Proper reference genes in gene expression analysis are crucial to mitigate experimental variances and ensure PCR efficacy. Unreliable reference genes can lead to erroneous gene expression quantification, resulting in data misinterpretation. This study focused on identifying suitable reference genes for mouse brown adipocyte research, utilizing brown adipocytes from the Ucp1-luciferase ThermoMouse model. Comparative analysis of gene expression data under adipogenesis and thermogenesis conditions was conducted, validating 13 housekeeping genes through various algorithms, including DeltaCq, BestKeeper, geNorm, Normfinder, and RefFinder. Tbp and Rer1 emerged as optimal references for Ucp1 and Pparg expression in brown adipogenesis, while Tbp and Ubc were ideal for the expression analysis of these target genes in thermogenesis. Conversely, certain conventional references, including Actb, Tubb5, and Gapdh, proved unstable as reference genes under both conditions. These findings stress the critical consideration of reference gene selection in gene expression analysis within specific biological systems to ensure accurate conclusions.

DAP12 interacts with RER1 and is retained in the secretory pathway before assembly with TREM2

DNAX-activating protein of 12 kDa (DAP12) is a transmembrane adapter protein expressed in lymphoid and myeloid lineage cells. It interacts with several immunoreceptors forming functional complexes that trigger intracellular signaling pathways. One of the DAP12 associated receptors is the triggering receptor expressed on myeloid cells 2 (TREM2). Mutations in both DAP12 and TREM2 have been linked to neurodegenerative diseases. However, mechanisms involved in the regulation of subcellular trafficking and turnover of these proteins are not well understood. Here, we demonstrate that proteasomal degradation of DAP12 is increased in the absence of TREM2. Interestingly, unassembled DAP12 is also retained in early secretory compartments, including the endoplasmic reticulum (ER) and the ER-Golgi intermediate compartment (ERGIC), thereby preventing its transport to the plasma membrane. We also show that unassembled DAP12 interacts with the retention in ER sorting receptor 1 (RER1). The deletion of endogenous RER1 decreases expression of functional TREM2-DAP12 complexes and membrane proximal signaling, and resulted in almost complete inhibition of phagocytic activity in THP-1 differentiated macrophage-like cells. These results indicate that RER1 acts as an important regulator of DAP12 containing immunoreceptor complexes and immune cell function.

RF, Mohan H, Annaert W, Chaudhary V. Maintenance of proteostasis by Drosophila Rer1 is essential for competitive cell survival and Myc-driven overgrowth

Defects in protein homeostasis can induce proteotoxic stress, affecting cellular fitness and, consequently, overall tissue health. In various growing tissues, cell competition based mechanisms facilitate detection and elimination of these compromised, often referred to as 'loser', cells by the healthier neighbors. The precise connection between proteotoxic stress and competitive cell survival remains largely elusive. Here, we reveal the function of an endoplasmic reticulum (ER) and Golgi localized protein Rer1 in the regulation of protein homeostasis in the developing Drosophila wing epithelium. Our results show that loss of Rer1 leads to proteotoxic stress and PERK-mediated phosphorylation of eukaryotic initiation factor 2α. Clonal analysis showed that rer1 mutant cells are identified as losers and eliminated through cell competition. Interestingly, we find that Rer1 levels are upregulated upon Myc-overexpression that causes overgrowth, albeit under high proteotoxic stress. Our results suggest that increased levels of Rer1 provide cytoprotection to Myc-overexpressing cells by alleviating the proteotoxic stress and thereby supporting Myc-driven overgrowth. In summary, these observations demonstrate that Rer1 acts as a novel regulator of proteostasis in Drosophila and reveal its role in competitive cell survival.

The structure of COPI vesicles and regulation of vesicle turnover

COPI-coated vesicles mediate transport between Golgi stacks and retrograde transport from the Golgi to the endoplasmic reticulum. The COPI coat exists as a stable heptameric complex in the cytosol termed coatomer and is recruited en bloc to the membrane for vesicle formation. Recruitment of COPI onto membranes is mediated by the Arf family of small GTPases, which, in their GTP-bound state, bind both membrane and coatomer. Arf GTPases also influence cargo selection, vesicle scission and vesicle uncoating. Guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs) regulate nucleotide binding by Arf GTPases. To understand the mechanism of COPI-coated vesicle trafficking, it is necessary to characterize the interplay between coatomer and Arf GTPases and their effectors. It is also necessary to understand interactions between coatomer and cargo, cargo adaptors/receptors and tethers facilitating binding to the target membrane. Here, we summarize current knowledge of COPI coat protein structure; we describe how structural and biochemical studies contributed to this knowledge; we review mechanistic insights into COPI vesicle biogenesis and disassembly; and we discuss the potential to answer open questions in the field.

Nedd4-2 Haploinsufficiency in Mice Impairs the Ubiquitination of Rer1 and Increases the Susceptibility to Endoplasmic Reticulum Stress and Seizures

Neural precursor cell expressed developmentally downregulated gene 4-like (NEDD4-2) is an epilepsy-associated gene encoding an E3 ligase that ubiquitinates neuroactive substrates. An involvement of NEDD4-2 in endoplasmic reticulum (ER) stress has been recently found with mechanisms needing further investigations. Herein, Nedd4-2+/− mice were found intolerant to thapsigargin (Tg) to develop ER stress in the brain. Pretreatment of Tg aggravated the pentylenetetrazole (PTZ)-induced seizures. Retention in endoplasmic reticulum 1 (Rer1), an ER retrieval receptor, was upregulated through impaired ubiquitination in Nedd4-2+/− mouse brain. Nedd4-2 interacted with Rer1 more strongly in mice with Tg administration. The negative regulation and NEDD4-2-mediated ubiquitination on RER1 were evaluated in cultured neurocytes and gliacytes by NEDD4-2 knockdown and overexpression. NEDD4-2 interacted with RER1 at higher levels in the cells with Tg treatment. Disruption of the 36STPY39 motif of RER1 attenuated the interaction with NEDD4-2, and the ubiquitinated RER1 underwent proteasomal degradation. Furthermore, the interactome of Rer1 was screened by immunoprecipitation-mass spectrometry in PTZ-induced mouse hippocampus, showing multiple potential ER retrieval cargoes that mediate neuroexcitability. The α1 subunit of the GABAA receptor was validated to interact with Rer1 and retain in ER more heavily in Nedd4-2+/− mouse brain by Endo-H digestion. In conclusion, Nedd4-2 deficiency in mice showed impaired ubiquitination of Rer1 and increased ER stress and seizures. These data indicate a protective effect of NEDD4-2 in ER stress and seizures possibly via RER1. We also provided potential ER retention cargoes of Rer1 awaiting further investigation.

The Golgi Apparatus and its Next-Door Neighbors

The Golgi apparatus represents a central compartment of membrane traffic. Its apparent architecture, however, differs considerably among species, from unstacked and scattered cisternae in the budding yeast Saccharomyces cerevisiae to beautiful ministacks in plants and further to gigantic ribbon structures typically seen in mammals. Considering the well-conserved functions of the Golgi, its fundamental structure must have been optimized despite seemingly different architectures. In addition to the core layers of cisternae, the Golgi is usually accompanied by next-door compartments on its cis and trans sides. The trans-Golgi network (TGN) can be now considered as a compartment independent from the Golgi stack. On the cis side, the intermediate compartment between the ER and the Golgi (ERGIC) has been known in mammalian cells, and its functional equivalent is now suggested for yeast and plant cells. High-resolution live imaging is extremely powerful for elucidating the dynamics of these compartments and has revealed amazing similarities in their behaviors, indicating common mechanisms conserved along the long course of evolution. From these new findings, I would like to propose reconsideration of compartments and suggest a new concept to describe their roles comprehensively around the Golgi and in the post-Golgi trafficking.

Trafficking-defective mutant PROKR2 cycles between endoplasmic reticulum and Golgi to attenuate endoplasmic reticulum stress

The endoplasmic reticulum (ER) possesses a quality control system that prevents misfolded proteins from leaving the ER for routing to the ER-associated degradation pathway. Some misfolded proteins can escape the ER to reach the Golgi, where they are then retrieved from the Golgi back to the ER for degradation, but why this occurs needs to be clarified. Studying a mutant prokineticin receptor 2 identified in patients with hypogonadotropic hypogonadism as a model, we find that the post-ER retrieval system provides another layer of quality control and also lowers the load of misfolded proteins in the ER to reduce ER stress. Our findings reveal the importance of a post-ER quality control mechanism in contributing to cellular homeostasis.

G protein–coupled receptors (GPCRs) play crucial roles in numerous physiological and pathological processes. Mutations in GPCRs that result in loss of function or alterations in signaling can lead to inherited or acquired diseases. Herein, studying prokineticin receptor 2 (PROKR2), we initially identify distinct interactomes for wild-type (WT) versus a mutant (P290S) PROKR2 that causes hypogonadotropic hypogonadism. We then find that both the WT and mutant PROKR2 are targeted for endoplasmic reticulum (ER)-associated degradation, but the mutant is degraded to a greater extent. Further analysis revealed that both forms can also leave the ER to reach the Golgi. However, whereas most of the WT is further transported to the cell surface, most of the mutant is retrieved to the ER. Thus, the post-ER itinerary plays an important role in distinguishing the ultimate fate of the WT versus the mutant. We have further discovered that this post-ER itinerary reduces ER stress induced by the mutant PROKR2. Moreover, we extend the core findings to another model GPCR. Our findings advance the understanding of disease pathogenesis induced by a mutation at a key residue that is conserved across many GPCRs and thus contributes to a fundamental understanding of the diverse mechanisms used by cellular quality control to accommodate misfolded proteins.

Chemoproteomic-enabled characterization of small GTPase Rab1a as a target of an N-arylbenzimidazole ligand's rescue of Parkinson's-associated cell toxicity

The development of phenotypic models of Parkinson's disease (PD) has enabled screening and identification of phenotypically active small molecules that restore complex biological pathways affected by PD toxicity.

Bring it back, bring it back, don't take it away from me - the sorting receptor RER1

The quote "bring it back, bring it back, don't take it away from me" from Queen's Love of my life describes the function of the sorting receptor RER1, a 23 kDa protein with four transmembrane domains (TMDs) that localizes to the intermediate compartment and the cis-Golgi. From there it returns escaped proteins that are not supposed to leave the endoplasmic reticulum (ER) back to it. Unique about RER1 is its ability to recognize its ligands through binding motifs in TMDs. Among its substrates are ER-resident proteins, as well as unassembled subunits of multimeric complexes that are retrieved back into the ER, this way guarding the full assembly of their respective complexes. The basic mechanisms for RER1-dependent retrieval have been already elucidated some years ago in yeast. More recently, several important cargoes of RER1 have been described in mammalian cells, and the in vivo role of RER1 is being unveiled by using mouse models. In this Review, we give an overview of the cell biology of RER1 in different models, discuss its controversial role in the brain and provide an outlook on future directions for RER1 research.

A novel mechanism for the retention of Golgi membrane proteins mediated by the Bre5p/Ubp3p deubiquitinase complex

The mechanisms employed in the retention of Golgi resident membrane proteins are diverse and include features such as the composition and length of the protein’s transmembrane domain and motifs that mediate direct or indirect associations with COPI-coatomer. However, in sum the current compendium of mechanisms cannot account for the localization of all Golgi membrane proteins, and this is particularly the case for proteins such as the glycosyltransferases. Here we describe a novel mechanism that mediates the steady-state retention of a subset of glycosyltransferases in the Golgi of budding yeast cells. This mechanism is mediated by a deubiquitinase complex composed of Bre5p and Ubp3p. We show that in the absence of this deubiquitinase certain glycosyltransferases are mislocalized to the vacuole, where they are degraded. We also show that Bre5p/Ubp3p clients bind to COPI-coatomer via a series of positively charged amino acids in their cytoplasmically exposed N-termini. Furthermore, we identify two proteins (Ktr3p and Mnn4p) that show a requirement for both Bre5p/Ubp3p as well as the COPI-coatomer–affiliated sorting receptor Vps74p. We also establish that some proteins show a nutrient-dependent role for Vps74p in their Golgi retention. This study expands the repertoire of mechanisms mediating the retention of Golgi membrane proteins.

Modulation of proteostasis and protein trafficking: a therapeutic avenue for misfolded G protein-coupled receptors causing disease in humans

Proteostasis refers to the process whereby the cell maintains in equilibrium the protein content of different compartments. This system consists of a highly interconnected network intended to efficiently regulate the synthesis, folding, trafficking, and degradation of newly synthesized proteins. Molecular chaperones are key players of the proteostasis network. These proteins assist in the assembly and folding processes of newly synthesized proteins in a concerted manner to achieve a three-dimensional structure compatible with export from the endoplasmic reticulum to other cell compartments. Pharmacologic interventions intended to modulate the proteostasis network and tackle the devastating effects of conformational diseases caused by protein misfolding are under development. These include small molecules called pharmacoperones, which are highly specific toward the target protein serving as a molecular framework to cause misfolded mutant proteins to fold and adopt a stable conformation suitable for passing the scrutiny of the quality control system and reach its correct location within the cell. Here, we review the main components of the proteostasis network and how pharmacoperones may be employed to correct misfolding of two G protein-coupled receptors, the vasopressin 2 receptor and the gonadotropin-releasing hormone receptor, whose mutations lead to X-linked nephrogenic diabetes insipidus and congenital hypogonadotropic hypogonadism in humans respectively.

RER1 enhances carcinogenesis and stemness of pancreatic cancer under hypoxic environment

Background

Increasing incidence and mortality rates of pancreatic cancer (PC) highlight an urgent need for novel and efficient drugs. Retention in endoplasmic reticulum 1 (RER1) is an important retention factor in the endoplasmic reticulum (ER). However, it remains elusive whether RER1 is involved in the retention of disease-related proteins.

Methods

We analyzed the expression level of RER1 in PC and adjacent tissues, and also employed Kaplan–Meier’s analysis to identify the correlation between RER1 expression and overall survival rate. Cell proliferation, colony formation, tumor formation, scratch test, and transwell invasion assays were performed in RER1 knockdown cells and negative control cells.

Results

We hereby reported the important functions of RER1 in tumorigenesis and metastasis of PC, evidenced by inhibitory effects of RER1 knockdown on PC cell proliferation, migration and aggressiveness. Tumor formation was also significantly repressed in RER1 knockdown cells compared to control. Hypoxia-inducible factor (HIF)-1α was found to be an upstream regulator of RER1. Knockdown HIF-1α cells exhibited similar repressive impact on cell proliferation as RER1, and showed diminished migratory and invasive abilities under hypoxic condition.

Conclusion

The present study has demonstrated that RER1 enhances the progression of PC through promoting cell proliferation, migration and aggressiveness.

TANGO1 and SEC12 are copackaged with procollagen I to facilitate the generation of large COPII carriers

Collagen is a major component of the extracellular matrix, and its secretion requires cytoplasmic proteins that assemble on the surface of the endoplasmic reticulum to bud ∼100-nm-diameter cargo transport vesicles (COPII). Bulky collagens, such as the 300-nm procollagen I (PC1), are too big to fit into normal COPII vesicles. Recently, large COPII-coated vesicles were found to act as PC1 carriers, but how these large COPII carriers are generated remains unclear. Here, we show copackaging of PC1 along with its cargo receptor TANGO1, a coreceptor protein, cTAGE5, and the COPII initiating factor SEC12. Because SEC12 is excluded from small COPII vesicles, we propose that TANGO1 targets SEC12 to PC1-containing endoplasmic reticulum and drives the formation of large COPII-coated vesicles.

Large coat protein complex II (COPII)-coated vesicles serve to convey the large cargo procollagen I (PC1) from the endoplasmic reticulum (ER). The link between large cargo in the lumen of the ER and modulation of the COPII machinery remains unresolved. TANGO1 is required for PC secretion and interacts with PC and COPII on opposite sides of the ER membrane, but evidence suggests that TANGO1 is retained in the ER, and not included in normal size (<100 nm) COPII vesicles. Here we show that TANGO1 is exported out of the ER in large COPII-coated PC1 carriers, and retrieved back to the ER by the retrograde coat, COPI, mediated by the C-terminal RDEL retrieval sequence of HSP47. TANGO1 is known to target the COPII initiation factor SEC12 to ER exit sites through an interacting protein, cTAGE5. SEC12 is important for the growth of COPII vesicles, but it is not sorted into small budded vesicles. We found both cTAGE5 and SEC12 were exported with TANGO1 in large COPII carriers. In contrast to its exclusion from small transport vesicles, SEC12 was particularly enriched around ER membranes and large COPII carriers that contained PC1. We constructed a split GFP system to recapitulate the targeting of SEC12 to PC1 via the luminal domain of TANGO1. The minimal targeting system enriched SEC12 around PC1 and generated large PC1 carriers. We conclude that TANGO1, cTAGE5, and SEC12 are copacked with PC1 into COPII carriers to increase the size of COPII, thus ensuring the capture of large cargo.

Rer1-mediated quality control system is required for neural stem cell maintenance during cerebral cortex development

Rer1 is a retrieval receptor for endoplasmic reticulum (ER) retention of various ER membrane proteins and unassembled or immature components of membrane protein complexes. However, its physiological functions during mammalian development remain unclear. This study aimed to investigate the role of Rer1-mediated quality control system in mammalian development. We show that Rer1 is required for the sufficient cell surface expression and activity of γ-secretase complex, which modulates Notch signaling during mouse cerebral cortex development. When Rer1 was depleted in the mouse cerebral cortex, the number of neural stem cells decreased significantly, and malformation of the cerebral cortex was observed. Rer1 loss reduced γ-secretase activity and downregulated Notch signaling in the developing cerebral cortex. In Rer1-deficient cells, a subpopulation of γ-secretase complexes and components was transported to and degraded in lysosomes, thereby significantly reducing the amount of γ-secretase complex on the cell surface. These results suggest that Rer1 maintains Notch signaling by maintaining sufficient expression of the γ-secretase complex on the cell surface and regulating neural stem cell maintenance during cerebral cortex development.

We showed that Rer1 functions as an early-Golgi quality control pathway that maintains γ-secretase activity by maintaining sufficient cell surface expression of γ-secretase complex during cerebral cortex development, thereby modulating Notch signaling.

The degradation pathway of a model misfolded protein is determined by aggregation propensity

Protein homeostasis in the secretory pathway is maintained by a hierarchy of quality control checkpoints, including endoplasmic reticulum–associated degradation (ERAD), which leads to the destruction of misfolded proteins in the ER, as well as post-ER proteolysis. Although most aberrant proteins are degraded by ERAD, some misfolded proteins escape the ER and are degraded instead by lysosomal/vacuolar proteases. To date, it remains unclear how misfolded membrane proteins are selected for these different fates. Here we designed a novel model substrate, SZ*, to investigate how substrate selection is mediated in yeast. We discovered that SZ* is degraded by both the proteasome and vacuolar proteases, the latter of which occurs after ER exit and requires the multivesicular body pathway. By interrogating how various conditions affect the fate of SZ*, we also discovered that heat-shock and substrate overexpression increase ERAD targeting. These conditions also increase substrate aggregation. We next found that aggregation of the membrane-free misfolded domain in SZ* is concentration dependent, and fusion of this misfolded domain to a post-ER quality control substrate instead targets the substrate for ERAD. Our data indicate that a misfolded membrane protein with a higher aggregation propensity is preferentially retained in the ER and targeted for ERAD.

The ER retention protein RER1 promotes alpha-synuclein degradation via the proteasome

Abnormal accumulation of α-synuclein (αSyn) has been linked to endoplasmic-reticulum (ER) stress, defective intracellular protein/vesicle trafficking, and cytotoxicity. Targeting factors involved in ER-related protein processing and trafficking may, therefore, be a key to modulating αSyn levels and associated toxicity. Recently retention in endoplasmic reticulum 1 (RER1) has been identified as an important ER retrieval/retention factor for Alzheimer's disease proteins and negatively regulates amyloid-β peptide levels. Here, we hypothesized that RER1 might also play an important role in retention/retrieval of αSyn and mediate levels. We expressed RER1 and a C-terminal mutant RER1Δ25, which lacks the ER retention/retrieval function, in HEK293 and H4 neuroglioma cells. RER1 overexpression significantly decreased levels of both wild type and A30P, A53T, and E46K disease causal mutants of αSyn, whereas the RER1Δ25 mutant had a significantly attenuated effect on αSyn. RER1 effects were specific to αSyn and had little to no effect on either βSyn or the Δ71-82 αSyn mutant, which both lack the NAC domain sequence critical for synuclein fibrillization. Tests with proteasomal and macroautophagy inhibitors further demonstrate that RER1 effects on αSyn are primarily mediated through the ubiquitin-proteasome system. RER1 also appears to interact with the ubiquitin ligase NEDD4. RER1 in human diseased brain tissues co-localizes with αSyn-positive Lewy bodies. Together, these findings provide evidence that RER1 is a novel and potential important mediator of elevated αSyn levels. Further investigation of the mechanism of RER1 and downstream effectors on αSyn may yield novel therapeutic targets for modulation in Parkinson disease and related synucleinopathies.

Remodeling of ER-exit sites initiates a membrane supply pathway for autophagosome biogenesis

Autophagosomes are double-membrane vesicles generated during autophagy. Biogenesis of the autophagosome requires membrane acquisition from intracellular compartments, the mechanisms of which are unclear. We previously found that a relocation of COPII machinery to the ER-Golgi intermediate compartment (ERGIC) generates ERGIC-derived COPII vesicles which serve as a membrane precursor for the lipidation of LC3, a key membrane component of the autophagosome. Here we employed super-resolution microscopy to show that starvation induces the enlargement of ER-exit sites (ERES) positive for the COPII activator, SEC12, and the remodeled ERES patches along the ERGIC A SEC12 binding protein, CTAGE5, is required for the enlargement of ERES, SEC12 relocation to the ERGIC, and modulates autophagosome biogenesis. Moreover, FIP200, a subunit of the ULK protein kinase complex, facilitates the starvation-induced enlargement of ERES independent of the other subunits of this complex and associates via its C-terminal domain with SEC12. Our data indicate a pathway wherein FIP200 and CTAGE5 facilitate starvation-induced remodeling of the ERES, a prerequisite for the production of COPII vesicles budded from the ERGIC that contribute to autophagosome formation.

ER retention is imposed by COPII protein sorting and attenuated by 4-phenylbutyrate

Native cargo proteins exit the endoplasmic reticulum (ER) in COPII-coated vesicles, whereas resident and misfolded proteins are substantially excluded from vesicles by a retention mechanism that remains unresolved. We probed the ER retention process using the proteostasis regulator 4-phenylbutyrate (4-PBA), which we show targets COPII protein to reduce the stringency of retention. 4-PBA competes with p24 proteins to bind COPII. When p24 protein uptake is blocked, COPII vesicles package resident proteins and an ER-trapped mutant LDL receptor. We further show that 4-PBA triggers the secretion of a KDEL-tagged luminal resident, implying that a compromised retention mechanism causes saturation of the KDEL retrieval system. The results indicate that stringent ER retention requires the COPII coat machinery to actively sort biosynthetic cargo from diffusible misfolded and resident ER proteins.

DOI:http://dx.doi.org/10.7554/eLife.26624.001

Transmembrane domain quality control systems operate at the endoplasmic reticulum and Golgi apparatus

Multiple protein quality control systems operate to ensure that misfolded proteins are efficiently cleared from the cell. While quality control systems that assess the folding status of soluble domains have been extensively studied, transmembrane domain (TMD) quality control mechanisms are poorly understood. Here, we have used chimeras based on the type I plasma membrane protein CD8 in which the endogenous TMD was substituted with transmembrane sequences derived from different polytopic membrane proteins as a mode to investigate the quality control of unassembled TMDs along the secretory pathway. We find that the three TMDs examined prevent trafficking of CD8 to the cell surface via potentially distinct mechanisms. CD8 containing two distinct non-native transmembrane sequences escape the ER and are subsequently retrieved from the Golgi, possibly via Rer1, leading to ER localisation at steady state. A third chimera, containing an altered transmembrane domain, was predominantly localised to the Golgi at steady state, indicating the existence of an additional quality control checkpoint that identifies non-native transmembrane domains that have escaped ER retention and retrieval. Preliminary experiments indicate that protein retained by quality control mechanisms at the Golgi are targeted to lysosomes for degradation.

Golgi membrane-associated degradation pathway in yeast and mammals

Autophagy is a cellular process that degrades subcellular constituents, and is conserved from yeast to mammals. Although autophagy is believed to be essential for living cells, cells lacking Atg5 or Atg7 are healthy, suggesting that a non-canonical degradation pathway exists to compensate for the lack of autophagy. In this study, we show that the budding yeast Saccharomyces cerevisiae, which lacks Atg5, undergoes bulk protein degradation using Golgi-mediated structures to compensate for autophagy when treated with amphotericin B1, a polyene antifungal drug. We named this mechanism Golgi membrane-associated degradation (GOMED) pathway. This process is driven by the disruption of PI(4)P-dependent anterograde trafficking from the Golgi, and it also exists in Atg5-deficient mammalian cells. Biologically, when an Atg5-deficient β-cell line and Atg7-deficient β-cells were cultured in glucose-deprived medium, a disruption in the secretion of insulin granules from the Golgi occurred, and GOMED was induced to digest these (pro)insulin granules. In conclusion, GOMED is activated by the disruption of PI(4)P-dependent anterograde trafficking in autophagy-deficient yeast and mammalian cells.

Formation and maintenance of the Golgi apparatus in plant cells

The Golgi apparatus plays essential roles in intracellular trafficking, protein and lipid modification, and polysaccharide synthesis in eukaryotic cells. It is well known for its unique stacked structure, which is conserved among most eukaryotes. However, the mechanisms of biogenesis and maintenance of the structure, which are deeply related to ER-Golgi and intra-Golgi transport systems, have long been mysterious. Now having extremely powerful microscopic technologies developed for live-cell imaging, the plant Golgi apparatus provides an ideal system to resolve the question. The plant Golgi apparatus has unique features that are not conserved in other kingdoms, which will also give new insights into the Golgi functions in plant life. In this review, we will summarize the features of the plant Golgi apparatus and transport mechanisms around it, with a focus on recent advances in Golgi biogenesis by live imaging of plants cells.

Rer1 and calnexin regulate endoplasmic reticulum retention of a peripheral myelin protein 22 mutant that causes type 1A Charcot-Marie-Tooth disease

Peripheral myelin protein 22 (PMP22) resides in the plasma membrane and is required for myelin formation in the peripheral nervous system. Many PMP22 mutants accumulate in excess in the endoplasmic reticulum (ER) and lead to the inherited neuropathies of Charcot-Marie-Tooth (CMT) disease. However, the mechanism through which PMP22 mutants accumulate in the ER is unknown. Here, we studied the quality control mechanisms for the PMP22 mutants L16P and G150D, which were originally identified in mice and patients with CMT. We found that the ER-localised ubiquitin ligase Hrd1/SYVN1 mediates ER-associated degradation (ERAD) of PMP22(L16P) and PMP22(G150D), and another ubiquitin ligase, gp78/AMFR, mediates ERAD of PMP22(G150D) as well. We also found that PMP22(L16P), but not PMP22(G150D), is partly released from the ER by loss of Rer1, which is a Golgi-localised sorting receptor for ER retrieval. Rer1 interacts with the wild-type and mutant forms of PMP22. Interestingly, release of PMP22(L16P) from the ER was more prominent with simultaneous knockdown of Rer1 and the ER-localised chaperone calnexin than with the knockdown of each gene. These results suggest that CMT disease-related PMP22(L16P) is trapped in the ER by calnexin-dependent ER retention and Rer1-mediated early Golgi retrieval systems and partly degraded by the Hrd1-mediated ERAD system.

Endoplasmic reticulum chaperones and oxidoreductases: critical regulators of tumor cell survival and immunorecognition

Endoplasmic reticulum (ER) chaperones and oxidoreductases are abundant enzymes that mediate the production of fully folded secretory and transmembrane proteins. Resisting the Golgi and plasma membrane-directed “bulk flow,” ER chaperones and oxidoreductases enter retrograde trafficking whenever they are pulled outside of the ER by their substrates. Solid tumors are characterized by the increased production of reactive oxygen species (ROS), combined with reduced blood flow that leads to low oxygen supply and ER stress. Under these conditions, hypoxia and the unfolded protein response upregulate their target genes. When this occurs, ER oxidoreductases and chaperones become important regulators of tumor growth. However, under these conditions, these proteins not only promote the folding of proteins, but also alter the properties of the plasma membrane and hence modulate tumor immune recognition. For instance, high levels of calreticulin serve as an “eat-me” signal on the surface of tumor cells. Conversely, both intracellular and surface BiP/GRP78 promotes tumor growth. Other ER folding assistants able to modulate the properties of tumor tissue include protein disulfide isomerase (PDI), Ero1α and GRP94. Understanding the roles and mechanisms of ER chaperones in regulating tumor cell functions and immunorecognition will lead to important insight for the development of novel cancer therapies.

Rer1p regulates the ER retention of immature rhodopsin and modulates its intracellular trafficking

Rhodopsin is a pigment in photoreceptor cells. Some rhodopsin mutations cause the protein to accumulate in the endoplasmic reticulum (ER), leading to photoreceptor degeneration. Although several mutations have been reported, how mutant rhodopsin is retained in the ER remains unclear. In this study, we identified Rer1p as a modulator of ER retention and rhodopsin trafficking. Loss of Rer1p increased the transport of wild-type rhodopsin to post-Golgi compartments. Overexpression of Rer1p caused immature wild-type rhodopsin to accumulate in the ER. Interestingly, the G51R rhodopsin mutant, which has a mutation in the first transmembrane domain and accumulates in the ER, was released to the plasma membrane or lysosomes in Rer1-knockdown cells. Consistent with these results, Rer1p interacted with both wild-type and mutant rhodopsin. These results suggest that Rer1p regulates the ER retention of immature or misfolded rhodopsin and modulates its intracellular trafficking through the early secretory pathway.

Intracellular retention of the extracellular domain of the (pro)renin receptor in mammalian cells

As a component of the renin-angiotensin system, the (pro)renin receptor [(P)RR] activates prorenin along with intracellular signaling pathways. In this study, the glutathione S-transferase-fused extracellular domain of (P)RR expressed in mammalian cells was recovered in the detergent phase in detergent-based two-phase separation experiments, and intracellular localization was observed by immunocytochemistry, suggesting retention inside the cell through stable membrane association.

Fluorescence-based visualization of autophagic activity predicts mouse embryo viability

Embryo quality is a critical parameter in assisted reproductive technologies. Although embryo quality can be evaluated morphologically, embryo morphology does not correlate perfectly with embryo viability. To improve this, it is important to understand which molecular mechanisms are involved in embryo quality control. Autophagy is an evolutionarily conserved catabolic process in which cytoplasmic materials sequestered by autophagosomes are degraded in lysosomes. We previously demonstrated that autophagy is highly activated after fertilization and is essential for further embryonic development. Here, we developed a simple fluorescence-based method for visualizing autophagic activity in live mouse embryos. Our method is based on imaging of the fluorescence intensity of GFP-LC3, a versatile marker for autophagy, which is microinjected into the embryos. Using this method, we show that embryonic autophagic activity declines with advancing maternal age, probably due to a decline in the activity of lysosomal hydrolases. We also demonstrate that embryonic autophagic activity is associated with the developmental viability of the embryo. Our results suggest that embryonic autophagic activity can be utilized as a novel indicator of embryo quality.

Protein quality control by Rer1p in the early secretory pathway: from mechanism to implication in Alzheimer's disease

γ-Secretase-mediated production of amyloid β from the amyloid precursor protein is recognized as a central player in the neuropathogenesis of Alzheimer’s disease (AD). One of the most peculiar features of this enzymatic activity is the fact that it targets transmembrane domains of mostly type I integral membrane proteins and thus manages to proteolyse peptide bonds within the hydrophobic lipid bilayers. In addition, γ-secretase does not exert its activity solely towards amyloid precursor protein, but to an increasing number of membrane proteins, including Notch, cadherins, syndecans, and so on. Because of the requirement of intramembrane proteolysis for a plethora of signaling pathways and cellular processes during embryonic development and organ physiology, this enzyme has drawn a lot of attention in the past 20 years. γ-Secretase is a multimeric transmembrane complex consisting of the catalytic presenilin, nicastrin, presenilin enhancer 2 (PEN2) and anterior-pharynx defective-1 (APH1) subunits. Proper assembly into functional complexes requires quality control mechanisms associated with the early biosynthetic compartments and allows mature complexes to transit to distal compartments where its activity is required. We previously identified Retrieval to ER protein 1 (Rer1p) as the first negative regulator of the stepwise assembly of γ-secretase during endoplasmic reticulum-to-Golgi transport. We review here the state of the art on how Rer1p regulates complex assembly, particularly γ-secretase, and evaluate the therapeutic potential of such regulatory processes in the context of AD.

The small GTPase Rab5 homologue Ypt5 regulates cell morphology, sexual development, ion-stress response and vacuolar formation in fission yeast

Inner-membrane transport is critical to cell function. Rab family GTPases play an important role in vesicle transport. In mammalian cells, Rab5 is reported to be involved in the regulation of endosome formation, phagocytosis and chromosome alignment. Here, we examined the role of the fission yeast Rab5 homologue Ypt5 using a point mutant allele. Mutant cells displayed abnormal cell morphology, mating, sporulation, endocytosis, vacuole fusion and responses to ion stress. Our data strongly suggest that fission yeast Rab5 is involved in the regulation of various types of cellular functions.

Secretory protein biogenesis and traffic in the early secretory pathway

The secretory pathway is responsible for the synthesis, folding, and delivery of a diverse array of cellular proteins. Secretory protein synthesis begins in the endoplasmic reticulum (ER), which is charged with the tasks of correctly integrating nascent proteins and ensuring correct post-translational modification and folding. Once ready for forward traffic, proteins are captured into ER-derived transport vesicles that form through the action of the COPII coat. COPII-coated vesicles are delivered to the early Golgi via distinct tethering and fusion machineries. Escaped ER residents and other cycling transport machinery components are returned to the ER via COPI-coated vesicles, which undergo similar tethering and fusion reactions. Ultimately, organelle structure, function, and cell homeostasis are maintained by modulating protein and lipid flux through the early secretory pathway. In the last decade, structural and mechanistic studies have added greatly to the strong foundation of yeast genetics on which this field was built. Here we discuss the key players that mediate secretory protein biogenesis and trafficking, highlighting recent advances that have deepened our understanding of the complexity of this conserved and essential process.

The secretory pathway: exploring yeast diversity

Protein secretion is an essential process for living organisms. In eukaryotes, this encompasses numerous steps mediated by several hundred cellular proteins. The core functions of translocation through the endoplasmic reticulum membrane, primary glycosylation, folding and quality control, and vesicle-mediated secretion are similar from yeasts to higher eukaryotes. However, recent research has revealed significant functional differences between yeasts and mammalian cells, and even among diverse yeast species. This review provides a current overview of the canonical protein secretion pathway in the model yeast Saccharomyces cerevisiae, highlighting differences to mammalian cells as well as currently unresolved questions, and provides a genomic comparison of the S. cerevisiae pathway to seven other yeast species where secretion has been investigated due to their attraction as protein production platforms, or for their relevance as pathogens. The analysis of Candida albicans, Candida glabrata, Kluyveromyces lactis, Pichia pastoris, Hansenula polymorpha, Yarrowia lipolytica, and Schizosaccharomyces pombe reveals that many - but not all - secretion steps are more redundant in S. cerevisiae due to duplicated genes, while some processes are even absent in this model yeast. Recent research obviates that even where homologous genes are present, small differences in protein sequence and/or differences in the regulation of gene expression may lead to quite different protein secretion phenotypes.

Localization of lipid raft proteins to the plasma membrane is a major function of the phospholipid transfer protein Sec14

The Sec14 protein domain is a conserved tertiary structure that binds hydrophobic ligands. The Sec14 protein from Saccharomyces cerevisiae is essential with studies of S. cerevisiae Sec14 cellular function facilitated by a sole temperature sensitive allele, sec14^ts. The sec14^ts allele encodes a protein with a point mutation resulting in a single amino acid change, Sec14^G266D. In this study results from a genome-wide genetic screen, and pharmacological data, provide evidence that the Sec14^G266D protein is present at a reduced level compared to wild type Sec14 due to its being targeted to the proteosome. Increased expression of the sec14^ts allele ameliorated growth arrest, but did not restore the defects in membrane accumulation or vesicular transport known to be defective in sec14^ts cells. We determined that trafficking and localization of two well characterized lipid raft resident proteins, Pma1 and Fus-Mid-GFP, were aberrant in sec14^ts cells. Localization of both lipid raft proteins was restored upon increased expression of the sec14^ts allele. We suggest that a major function provided by Sec14 is trafficking and localization of lipid raft proteins.

Retention in endoplasmic reticulum 1 (RER1) modulates amyloid-β (Aβ) production by altering trafficking of γ-secretase and amyloid precursor protein (APP)

BACKGROUND

Aβ production is influenced by intracellular trafficking of secretases and amyloid precursor protein (APP).

RESULTS

Retention in endoplasmic reticulum 1 (RER1) regulates the trafficking of γ-secretase and APP, thereby influences Aβ production.

CONCLUSION

RER1, an ER retention/retrieval factor for γ-secretase and APP, modulates Aβ production.

SIGNIFICANCE

RER1 and its influence on γ-secretase and APP may be implicated for a safe strategy to target Aβ production. The presence of neuritic plaques containing aggregated amyloid-β (Aβ) peptides in the brain parenchyma is a pathological hallmark of Alzheimer disease (AD). Aβ is generated by sequential cleavage of the amyloid β precursor protein (APP) by β- and γ-secretase, respectively. As APP processing to Aβ requires transport through the secretory pathway, trafficking of the substrate and access to the secretases are key factors that can influence Aβ production (Thinakaran, G., and Koo, E. H. (2008) Amyloid precursor protein trafficking, processing, and function. J. Biol. Chem. 283, 29615-29619). Here, we report that retention in endoplasmic reticulum 1 (RER1) associates with γ-secretase in early secretory compartments and regulates the intracellular trafficking of γ-secretase. RER1 overexpression decreases both γ-secretase localization on the cell surface and Aβ secretion and conversely RER1 knockdown increases the level of cell surface γ-secretase and increases Aβ secretion. Furthermore, we find that increased RER1 levels decrease mature APP and increase immature APP, resulting in less surface accumulation of APP. These data show that RER1 influences the trafficking and localization of both γ-secretase and APP, thereby regulating the production and secretion of Aβ peptides.

cis-Golgi proteins accumulate near the ER exit sites and act as the scaffold for Golgi regeneration after brefeldin A treatment in tobacco BY-2 cells

The Golgi apparatus forms stacks of cisternae in many eukaryotic cells. However, little is known about how such a stacked structure is formed and maintained. To address this question, plant cells provide a system suitable for live-imaging approaches because individual Golgi stacks are well separated in the cytoplasm. We established tobacco BY-2 cell lines expressing multiple Golgi markers tagged by different fluorescent proteins and observed their responses to brefeldin A (BFA) treatment and BFA removal. BFA treatment disrupted cis, medial, and trans cisternae but caused distinct relocalization patterns depending on the proteins examined. Medial- and trans-Golgi proteins, as well as one cis-Golgi protein, were absorbed into the endoplasmic reticulum (ER), but two other cis-Golgi proteins formed small punctate structures. After BFA removal, these puncta coalesced first, and then the Golgi stacks regenerated from them in the cis-to-trans order. We suggest that these structures have a property similar to the ER-Golgi intermediate compartment and function as the scaffold of Golgi regeneration.

Secretory pathway genes assessed by high-throughput microscopy and synthetic genetic array analysis

We developed a procedure for automated confocal microscopy to image the effect of the non-essential yeast gene deletion set on the localisation of the plasma membrane GFP-labelled protein Mrh1p-GFP. To achieve this it was necessary to devise an expression system expressing Redstar2 RFP-fluorescence specifically in the nucleus, mCherry RFP at a lower intensity in the cytoplasm and Mrh1p-GFP in the plasma membrane. This fluorescence labelling scheme utilising specifically designed image analysis scripts allowed automated segmentation of the cells into sub-regions comprising nuclei, cytoplasm and cell-surface. From this high-throughput high content screening approach we were able to determine that gene deletions including emc1Δ, emc2Δ, emc3Δ, emc4Δ, emc5Δ and emc6Δ, caused intracellular mislocalisation at the ER of a plasma membrane protein Mrh1p-GFP. CPY processing patterns were unaffected in these mutants and collectively our data suggest a transport role for the EMC genes within the early secretory pathway. HAC1 is central to the unfolded protein response (UPR) and in its absence, i.e. the absence of UPR, emc1Δ-, emc3Δ-, emc4Δ-, emc5Δ-hac1Δ double mutants were specifically hypersensitive to ER-stress (tunicamycin) lending credence to the usefulness of the high content microscope screening for discovery of functional effects of single mutants.

Expression of Ttyh1, a member of the Tweety family in neurons in vitro and in vivo and its potential role in brain pathology

We have previously shown that Ttyh1 mRNA is expressed in neurons and its expression is up-regulated in the brain during epileptogenesis and epilepsy. In this study, we aimed to elucidate the role of Ttyh1 in neurons. We found widespread expression of Ttyh1 protein in neurons in vivo and in vitro. Ttyh1 immunoreactivity in vitro was frequently found in invaginations of dendritic spines; however, Ttyh1, seldom co-localized with synaptic markers in vivo. Silencing Ttyh1 expression with siRNA in hippocampal cultures resulted in alterations of MAP2 distribution along neurites causing it to appear in the form of chains of beads. Over-expression of Ttyh1 caused intense neuritogenesis and the formation of numerous filopodia-like protrusions. Similar protrusions were also produced in SH-SY5Y neuroblastoma cells over-expressing Ttyh1. Using a biotin-streptavidin pull-down assay and mass spectrometry, we identified proteins that can form complexes with Ttyh1 in the brain. Ttyh1 binding proteins are often expressed in the endoplasmic reticulum or the Golgi apparatus or are localized at synapses. Finally, we found increased expression of Ttyh1 in the inner molecular layer of the dentate gyrus in an animal model of epilepsy. On the basis of our findings, we propose Ttyh1 involvement in brain pathology.

Identification of the neuroblastoma-amplified gene product as a component of the syntaxin 18 complex implicated in Golgi-to-endoplasmic reticulum retrograde transport

Syntaxin 18, a soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein receptor (SNARE) protein implicated in endoplasmic reticulum (ER) membrane fusion, forms a complex with other SNAREs (BNIP1, p31, and Sec22b) and several peripheral membrane components (Sly1, ZW10, and RINT-1). In the present study, we showed that a peripheral membrane protein encoded by the neuroblastoma-amplified gene (NAG) is a subunit of the syntaxin 18 complex. NAG encodes a protein of 2371 amino acids, which exhibits weak similarity to yeast Dsl3p/Sec39p, an 82-kDa component of the complex containing the yeast syntaxin 18 orthologue Ufe1p. Under conditions favoring SNARE complex disassembly, NAG was released from syntaxin 18 but remained in a p31-ZW10-RINT-1 subcomplex. Binding studies showed that the extreme N-terminal region of p31 is responsible for the interaction with NAG and that the N- and the C-terminal regions of NAG interact with p31 and ZW10-RINT-1, respectively. Knockdown of NAG resulted in a reduction in the expression of p31, confirming their intimate relationship. NAG depletion did not substantially affect Golgi morphology and protein export from the ER, but it caused redistribution of Golgi recycling proteins accompanied by a defect in protein glycosylation. These results together suggest that NAG links between p31 and ZW10-RINT-1 and is involved in Golgi-to-ER transport.

Linking phospholipid flippases to vesicle-mediated protein transport

Type IV P-type ATPases (P4-ATPases) are a large family of putative phospholipid translocases (flippases) implicated in the generation of phospholipid asymmetry in biological membranes. P4-ATPases are typically the largest P-type ATPase subgroup found in eukaryotic cells, with five members in Saccharomyces cerevisiae, six members in Caenorhabditis elegans, 12 members in Arabidopsis thaliana and 14 members in humans. In addition, many of the P4-ATPases require interaction with a noncatalytic subunit from the CDC50 gene family for their transport out of the endoplasmic reticulum (ER). Deficiency of a P4-ATPase (Atp8b1) causes liver disease in humans, and studies in a variety of model systems indicate that P4-ATPases play diverse and essential roles in membrane biogenesis. In addition to their proposed role in establishing and maintaining plasma membrane asymmetry, P4-ATPases are linked to vesicle-mediated protein transport in the exocytic and endocytic pathways. Recent studies have also suggested a role for P4-ATPases in the nonvesicular intracellular trafficking of sterols. Here, we discuss the physiological requirements for yeast P4-ATPases in phospholipid translocase activity, transport vesicle budding and ergosterol metabolism, with an emphasis on Drs2p and its noncatalytic subunit, Cdc50p.

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.

Growth Control of Golgi Phosphoinositides by Reciprocal Localization of Sac1 Lipid Phosphatase and Pik1 4-Kinase

Compartment-specific control of phosphoinositide lipids is essential for cell function. The Sac1 lipid phosphatase regulates endoplasmic reticulum (ER) and Golgi phosphatidylinositol-4-phosphate [PI(4)P] in response to nutrient levels and cell growth stages. During exponential growth, Sac1p interacts with Dpm1p at the ER but shuttles to the Golgi during starvation. Here, we report that a C-terminal region in Sac1p is required for retention in the perinuclear ER, whereas the N-terminal domain is responsible for Golgi localization. We also show that starvation-induced shuttling of Sac1p to the Golgi depends on the coat protein complex II and the Rer1 adaptor protein. Starvation-induced shuttling of Sac1p to the Golgi specifically eliminates a pool of PI(4)P generated by the lipid kinase Pik1p. In addition, absence of nutrients leads to a rapid dissociation of Pik1p, together with its non-catalytical subunit Frq1p, from Golgi membranes. Reciprocal rounds of association/dissociation of the Sac1p lipid phosphatase and the Pik1p/Frq1p lipid kinase complex are responsible for growth-dependent control of Golgi phosphoinositides. Sac1p and Pik1p/Frq1p are therefore elements of a unique machinery that synchronizes ER and Golgi function in response to different growth conditions.

Rer1p competes with APH-1 for binding to nicastrin and regulates gamma-secretase complex assembly in the early secretory pathway

The γ-secretase complex, consisting of presenilin, nicastrin, presenilin enhancer-2 (PEN-2), and anterior pharynx defective-1 (APH-1) cleaves type I integral membrane proteins like amyloid precursor protein and Notch in a process of regulated intramembrane proteolysis. The regulatory mechanisms governing the multistep assembly of this “proteasome of the membrane” are unknown. We characterize a new interaction partner of nicastrin, the retrieval receptor Rer1p. Rer1p binds preferentially immature nicastrin via polar residues within its transmembrane domain that are also critical for interaction with APH-1. Absence of APH-1 substantially increased binding of nicastrin to Rer1p, demonstrating the competitive nature of these interactions. Moreover, Rer1p expression levels control the formation of γ-secretase subcomplexes and, concomitantly, total cellular γ-secretase activity. We identify Rer1p as a novel limiting factor that negatively regulates γ-secretase complex assembly by competing with APH-1 during active recycling between the endoplasmic reticulum (ER) and Golgi. We conclude that total cellular γ-secretase activity is restrained by a secondary ER control system that provides a potential therapeutic value.

Genetic and molecular interactions of the Erv41p-Erv46p complex involved in transport between the endoplasmic reticulum and Golgi complex

Erv41p and Erv46p are integral membrane proteins conserved across species. They were originally identified as abundant constituents of COPII-coated vesicles, and form a complex which cycles between the endoplasmic reticulum and Golgi complex. Yeast strains lacking these proteins are viable but display subtle secretory phenotypes. In order to obtain information about possible biological roles of this protein complex in endoplasmic reticulum to Golgi transport, we employed the Synthetic Genetic Array approach to screen for synthetic genetic interactions with the erv46 null mutation. We identified synthetic interactions with vma12, vma21, vma22 and vps1 deletion mutations. The vma21Δ mutation exacerbates transport defects caused by the erv46Δ mutation. Unexpectedly, yeast strains lacking Vma21p fail to sort the endoplasmic reticulum to Golgi v-SNARE, Bos1p, efficiently into COPII vesicles, yet these vesicles are fully fusion competent. In addition, we set out to identify, by a biochemical approach, proteins interacting with the Erv41p-Erv46p complex. We report a strong interaction between the Erv41p-Erv46p complex and endoplasmic reticulum glucosidase II. Strains lacking a cycling Erv41p-Erv46p complex display a mild glycoprotein processing defect.

Search and destroy: ER quality control and ER-associated protein degradation

Proteins synthesized in the endoplasmic reticulum (ER) encounter quality control checkpoints that verify their fitness to proceed in the secretory pathway. Molecules undergoing folding and assembly are kept out of the exocytic pathway until maturation is complete. Misfolded side products that inevitably form are removed from the mixture of conformers and returned to the cytosol for degradation. How unfolded proteins are recognized and how irreversibly misfolded proteins are sorted to ER-associated degradation pathways was poorly understood. Recent developments from a combination of genetic and biochemical analyses has revealed new insights into these mechanisms. The emerging view shows distinct pathways working in collaboration to filter the diverse range of unfolded proteins from the transport flow and to divert misfolded molecules for destruction.

Endoplasmic reticulum quality control of unassembled iron transporter depends on Rer1p-mediated retrieval from the golgi

Endoplasmic reticulum (ER) quality control is a conserved process by which misfolded or unassembled proteins are selectively retained in the endoplasmic reticulum (ER). Failure in oligomerization of multisubunit membrane proteins is one of the events that triggers ER quality control. The transmembrane domains (TMDs) of unassembled subunits are determinants of ER retention in many cases, although the mechanism of the TMD-mediated sorting of unassembled subunits remains elusive. We studied a yeast iron transporter complex on the cell surface as a new model system for ER quality control. When Fet3p, a transmembrane subunit, is not assembled with the other membrane subunit, Ftr1p, unassembled Fet3p is exclusively localized to the ER at steady state. The TMD of Fet3p contains a determinant for this process. However, pulse-chase analysis and in vitro budding assays indicate that unassembled Fet3p rapidly escapes from the ER. Furthermore, Rer1p, a retrieval receptor for ER-resident membrane proteins in the Golgi, is responsible for the TMD-dependent ER retrieval of unassembled Fet3p. These findings provide clear evidence that the ER quality control of unassembled membrane proteins can be achieved by retrieval from the Golgi and that Rer1p serves as a specific sorting receptor in this process.

Subcellular localization and oligomeric structure of the yeast putative stretch-activated Ca2+ channel component Mid1

The yeast Mid1 protein with an apparent molecular mass of 100 kDa is required for Ca2+ influx stimulated by the mating pheromone and by a capacitative calcium entrylike mechanism acting in response to Ca2+ depletion from the endoplasmic reticulum (ER) and functions as a stretch-activated Ca2+ -permeable channel when expressed in mammalian cells. Our previous work with protease protection experiments has indicated that Mid1 is present in the plasma membrane. In this study, we examined a possible intracellular localization of this protein by indirect fluorescence microscopy and found that Mid1 is present in the ER membrane as well as the plasma membrane. Intracellular fluorescence images for Mid1 were the same as those for the ER marker protein Sec71 but quite different from those of the Golgi protein Ypt1. The results were confirmed by membrane fractionation using Angiografin density gradient analysis. We also investigated the oligomeric structures and protein levels of Mid1 and found that Mid1 forms a 200-kDa oligomer by disulfide bonding. The protein level and modification of Mid1 in the plasma membrane and the ER membrane were unchanged by the mating pheromone. These findings provide new insight into the function of Mid1 in relation to localization, modification, and activation mechanisms.

Rer1p, a retrieval receptor for ER membrane proteins, recognizes transmembrane domains in multiple modes

The yeast Golgi membrane protein Rer1p is required for the retrieval of various endoplasmic reticulum (ER) membrane proteins such as Sec12p and Sec71p to the ER. We demonstrate here that the transmembrane domain (TMD) of Sec71p, a type-III membrane protein, contains an ER localization signal, which is required for physical recognition by Rer1p. The Sec71TMD-GFP fusion protein is efficiently retrieved to the ER by Rer1p. The structural feature of this TMD signal turns out to be the spatial location of polar residues flanking the highly hydrophobic core sequence but not the whole length of the TMD. On the Rer1p side, Tyr152 residue in the 4th TMD is important for the recognition of Sec12p but not Sec71p, suggesting that Rer1p interacts with its ligands at least in two modes. Sec71TMD-GFP expressed in the Deltarer1 mutant cells is mislocalized from the ER to the lumen of vacuoles via the multivesicular body (MVB) sorting pathway. In this case, not only the presence of polar residues in the Sec71TMD but also the length of the TMD is critical for the MVB sorting. Thus, the Rer1p-dependent ER retrieval and the MVB sorting in late endosomes both watch polar residues in the TMD but in a different manner.

Intracellular sorting and transport of proteins

The secretory and endocytic pathways of eukaryotic organelles consist of multiple compartments, each with a unique set of proteins and lipids. Specific transport mechanisms are required to direct molecules to defined locations and to ensure that the identity, and hence function, of individual compartments are maintained. The localisation of proteins to specific membranes is complex and involves multiple interactions. The recent dramatic advances in understanding the molecular mechanisms of membrane transport has been due to the application of a multi-disciplinary approach, integrating membrane biology, genetics, imaging, protein and lipid biochemistry and structural biology. The aim of this review is to summarise the general principles of protein sorting in the secretory and endocytic pathways and to highlight the dynamic nature of these processes. The molecular mechanisms involved in this transport along the secretory and endocytic pathways are discussed along with the signals responsible for targeting proteins to different intracellular locations.

ER retention may play a role in sorting of the nuclear pore membrane protein POM121

Integral membrane proteins of the nuclear envelope (NE) are synthesized on the rough endoplasmic reticulum (ER) and following free diffusion in the continuous ER/NE membrane system are targeted to their proper destinations due to interactions of specific domains with other components of the NE. By studying the intracellular distribution and dynamics of a deletion mutant of an integral membrane protein of the nuclear pores, POM121, which lacks the pore-targeting domain, we investigated if ER retention plays a role in sorting of integral membrane proteins to the nuclear envelope. A nascent membrane protein lacking sorting determinants is believed to diffuse laterally in the continuous ER/NE lipid bilayer and expected to follow vesicular traffic to the plasma membrane. The GFP-tagged deletion mutant, POM121(1-129)-GFP, specifically distributed within the ER membrane, but was completely absent from the Golgi compartment and the plasma membrane. Experiments using fluorescence recovery after photobleaching (FRAP) and fluorescence loss in photobleaching (FLIP) demonstrated that despite having very high mobility within the whole ER network (D = 0.41 +/- 0.11 micro m(2)/s) POM121(1-129)-GFP was unable to exit the ER. It was also not detected in post-ER compartments of cells incubated at 15 degrees C. Taken together, these experiments show that amino acids 1-129 of POM121 are able to retain GFP in the ER membrane and suggest that this retention occurs by a direct mechanism rather than by a retrieval mechanism. Our data suggest that ER retention might be important for sorting of POM121 to the nuclear pores.