Misfolding and aggregation of newly synthesized proteins in the endoplasmic reticulum

Variance and higher moments in the sigmoidal self-assembly of branched fibrils

Self-assembly of functional branched filaments, such as actin filaments and microtubules, or dysfunctional ones, such as amyloid fibrils, plays important roles in many biological processes. Here, based on the master equation approach, we study the kinetics of the formation of the branched fibrils. In our model, a branched fibril has one mother branch and several daughter branches. A daughter branch grows from the side of a pre-existing mother branch or daughter branch. In our model, we consider five basic processes for the self-assembly of the branched filaments, namely, the nucleation, the dissociation of the primary nucleus of fibrils, the elongation, the fragmentation, and the branching. The elongation of a mother branch from two ends and the elongation of a daughter branch from two ends can, in principle, occur with four different rate constants associated with the corresponding tips. This leads to a pronounced impact of the directionality of growth on the kinetics of the self-assembly. Here, we have unified and generalized our four previously presented models of branched fibrillogenesis in a single model. We have obtained a system of non-linear ordinary differential equations that give the time evolution of the polymer numbers and the mass concentrations along with the higher moments as observable quantities.

N-glycosylation as a eukaryotic protective mechanism against protein aggregation

The tendency for proteins to form aggregates is an inherent part of every proteome and arises from the self-assembly of short protein segments called aggregation-prone regions (APRs). While posttranslational modifications (PTMs) have been implicated in modulating protein aggregation, their direct role in APRs remains poorly understood. In this study, we used a combination of proteome-wide computational analyses and biophysical techniques to investigate the potential involvement of PTMs in aggregation regulation. Our findings reveal that while most PTM types are disfavored near APRs, N-glycosylation is enriched and evolutionarily selected, especially in proteins prone to misfolding. Experimentally, we show that N-glycosylation inhibits the aggregation of peptides in vitro through steric hindrance. Moreover, mining existing proteomics data, we find that the loss of N-glycans at the flanks of APRs leads to specific protein aggregation in Neuro2a cells. Our findings indicate that, among its many molecular functions, N-glycosylation directly prevents protein aggregation in higher eukaryotes.

miR-27b targets MAIP1 to mediate lipid accumulation in cultured human and mouse hepatic cells

Non-alcoholic liver disease (NAFLD) is a condition caused by excessive fat accumulation in the liver and developed via multiple pathways. miR-27b has been suggested to play crucial roles in the development of NAFLD, assuming via targeting genes involved in lipid catabolism and anabolism. However, other pathways regulated by miR-27b are largely unknown. Here we show that lipid accumulation was induced in miR-27b-transfected human and mouse hepatic cells and that knockdowns of three miR-27b-target genes, β-1,4-galactosyltransferase 3 (B4GALT3), matrix AAA peptidase interacting protein 1 (MAIP1) and PH domain and leucine rich repeat protein phosphatase 2 (PHLPP2), induced lipid accumulation. We also show that B4GALT3 and MAIP1 were direct targets of miR-27b and overexpression of MAIP1 ameliorated miR-27b-induced lipid accumulation. In addition, we show that hepatic Maip1 expression declined in mice fed a high-fat diet, suggesting the involvement of decreased Maip1 expression in the condition of fatty liver. Overall, we identified MAIP1/miR-27b axis as a mediator of hepatic lipid accumulation, a potential therapeutic target for NAFLD.

Rescue of α-synuclein aggregation in Parkinson's patient neurons by synergistic enhancement of ER proteostasis and protein trafficking

Neurodegenerative disorders are characterized by a collapse in proteostasis, as shown by the accumulation of insoluble protein aggregates in the brain. Proteostasis involves a balance of protein synthesis, folding, trafficking, and degradation, but how aggregates perturb these pathways is unknown. Using Parkinson's disease (PD) patient midbrain cultures, we find that aggregated α-synuclein induces endoplasmic reticulum (ER) fragmentation and compromises ER protein folding capacity, leading to misfolding and aggregation of immature lysosomal β-glucocerebrosidase. Despite this, PD neurons fail to initiate the unfolded protein response, indicating perturbations in sensing or transducing protein misfolding signals in the ER. Small molecule enhancement of ER proteostasis machinery promotes β-glucocerebrosidase solubility, while simultaneous enhancement of trafficking improves ER morphology, lysosomal function, and reduces α-synuclein. Our studies suggest that aggregated α-synuclein perturbs the ability of neurons to respond to misfolded proteins in the ER, and that synergistic enhancement of multiple proteostasis branches may provide therapeutic benefit in PD.

Chaperone therapy for molecular pathology in lysosomal diseases

In lysosomal diseases, enzyme deficiency is caused by misfolding of mutant enzyme protein with abnormal steric structure that is expressed by gene mutation. Chaperone therapy is a new molecular therapeutic approach primarily for lysosomal diseases. The misfolded mutant enzyme is digested rapidly or aggregated to induce endoplasmic reticulum stress. As a result, the catalytic activity is lost. The following sequence of events results in chaperone therapy to achieve correction of molecular pathology. An orally administered low molecular competitive inhibitor (chaperone) is absorbed into the bloodstream and reaches the target cells and tissues. The mutant enzyme is stabilized by the chaperone and subjected to normal enzyme proteinfolding (proteostasis). The first chaperone drug was developed for Fabry disease and is currently available in medical practice. At present three types of chaperones are available: competitive chaperone with enzyme inhibitory bioactivity (exogenous), non-competitive (or allosteric) chaperone without inhibitory bioactivity (exogenous), and molecular chaperone (heat shock protein; endogenous). The third endogenous chaperone would be directed to overexpression or activated by an exogenous low-molecular inducer. This new molecular therapeutic approach, utilizing the three types of chaperone, is expected to apply to a variety of diseases, genetic or non-genetic, and neurological or non-neurological, in addition to lysosomal diseases.

Endoplasmic Reticulum Associated Degradation of Spinocerebellar Ataxia-Related CD10 Cysteine Mutant

Spinocerebellar ataxia (SCA) is one of the most severe neurodegenerative diseases and is often associated with misfolded protein aggregates derived from the genetic mutation of related genes. Recently, mutations in CD10 such as C143Y have been identified as SCA type 43. CD10, also known as neprilysin or neuroendopeptidase, digests functional neuropeptides, such as amyloid beta, in the extracellular region. In this study, we explored the cellular behavior of CD10 C143Y to gain an insight into the functional relationship of the mutation and SCA pathology. We found that wild-type CD10 is expressed on the plasma membrane and exhibits endopeptidase activity in a cultured cell line. CD10 C143Y, however, forms a disulfide bond-mediated oligomer that does not appear by the wild-type CD10. Furthermore, the CD10 C143Y mutant was retained in the endoplasmic reticulum (ER) by the molecular chaperone BiP and was degraded through the ER-associated degradation (ERAD) process, in which representative ERAD factors including EDEM1, SEL1L, and Hrd1 participate in the degradation. Suppression of CD10 C143Y ERAD recovers intracellular transport but not enzymatic activity. Our results indicate that the C143Y mutation in CD10 negatively affects protein maturation and results in ER retention and following ERAD. These findings provide beneficial insight into SCA type 43 pathology.

Assistance for Folding of Disease-Causing Plasma Membrane Proteins

An extensive catalog of plasma membrane (PM) protein mutations related to phenotypic diseases is associated with incorrect protein folding and/or localization. These impairments, in addition to dysfunction, frequently promote protein aggregation, which can be detrimental to cells. Here, we review PM protein processing, from protein synthesis in the endoplasmic reticulum to delivery to the PM, stressing the main repercussions of processing failures and their physiological consequences in pathologies, and we summarize the recent proposed therapeutic strategies to rescue misassembled proteins through different types of chaperones and/or small molecule drugs that safeguard protein quality control and regulate proteostasis.

Genome-wide CRISPR screen identifies suppressors of endoplasmic reticulum stress-induced apoptosis

Sensing misfolded proteins in the endoplasmic reticulum (ER), cells initiate the ER stress response and, when overwhelmed, undergo apoptosis. However, little is known about how cells prevent excessive ER stress response and cell death to restore homeostasis. Here, we report the identification and characterization of cellular suppressors of ER stress-induced apoptosis. Using a genome-wide CRISPR library, we screen for genes whose inactivation further increases ER stress-induced up-regulation of C/EBP homologous protein 10 (CHOP)-the transcription factor central to ER stress-associated apoptosis. Among the top validated hits are two interacting components of the polycomb repressive complex (L3MBTL2 [L(3)Mbt-Like 2] and MGA [MAX gene associated]), and microRNA-124-3 (miR-124-3). CRISPR knockout of these genes increases CHOP expression and sensitizes cells to apoptosis induced by multiple ER stressors, while overexpression confers the opposite effects. L3MBTL2 associates with the CHOP promoter in unstressed cells to repress CHOP induction but dissociates from the promoter in the presence of ER stress, whereas miR-124-3 directly targets the IRE1 branch of the ER stress pathway. Our study reveals distinct mechanisms that suppress ER stress-induced apoptosis and may lead to a better understanding of diseases whose pathogenesis is linked to overactive ER stress response.

Biosynthesis, structure, and folding of the insulin precursor protein

Insulin synthesis in pancreatic β-cells is initiated as preproinsulin. Prevailing glucose concentrations, which oscillate pre- and postprandially, exert major dynamic variation in preproinsulin biosynthesis. Accompanying upregulated translation of the insulin precursor includes elements of the endoplasmic reticulum (ER) translocation apparatus linked to successful orientation of the signal peptide, translocation and signal peptide cleavage of preproinsulin-all of which are necessary to initiate the pathway of proper proinsulin folding. Evolutionary pressures on the primary structure of proinsulin itself have preserved the efficiency of folding ("foldability"), and remarkably, these evolutionary pressures are distinct from those protecting the ultimate biological activity of insulin. Proinsulin foldability is manifest in the ER, in which the local environment is designed to assist in the overall load of proinsulin folding and to favour its disulphide bond formation (while limiting misfolding), all of which is closely tuned to ER stress response pathways that have complex (beneficial, as well as potentially damaging) effects on pancreatic β-cells. Proinsulin misfolding may occur as a consequence of exuberant proinsulin biosynthetic load in the ER, proinsulin coding sequence mutations, or genetic predispositions that lead to an altered ER folding environment. Proinsulin misfolding is a phenotype that is very much linked to deficient insulin production and diabetes, as is seen in a variety of contexts: rodent models bearing proinsulin-misfolding mutants, human patients with Mutant INS-gene-induced Diabetes of Youth (MIDY), animal models and human patients bearing mutations in critical ER resident proteins, and, quite possibly, in more common variety type 2 diabetes.

Human cytomegalovirus-encoded US9 targets MAVS and STING signaling to evade type I interferon immune responses

Human cytomegalovirus (HCMV) has evolved sophisticated immune evasion mechanisms that target both the innate and adaptive immune responses. However, how HCMV encoded proteins are involved in this immune escape is not clear. Here, we show that HCMV glycoprotein US9 inhibits the IFN-β response by targeting the mitochondrial antiviral-signaling protein (MAVS) and stimulator of interferon genes (STING)-mediated signaling pathways. US9 accumulation in mitochondria attenuates the mitochondrial membrane potential, leading to promotion of MAVS leakage from the mitochondria. Furthermore, US9 disrupts STING oligomerization and STING–TBK1 association through competitive interaction. Intriguingly, US9 blocks interferon regulatory factor 3 (IRF3) nuclear translocation and its cytoplasmic domain is essential for inhibiting IRF3 activation. Mutant HCMV lacking US7-16 is impaired in antagonism of MAVS/STING-mediated IFN-β expression, an effect that is reversible by the introduction of US9. Our findings indicate that HCMV US9 is an antagonist of IFN signaling to persistently evade host innate antiviral responses.

MAVS and STING signaling are central to interferon-inducing antiviral immunity. Here, the authors show how the human cytomegalovirus protein US9 can evade this immunity by antagonizing these pathways.

WITHDRAWN: Non-infectious stressors and innate immune response

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PIGN prevents protein aggregation in the endoplasmic reticulum independently of its function in the GPI synthesis

Quality control of proteins in the endoplasmic reticulum (ER) is essential for ensuring the integrity of secretory proteins before their release into the extracellular space. Secretory proteins that fail to pass quality control form aggregates. Here we show the PIGN-1/PIGN is required for quality control in Caenorhabditis elegans and in mammalian cells. In C. elegans pign-1 mutants, several proteins fail to be secreted and instead form abnormal aggregation. PIGN-knockout HEK293 cells also showed similar protein aggregation. Although PIGN-1/PIGN is responsible for glycosylphosphatidylinositol (GPI)-anchor biosynthesis in the ER, certain mutations in C. elegans pign-1 caused protein aggregation in the ER without affecting GPI-anchor biosynthesis. These results show that PIGN-1/PIGN has a conserved and non-canonical function to prevent deleterious protein aggregation in the ER independently of the GPI-anchor biosynthesis. PIGN is a causative gene for some human diseases including multiple congenital seizure-related syndrome (MCAHS1). Two pign-1 mutations created by CRISPR/Cas9 that correspond to MCAHS1 also cause protein aggregation in the ER, implying that the dysfunction of the PIGN non-canonical function might affect symptoms of MCAHS1 and potentially those of other diseases.

Molecular architecture of the human sperm IZUMO1 and egg JUNO fertilization complex

Fertilization is an essential biological process in sexual reproduction and comprises a series of molecular interactions between the sperm and egg. The fusion of the haploid spermatozoon and oocyte is the culminating event in mammalian fertilization, enabling the creation of a new, genetically distinct diploid organism. The merger of two gametes is achieved through a two-step mechanism in which the sperm protein IZUMO1 on the equatorial segment of the acrosome-reacted sperm recognizes its receptor, JUNO, on the egg surface. This recognition is followed by the fusion of the two plasma membranes. IZUMO1 and JUNO proteins are indispensable for fertilization, as constitutive knockdown of either protein results in mice that are healthy but infertile. Despite their central importance in reproductive medicine, the molecular architectures of these proteins and the details of their functional roles in fertilization are not known. Here we present the crystal structures of human IZUMO1 and JUNO in unbound and bound conformations. The human IZUMO1 structure exhibits a distinct boomerang shape and provides structural insights into the IZUMO family of proteins. Human IZUMO1 forms a high-affinity complex with JUNO and undergoes a major conformational change within its N-terminal domain upon binding to the egg-surface receptor. Our results provide insights into the molecular basis of sperm-egg recognition, cross-species fertilization, and the barrier to polyspermy, thereby promising benefits for the rational development of non-hormonal contraceptives and fertility treatments for humans and other mammals.

Co- and Post-Translational Protein Folding in the ER

The biophysical rules that govern folding of small, single-domain proteins in dilute solutions are now quite well understood. The mechanisms underlying co-translational folding of multidomain and membrane-spanning proteins in complex cellular environments are often less clear. The endoplasmic reticulum (ER) produces a plethora of membrane and secretory proteins, which must fold and assemble correctly before ER exit - if these processes fail, misfolded species accumulate in the ER or are degraded. The ER differs from other cellular organelles in terms of the physicochemical environment and the variety of ER-specific protein modifications. Here, we review chaperone-assisted co- and post-translational folding and assembly in the ER and underline the influence of protein modifications on these processes. We emphasize how method development has helped advance the field by allowing researchers to monitor the progression of folding as it occurs inside living cells, while at the same time probing the intricate relationship between protein modifications during folding.

Orchestration of secretory protein folding by ER chaperones

The endoplasmic reticulum is a major compartment of protein biogenesis in the cell, dedicated to production of secretory, membrane and organelle proteins. The secretome has distinct structural and post-translational characteristics, since folding in the ER occurs in an environment that is distinct in terms of its ionic composition, dynamics and requirements for quality control. The folding machinery in the ER therefore includes chaperones and folding enzymes that introduce, monitor and react to disulfide bonds, glycans, and fluctuations of luminal calcium. We describe the major chaperone networks in the lumen and discuss how they have distinct modes of operation that enable cells to accomplish highly efficient production of the secretome. This article is part of a Special Issue entitled: Functional and structural diversity of endoplasmic reticulum.

Quantifying Changes in the Cellular Thiol-Disulfide Status during Differentiation of B Cells into Antibody-Secreting Plasma Cells

Plasma cells produce and secrete massive amounts of disulfide-containing antibodies. To accommodate this load on the secretory machinery, the differentiation of resting B cells into antibody-secreting plasma cells is accompanied by a preferential expansion of the secretory compartments of the cells and by an up-regulation of enzymes involved in redox regulation and protein folding. We have quantified the absolute levels of protein thiols, protein disulfides, and glutathionylated proteins in whole cells. The results show that while the global thiol-disulfide state is affected to some extent by the differentiation, steady-state levels of glutathionylated protein thiols are less than 0.3% of the total protein cysteines, even in fully differentiated cells, and the overall protein redox state is not affected until late in differentiation, when large-scale IgM production is ongoing. A general expansion of the ER does not affect global protein redox status until an extensive production of cargo proteins has started.

The cytosolic chaperone α-crystallin B rescues folding and compartmentalization of misfolded multispan transmembrane proteins

The α-crystallin B chain (CRYAB or HspB5) is a cytosolic chaperone belonging to the small heat shock protein family, which is known to help in the folding of cytosolic proteins. Here we show that CRYAB binds the mutant form of at least two multispan transmembrane proteins (TMPs), exerting an anti-aggregation activity. It rescues the folding of mutant Frizzled4, which is responsible for a rare autosomal dominant form of familial exudative vitreoretinopathy (Fz4-FEVR), and the mutant ATP7B Cu transporter (ATP7B-H1069Q) associated with a common form of Wilson's disease. In the case of Fz4-FEVR, CRYAB prevents the formation of inter-chain disulfide bridges between the lumenal ectodomains of the aggregated mutant chains, which enables correct folding and promotes appropriate compartmentalization on the plasma membrane. ATP7B-H1069Q, with help from CRYAB, folds into the proper conformation, moves to the Golgi complex, and responds to copper overload in the same manner as wild-type ATP7B. These findings strongly suggest that CRYAB plays a pivotal role, previously undetected, in the folding of multispan TMPs and, from the cytosol, is able to orchestrate folding events that take place in the lumen of the ER. Our results contribute to the explanation of the complex scenario behind multispan TMP folding; additionally, they serve to expose interesting avenues for novel therapeutic approaches.

Dominant protein interactions that influence the pathogenesis of conformational diseases

Misfolding of exportable proteins can trigger endocrinopathies. For example, misfolding of insulin can result in autosomal dominant mutant INS gene-induced diabetes of youth, and misfolding of thyroglobulin can result in autosomal recessive congenital hypothyroidism with deficient thyroglobulin. Both proinsulin and thyroglobulin normally form homodimers; the mutant versions of both proteins misfold in the ER, triggering ER stress, and, in both cases, heterozygosity creates potential for cross-dimerization between mutant and WT gene products. Here, we investigated these two ER-retained mutant secretory proteins and the selectivity of their interactions with their respective WT counterparts. In both cases and in animal models of these diseases, we found that conditions favoring an increased stoichiometry of mutant gene product dominantly inhibited export of the WT partner, while increased relative level of the WT gene product helped to rescue secretion of the mutant partner. Surprisingly, the bidirectional consequences of secretory blockade and rescue occur simultaneously in the same cells. Thus, in the context of heterozygosity, expression level and stability of WT subunits may be a critical factor influencing the effect of protein misfolding on clinical phenotype. These results offer new insight into dominant as well as recessive inheritance of conformational diseases and offer opportunities for the development of new therapies.

Loss of the oxidative stress sensor NPGPx compromises GRP78 chaperone activity and induces systemic disease

NPGPx is a member of the glutathione peroxidase (GPx) family; however, it lacks GPx enzymatic activity due to the absence of a critical selenocysteine residue, rendering its function an enigma. Here, we show that NPGPx is a newly identified stress sensor that transmits oxidative stress signals by forming the disulfide bond between its Cys57 and Cys86 residues. This oxidized form of NPGPx binds to glucose-regulated protein (GRP)78 and forms covalent bonding intermediates between Cys86 of NPGPx and Cys41/Cys420 of GRP78. Subsequently, the formation of the disulfide bond between Cys41 and Cys420 of GRP78 enhances its chaperone activity. NPGPx-deficient cells display increased reactive oxygen species, accumulated misfolded proteins, and impaired GRP78 chaperone activity. Complete loss of NPGPx in animals causes systemic oxidative stress, increases carcinogenesis, and shortens life span. These results suggest that NPGPx is essential for releasing excessive ER stress by enhancing GRP78 chaperone activity to maintain physiological homeostasis.

Chaperone and foldase coexpression in the baculovirus-insect cell expression system

CONCLUSIONS

The BEVS has become widely utilized for production of recombinant proteins. However, protein aggregation and inefficient processing often limit yields, especially for secreted and membrane proteins. Since many proteins of pharmaceutical interest require similar posttranslational processing steps, engineering the folding, assembly, and secretion pathway may enhance the production of a wide variety of valuable complex proteins. Efforts should be undertaken to coexpress the relevant chaperones or foldases at low levels in concert with the final product to ensure the ideal folding and assembly environment. In the future, expression of oligosaccharide modifying enzymes and secretion factors may further improve secretion rates of assembled proteins and provide heterologous proteins with altered glycoforms. Also significant is the use of BEVS as an in vivo eucaryotic laboratory to study the fundamental roles of differnt chaperones, foldases, and secretion factors. The coexpression of chaperones and foldases will complement other approaches such as the development of alternative insect cell lines, promoters, and signal peptides to optimize the baculovirus-insect cell expression system for generating high yields of valuable proteins.

Specific and nonspecific effects of glycosylation

Glycosylation regulates vital cellular processes and dramatically influences protein folding and stability. In particular, experiments have demonstrated that asparagine (N)-linked disaccharides drive a "conformational switch" in a model peptide. The present work investigates this conformational switch via extensive atomically detailed replica exchange molecular dynamics simulations in explicit solvent. To distinguish the effects of specific and nonspecific interactions upon the peptide conformational ensemble, these simulations considered model peptides that were N-linked to a disaccharide and to a steric crowder of the same shape. The simulations are remarkably consistent with experiment and provide detailed insight into the peptide structure ensemble. They suggest that steric crowding by N-linked disaccharides excludes extended conformations, but does not significantly impact the tetrahedral structure of the surrounding solvent or otherwise alter the peptide free energy surface. However, the combination of steric crowding with specific hydrogen bonds and hydrophobic stacking interactions more dramatically impacts the peptide ensemble and stabilizes new structures.

Convergent modulation of Kv4.2 channel alpha subunits by structurally distinct DPPX and KChIP auxiliary subunits

Kv4.2 is the major voltage-gated K(+) (Kv) channel alpha subunit responsible for the somatodendritic transient or A-type current I(SA) that activates at subthreshold membrane potentials. Stable association of Kv4.2 with diverse auxiliary subunits and reversible Kv4.2 phosphorylation regulate I(SA) function. Two classes of auxiliary subunits play distinct roles in modulating the biophysical properties of Kv4.2: dipeptidyl-peptidase-like type II transmembrane proteins typified by DPPX-S, and cytoplasmic Ca(2+) binding proteins known as K(+) channel interacting proteins (KChIPs). Here, we characterize the convergent roles that DPPX-S and KChIPs play as component subunits of Kv4.2 channel complexes. We coexpressed DPPX-S with Kv4.2 in heterologous cells and found a dramatic redistribution of Kv4.2, releasing it from intracellular retention and allowing plasma membrane expression, as well as altered Kv4.2 phosphorylation, detergent solubility, and stability. These changes are remarkably similar to those obtained upon coexpression of Kv4.2 with the structurally distinct KChIPs1-3 auxiliary subunits. KChIP4a, which negatively affects the impact of other KChIPs on Kv4.2, also inhibits the effects of DPPX-S, consistent with the formation of a ternary complex of Kv4.2, DPPX-S, and KChIPs early in channel biosynthesis. Tandem MS analyses reveal that coexpression with DPPX-S or KChIP2 leads to a pattern of Kv4.2 phosphorylation in heterologous cells similar to that observed in brain, but lacking in cells expressing Kv4.2 alone. In conclusion, transmembrane DPPX-S and cytoplasmic KChIPs, despite having distinct structures and binding sites on Kv4.2, exert similar effects on Kv4.2 trafficking, but distinct effects on Kv4.2 gating.

Evidence for a human leucocyte antigen-DM-induced structural change in human leucocyte antigen-DObeta

Human leucocyte antigen (HLA)-DO is a non-classical major histocompatibility complex class II molecule which modulates the function of HLA-DM and the loading of antigenic peptides on molecules such as HLA-DR. The bulk of HLA-DO associates with HLA-DM and this interaction is critical for HLA-DO egress from the endoplasmic reticulum. HLA-DM assists the early steps of HLA-DO maturation presumably through the stabilization of the interactions between the N-terminal regions of the alpha and beta chains. To evaluate a possible role for HLA-DM in influencing the conformation of HLA-DO, we made use of a monoclonal antibody, Mags.DO5, that was raised against HLA-DO/DM complexes. Using transfected cells expressing mismatched heterodimers between HLA-DR and -DO chains, we found that the epitope for Mags.DO5 is located on the DObeta chain and that Mags.DO5 reactivity was increased upon cotransfection with HLA-DM. Our results suggest that HLA-DM influences the folding of HLA-DO in the endoplasmic reticulum. A mutant HLA-DO showing reduced capacity for endoplasmic reticulum egress was better recognized by Mags.DO5 in the presence of HLA-DM. On the other hand, an HLA-DO mutant capable of endoplasmic reticulum egress on its own was efficiently recognized by Mags.DO5, irrespective of the presence of HLA-DM. Taken together, our results suggest that HLA-DM acts as a private chaperone, directly assisting the folding of HLA-DO to promote egress from the endoplasmic reticulum.

In and out of the ER: protein folding, quality control, degradation, and related human diseases

A substantial fraction of eukaryotic gene products are synthesized by ribosomes attached at the cytosolic face of the endoplasmic reticulum (ER) membrane. These polypeptides enter cotranslationally in the ER lumen, which contains resident molecular chaperones and folding factors that assist their maturation. Native proteins are released from the ER lumen and are transported through the secretory pathway to their final intra- or extracellular destination. Folding-defective polypeptides are exported across the ER membrane into the cytosol and destroyed. Cellular and organismal homeostasis relies on a balanced activity of the ER folding, quality control, and degradation machineries as shown by the dozens of human diseases related to defective maturation or disposal of individual polypeptides generated in the ER.

Tyrosinase and ocular diseases: Some novel thoughts on the molecular basis of oculocutaneous albinism type 1

Tyrosinase (TYR) is a multifunctional copper-containing glycoenzyme (approximately 80 kDa), which plays a key role in the rate-limiting steps of the melanin biosynthetic pathway. This membrane-bound protein, possibly evolved by the fusion of two different copper-binding proteins, is mainly expressed in epidermal, ocular and follicular melanocytes. In the melanocytes, TYR functions as an integrated unit with other TYR-related proteins (TYRP1, TYRP2), lysosome-associated membrane protein 1 (LAMP1) and melanocyte-stimulating hormone receptors; thus forming a melanogenic complex. Mutations in the TYR gene (TYR, 11q14-21, MIM 606933) cause oculocutaneous albinism type 1 (OCA1, MIM 203100), a developmental disorder having an autosomal recessive mode of inheritance. In addition, TYR can act as a modifier locus for primary congenital glaucoma (PCG) and it also contributes significantly in the eye developmental process. Expression of TYR during neuroblast division helps in later pathfinding by retinal ganglion cells from retina to the dorsal lateral geniculate nucleus. However, mutation screening of TYR is complicated by the presence of a pseudogene-TYR like segment (TYRL, 11p11.2, MIM 191270), sharing approximately 98% sequence identity with the 3' region of TYR. Thus, in absence of a full-proof strategy, any nucleotide variants identified in the 3' region of TYR could actually be present in TYRL. Interestingly, despite extensive search, the second TYR mutation in 15% of the OCA1 cases remains unidentified. Several possible locations of these "uncharacterized mutations" (UCMs) have been speculated so far. Based on the structure of TYR gene, its sequence context and some experimental evidences, we propose two additional possibilities, which on further investigations might shed light on the molecular basis of UCMs in TYR of OCA1 patients; (i) partial deletion of the exons 4 and 5 region of TYR that is homologous with TYRL and (ii) variations in the polymorphic GA complex repeat located between distal and proximal elements of the human TYR promoter that can modulate the expression of the gene leading to disease pathogenesis.

Endoplasmic Reticulum Vacuolization and Valosin-Containing Protein Relocalization Result from Simultaneous Hsp90 Inhibition by Geldanamycin and Proteasome Inhibition by Velcade

Geldanamycin and Velcade, new anticancer drugs with novel mechanisms of action, are currently undergoing extensive clinical trials. Geldanamycin interrupts Hsp90 chaperone activity and causes down-regulation of its many client proteins by the ubiquitin-proteasome pathway; Velcade is a specific proteasome inhibitor. Misfolded Hsp90 clients within the endoplasmic reticulum (ER) lumen are cleared by ER--associated protein degradation, a sequential process requiring valosin-containing protein (VCP)-dependent retrotranslocation followed by ubiquitination and proteasomal proteolysis. Cotreatment of cells with geldanamycin and Velcade prevents destruction of destabilized, ubiquitinated Hsp90 client proteins, causing them to accumulate. Here, we report that misfolded protein accumulation within the ER resulting from geldanamycin and Velcade exposure overwhelms the ability of the VCP--centered machine to maintain the ER secretory pathway, causing the ER to distend into conspicuous vacuoles. Overexpression of dominant-negative VCP or the "small VCP--interacting protein" exactly recapitulated the vacuolated phenotype provoked by the drugs, associating loss of VCP function with ER vacuolization. In cells transfected with a VCP--enhanced yellow fluorescent protein fluorescent construct, geldanamycin plus Velcade treatment redistributed VCP--enhanced yellow fluorescent protein from the cytoplasm and ER into perinuclear aggresomes. In further support of the view that compromise of VCP function is responsible for ER vacuolization, small interfering RNA interference of VCP expression induced ER vacuolization that was markedly increased by Velcade. VCP knockdown by small interfering RNA eventually deconstructed both the ER and Golgi and interdicted protein trafficking through the secretory pathway to the plasma membrane. Thus, simultaneous geldanamycin and Velcade treatment has far-reaching secondary cytotoxic consequences that likely contribute to the cytotoxic activity of this anticancer drug combination.

Hantavirus Gc glycoprotein: evidence for a class II fusion protein

Hantavirus cell entry is promoted by its envelope glycoproteins, Gn and Gc, through cell attachment and by fusion between viral and endosomal membranes at low pH. However, the role of Gn and Gc in receptor binding and cell fusion has not yet been defined. In this work, a sequence presenting characteristics similar to those of class II fusion peptides (FPs) of alphavirus E1 and flavivirus E proteins is identified within the hantavirus Gc glycoprotein. A three-dimensional comparative molecular model based on crystallographic data of tick-borne encephalitis virus E protein is proposed for the Andes virus (ANDV) Gc ectodomain, which supports a feasible class II fusion-protein fold. In vitro experimental evidence is provided for the binding activity of the ANDV FP candidate to artificial membranes, as demonstrated by fluorescence anisotropy assays. Taken together, these results support the hypothesis that the Gc glycoprotein of hantaviruses and of other members of the family Bunyaviridae directs the viral fusion activity and that it may be classified as a class II viral fusion protein.

The Oxidative Folding Rate of Bovine Pancreatic Ribonuclease Is Enhanced by a Covalently Attached Oligosaccharide

Bovine pancreatic ribonuclease B (RNase B) differs from RNase A by the presence of an oligosaccharide moiety covalently attached to Asn 34. Oxidative folding studies of RNase B were carried out at different temperatures using DTT(ox) as the oxidizing agent, and the results were compared with those for RNase A. The oxidative folding rates of RNase B are between 1.7 and 1.3 times faster than those of RNase A at the temperatures that were investigated. The folding pathways of RNase B were determined to be similar to those of RNase A in that two structured intermediates, each lacking one native disulfide bond, were found to populate the regeneration pathways at 25 degrees C and pH 8.3. The thermodynamic stabilities of these two glycosylated intermediates, and their rates of formation from their unstructured precursors in the rate-determining step, were found to be higher than those of their unglycosylated counterparts from RNase A. Thus, the underlying cause for the faster rate of oxidative regeneration of native RNase B appears to be both thermodynamic and kinetic due to the higher stability, and faster rate of formation, of the intermediates of RNase B compared to those of RNase A.

The endoplasmic reticulum as a protein-folding compartment

The lumen of the endoplasmic reticulum (ER) provides a dynamic and efficient environment for the folding of proteins destined for secretion and for a variety of cellular compartments and membranes. Usually, the folding process begins on the nascent chains and is completed minutes or hours later during assembly of oligomers. It is assisted by molecular chaperones and folding enzymes, some of which are unique to the ER. Quality control and selective degradation systems ensure only conformationally mature proteins are transported from the ER.

N-linked oligosaccharides affect the enzymatic activity of CD39: diverse interactions between seven N-linked glycosylation sites

Rat CD39, a membrane-bound ectonucleoside triphosphate diphosphohydrolase that hydrolyzes extracellular nucleoside tri- and diphosphates, has seven potential N-glycosylation sites at asparagine residues 73, 226, 291, 333, 375, 429, and 458. To determine their roles in the structure and function of CD39, we mutated these sites individually or in combination by replacing asparagine with serine or glutamine and analyzed the surface expression and the enzymatic activity of the mutants. The results indicate that rat CD39 can be glycosylated at all seven sites when expressed in COS7 cells. Glycosylation sites 73 at the N terminus, 333 in the middle, and 429 and 458 at the C terminus were principally required for cell surface appearance of enzymatically active CD39. Whereas deletion of these sites individually had modest effects on surface ATPase activity, some double deletions of these sites had major effects on both surface activity and expression. The importance of these N-glycosylation sites is recognizable in other members of the ectonucleoside triphosphate diphosphohydrolase family.

Divergent stress responses to IL-1β, nitric oxide, and tunicamycin by chondrocytes

As the only cell in cartilage responsible for matrix synthesis, the chondrocyte's viability is crucial to healthy tissue. It must tolerate stresses from both mechanical and cellular sources. This study examines the endoplasmic reticulum (ER) stress response in chondrocytes after exposure to IL-1beta, nitric oxide, or tunicamycin in order to determine whether this form of stress causes cell death. Cultures of the immortalized human juvenile costal chondrocyte cell line, C-28/I2, were treated with IL-1beta, S-nitroso-N-acetylpenicillamine (SNAP), and tunicamycin. Increasing intracellular nitric oxide levels by SNAP treatment or inhibiting protein folding in the ER lumen by tunicamycin induced the ER stress response as evidenced by increased protein and gene expression of GADD153 as well as PERK and eIF2-alpha phosphorylation, and resulted in apoptosis. IL-1beta treatment induced PERK and eIF2-alpha phosphorylation, but not GADD153 expression or apoptosis. The ER stress signaling pathway of IL-1beta involved iNOS because blocking its expression, inhibited ER stress gene expression. Therefore, inducing the ER stress response in chondrocytes results in divergent responses depending on the agent used. Even though IL-1beta, a common proinflammatory cytokine, induces the ER stress response, it is not proapoptotic to chondrocytes. On the other hand, exposure to high levels of intracellular nitric oxide induce chondrocyte apoptosis as part of the ER stress response.

Glycan-independent role of calnexin in the intracellular retention of Charcot-Marie-tooth 1A Gas3/PMP22 mutants

Missense point mutations in Gas3/PMP22 are responsible for the peripheral neuropathies Charcot-Marie-Tooth 1A and Dejerine Sottas syndrome. These mutations induce protein misfolding with the consequent accumulation of the proteins in the endoplasmic reticulum and the formation of aggresomes. During folding, Gas3/PMP22 associates with the lectin chaperone calnexin. Here, we show that calnexin interacts with the misfolded transmembrane domains of Gas3/PMP22, fused to green fluorescent protein, in a glycan-independent manner. In addition, photobleaching experiments in living cells revealed that Gas3/PMP22-green fluorescent protein mutants are mobile but diffuse at almost half the diffusion coefficient of wild type protein. Our results support emerging models for a glycan-independent chaperone role for calnexin and for the mechanism of retention of misfolded membrane proteins in the endoplasmic reticulum.

Evidence for stabilization of aquaporin-2 folding mutants byN-linked glycosylation in endoplasmic reticulum

Aquaporin-2 (AQP2) is the vasopressin-sensitive water channel that regulates water reabsorption in the distal nephron collecting duct. Inherited AQP2 mutations that disrupt folding lead to nephrogenic diabetes insipidus (NDI) by targeting newly synthesized protein for degradation in the endoplasmic reticulum (ER). During synthesis, a subset of wild-type (WT) AQP2 is covalently modified by N-linked glycosylation at residue Asn123. To investigate the affect of glycosylation, we expressed WT AQP2 and four NDI-related mutants in Xenopus laevis oocytes and compared stability of glycosylated and nonglycosylated isoforms. In all constructs, ∼15–20% of newly synthesized AQP2 was covalently modified by N-linked glycosylation. At steady state, however, core glycosylated WT protein was nearly undetectable, whereas all mutants were found predominantly in the glycosylated form (60–70%). Pulse-chase metabolic labeling studies revealed that glycosylated isoforms of mutant AQP2 were significantly more stable than their nonglycosylated counterparts. For nonglycosylated isoforms, the half-life of WT AQP2 was significantly greater (>48 h) than that of mutant AQP2 (T126M 4.1 ± 1.0 h, A147T 4.2 ± 0.60 h, C181W 4.5 ± 0.50 h, R187C 6.8 ± 1.2 h). This is consistent with rapid turnover in the ER as previously reported. In contrast, the half-lives of mutant proteins containing N-linked glycans were similar to WT (∼25 h), indicating that differences in steady-state glycosylation profiles are caused by increased stability of glycosylated mutant proteins. These results suggest that addition of a single N-linked oligosaccharide moiety can partially compensate for ER folding defects induced by disease-related mutations.

An endoplasmic reticulum trafficking signal prevents surface expression of a voltage- and Ca2+-activated K+ channel splice variant

Protein delivery to restricted plasma membrane domains is exquisitely regulated at different stages of the cell trafficking machinery. Traffic control involves the recognition of export/retention/retrieval signals in the endoplasmic reticulum (ER)/Golgi complex that will determine protein fate. A splice variant (SV), SV1, of the voltage- and Ca(2+)-activated K(+) channel alpha-subunit accumulates the channel in the ER, preventing its surface expression. We show that SV1 insert contains a nonbasic, hydrophobic retention/retrieval motif, CVLF, that does not interfere with proper folding and tetramerization of SV1. Localization of proteins in the ER by CVLF is independent of its position; originally, on the first internal loop, SV1 insert or CVLF perform equally well if placed at the middle or end of the alpha-subunit intracellular carboxyl terminus. Also, CVLF is able to restrict the traffic of an independently expressed transmembrane protein, beta 1-subunit. CVLF is present in proteins across species and in lower organisms. Thus, CVLF may have evolved to serve as a regulator of cellular traffic.

Cell-cell fusion induced by the avian reovirus membrane fusion protein is regulated by protein degradation

The p10 fusion-associated small transmembrane protein of avian reovirus induces extensive syncytium formation in transfected cells. Here we show that p10-induced cell-cell fusion is restricted by rapid degradation of the majority of newly synthesized p10. The small ectodomain of p10 targets the protein for degradation following p10 insertion into an early membrane compartment. Paradoxically, conservative amino acid substitutions in the p10 ectodomain hydrophobic patch that eliminate fusion activity also increase p10 stability. The small amount of p10 that escapes intracellular degradation accumulates at the cell surface in a relatively stable form, where it mediates cell-cell fusion as a late-stage event in the virus replication cycle. The unusual relationship between a nonstructural viral membrane fusion protein and the replication cycle of a nonenveloped virus has apparently contributed to the evolution of a novel mechanism for restricting the extent of virus-induced cell-cell fusion.

Multistep regulation of membrane insertion of the fusion peptide of Semliki Forest virus

A prevailing model for virus membrane fusion proteins has been that the hydrophobic fusion peptide is hidden in the prefusion conformation, becomes exposed once the fusion reaction is triggered, and then either inserts into target membranes or is rapidly inactivated. This model is in general agreement with the structure and mechanism of class I fusion proteins, such as the influenza virus hemagglutinin. We here describe studies of the class II fusion protein E1 from the alphavirus Semliki Forest virus (SFV). SFV fusion is triggered by low pH, which releases E1 from its heterodimeric interaction with the E2 protein and induces the formation of a stable E1 homotrimer. The exposure and target membrane interaction of the E1 fusion peptide (residues 83 to 100) were followed using a monoclonal antibody (MAb E1f) mapping to E1 residues 85 to 95. In agreement with the known structure of SFV and other alphaviruses, the fusion peptide was shielded in native SFV particles and exposed when E1-E2 dimer dissociation was triggered by acidic pH. In contrast, the fusion peptide on purified E1 ectodomains (E1(*)) was fully accessible at neutral pH. Functional assays showed that MAb E1f binding at neutral pH prevented subsequent low-pH-triggered E1(*) interaction with target membranes and trimerization. E1(*) was not inactivated by low pH when treated either in the absence of target membranes or in the presence of fusion-inactive cholesterol-deficient liposomes. Thus, the membrane insertion of the E1 fusion peptide is regulated by additional low-pH-dependent steps after exposure, perhaps involving an E1-cholesterol interaction.

Quality Control and Protein Folding in the Secretory Pathway

The biosynthesis of secretory and membrane proteins in the endoplasmic reticulum (ER) yields mostly properly folded and assembled structures with full biological activity. Such fidelity is maintained by quality control (QC) mechanisms that avoid the production of nonnative structures. QC relies on chaperone systems in the ER that monitor and assist in the folding process. When folding promotion is not sufficient, proteins are retained in the ER and eventually retranslocated to the cytosol for degradation by the ubiquitin proteasome pathway. Retention of proteins that fail QC can sometimes occur beyond the ER, and degradation can take place in lysosomes. Several diseases are associated with proteins that do not pass QC, fail to be degraded efficiently, and accumulate as aggregates. In other cases, pathology arises from the downregulation of mutated but potentially functional proteins that are retained and degraded by the QC system.

A fundamental role for KChIPs in determining the molecular properties and trafficking of Kv4.2 potassium channels

Kv4 potassium channels regulate action potentials in neurons and cardiac myocytes. Co-expression of EF hand-containing Ca2+-binding proteins termed KChIPs with pore-forming Kv4 alpha subunits causes changes in the gating and amplitude of Kv4 currents (An, W. F., Bowlby, M. R., Betty, M., Cao, J., Ling, H. P., Mendoza, G., Hinson, J. W., Mattsson, K. I., Strassle, B. W., Trimmer, J. S., and Rhodes, K. J. (2000) Nature 403, 553-556). Here we show that KChIPs profoundly affect the intracellular trafficking and molecular properties of Kv4.2 alpha subunits. Co-expression of KChIPs1-3 causes a dramatic redistribution of Kv4.2, releasing intrinsic endoplasmic reticulum retention and allowing for trafficking to the cell surface. KChIP co-expression also causes fundamental changes in Kv4.2 steady-state expression levels, phosphorylation, detergent solubility, and stability that reconstitute the molecular properties of Kv4.2 in native cells. Interestingly, the KChIP4a isoform, which exhibits unique effects on Kv4 channel gating, does not exert these effects on Kv4.2 and negatively influences the impact of other KChIPs. We provide evidence that these KChIP effects occur through the masking of an N-terminal Kv4.2 hydrophobic domain. These studies point to an essential role for KChIPs in determining both the biophysical and molecular characteristics of Kv4 channels and provide a molecular basis for the dramatic phenotype of KChIP knockout mice.

Effects of inactivation and constitutive expression of the unfolded- protein response pathway on protein production in the yeast Saccharomyces cerevisiae

One strategy to obtain better yields of secreted proteins has been overexpression of single endoplasmic reticulum-resident foldases or chaperones. We report here that manipulation of the unfolded-protein response (UPR) pathway regulator, HAC1, affects production of both native and foreign proteins in the yeast Saccharomyces cerevisiae. The effects of HAC1 deletion and overexpression on the production of a native protein, invertase, and two foreign proteins, Bacillus amyloliquefaciens alpha-amylase and Trichoderma reesei endoglucanase EGI, were studied. Disruption of HAC1 caused decreases in the secretion of both alpha-amylase (70 to 75% reduction) and EGI (40 to 50% reduction) compared to the secretion by the parental strain. Constitutive overexpression of HAC1 caused a 70% increase in alpha-amylase secretion but had no effect on EGI secretion. The invertase levels were twofold higher in the strain overexpressing HAC1. Also, the effect of the active form of T. reesei hac1 was tested in S. cerevisiae. hac1 expression caused a 2.4-fold increase in the secretion of alpha-amylase in S. cerevisiae and also slight increases in invertase and total protein production. Overexpression of both S. cerevisiae HAC1 and T. reesei hac1 caused an increase in the expression of the known UPR target gene KAR2 at early time points during cultivation.

Fusion with HDEL protects cell wall invertase from early degradation when N-glycosylation is inhibited

Previous data obtained in different suspension-cultured plant cells have clearly illustrated that N-glycans are absolutely required for transport of glycoproteins to the extracellular compartment, regardless of their oligosaccharide structure [see Lerouge et al. (1998) Plant Mol. Biol. 38: 31 for review]. In the present study the role of N-glycosylation in the transport of glycoproteins to the cell surface was studied in BY2 tobacco cells using both endogenous and recombinant cell wall invertases as markers. When synthesized without their N-glycans, both invertases were very rapidly degraded. This degradation did not occur in an acidic compartment and was brefeldin A-insensitive. Therefore, it most probably represents a pre-Golgi event. However, the low efficiency of specific inhibitors did not favor a strong contribution of proteasomes in this proteolysis. In contrast, addition of a C-terminal His-Asp-Glu-Leu (HDEL) extension prevented arrival of these non-glycosylated glycoproteins in the compartment where they are degraded. These results argue for the presence of an endoplasmic reticulum (ER) domain specialized in protein degradation. Consistent with our results and the well-known stabilization of recombinant proteins retained in the ER, the addition of an ER retention signal to a protein would prevent its targeting to an ER domain devoted to degradation.

N-glycosylation of recombinant human fucosyltransferase III is required for its in vivo folding in mammalian and insect cells

Human alpha3/4fucosyltransferase (FT3) catalyses the synthesis of fucosylated glycoconjugates involved in cell-cell interactions. FT3 has two potential N-glycosylation sites at Asn(154) and Asn(185). Soluble secretory forms of the enzyme (SFT3) and mutant forms with the first, second and both glycosylation sites (SFT3DN1, SFT3DN2, SFT3DN) mutated have been expressed in baby hamster kidney (BHK) and Spodoptera frugiperda (Sf9) cells. Deletion of the first or both sites caused total enzyme inactivation. Deletion of the second site caused 99% and 75% decrease of secretory enzyme expression in BHK and Sf9 cells, respectively. Sf9 cells produced 1 mg/l SFT3 and 0.3 mg/l SFT3DN2; these values were 175- and 3750-fold higher, respectively, than those observed for BHK cells. A significant amount of protein was accumulated intracellularly in Sf9 cells which for SFT3 was active and for SFT3DN2 was inactive, indicating the importance of the glycans from the second glycosylation site for protein folding. The corresponding full-length forms FT3, FT3DN1 and FT3DN2 associated with calnexin as observed by immunoprecipitation studies, which indicated the possible role of this chaperon in the folding of glycosylated glycosyltransferases.

Endoplasmic reticulum signaling as a determinant of recombinant protein expression

Generation of functional recombinant proteins requires efficient and undisturbed functioning of the ER-Golgi secretory pathway in host cells. In large-scale production, where target proteins are highly overexpressed, this pathway can be easily congested with unfolded or misfolded proteins. Accumulating evidence suggests that, in addition to responsibility for protein processing, ER is also an important signaling compartment and a sensor of cellular stress. Two ER responses have been described to arise from the overaccumulation of proteins: unfolded protein response (UPR) and ER overload response (EOR). UPR and EOR employ various mechanisms at the transcriptional and the translational levels to deal efficiently and appropriately with encountered stress. This review will outline the molecular bases of ER functioning and stress response, highlight the relevance of ER signaling to the large-scale cell culture productivity and discuss possible approaches to the improvement of the secretion capacities of recombinant cells.

Expression and Structure-Function Analysis of DE, a Sperm Cysteine-Rich Secretory Protein That Mediates Gamete Fusion1

Rat sperm epididymal glycoprotein DE belongs to the cysteine-rich secretory protein (CRISP) family and participates in sperm-egg fusion through its binding to complementary sites on the egg surface. To investigate the molecular mechanisms underlying the role of DE in gamete fusion, in the present work we expressed DE in a prokaryotic system, and examined the relevance of carbohydrates and disulfide bonds for the biological activity of the protein. Immunofluorescence and sperm-egg fusion assays carried out in the presence of recombinant DE (recDE) revealed that this protein exhibits the ability to bind to the DE-egg binding sites and to inhibit gamete fusion, as does native DE (nDE). Comparison of the proteins indicated, however, that the inhibitory ability of recDE was significantly lower than that of nDE. This difference would not be due to the lack of carbohydrates in the bacterially expressed protein because enzymatically deglycosylated nDE was as able as the untreated protein to inhibit gamete fusion. To examine whether disulfide bridges are involved in DE activity, the presence of sulfhydryls in nDE and recDE was evaluated by the biotin-maleimide technique. Results indicated that, unlike nDE, in which all cysteines are involved in disulfide bonds, recDE contains free thiol groups. Subsequent experiments showed that reduction of nDE with dithiothreitol significantly decreased the ability of the protein to inhibit gamete fusion. Together, these results indicate that whereas carbohydrates do not have a role in DE-mediated gamete fusion, disulfide bridges are required for full biological activity of the protein. To our knowledge, this is the first study reporting the relevance of structural components for the function of a CRISP member.

The role of constant region carbohydrate in the assembly and secretion of human IgD and IgA1

Immunoglobulins are glycoproteins, containing N- linked carbohydrates in the heavy chain constant regions of all isotypes and O-linked carbohydrates in the hinge regions of human IgA1 and IgD. A previous study showed that IgD synthesized in the presence of tunicamycin and lacking the three N-linked glycans on the heavy chain was not secreted (Shin, S. U., Wei, D. F., Amin, A. R., Thorbecke, G. J., and Morrison, S. L. (1992) Hum. Antibodies 3, 65-74). The contribution of each of the carbohydrates in the Fc of IgD to assembly and secretion was now analyzed by eliminating the carbohydrate addition sequence, Asn-X-Ser/Thr, through site-directed mutagenesis. Only the carbohydrate nearest the sole disulfide bond between heavy chains, which remained high mannose and appeared to be buried within the folded molecule, was found to be essential for secretion. When IgD lacked that glycan, assembly reached only the heavy/light chain half-molecule stage, and heavy chains were held inside the endoplasmic reticulum. Using benzyl 2-acetamido-2-deoxy-alpha-d-galactopyranoside (BADG) to inhibit complete O-linked glycosylation, we found that IgA1 and IgD with incomplete hinge carbohydrates were assembled and secreted from cells. Thus, one N-linked glycan plays a structural role in IgD and is required for proper assembly and secretion, but the O-linked carbohydrates in the hinge of IgD and IgA1 are not required for folding and export.

Autosomal dominant neurohypophyseal diabetes insipidus due to substitution of histidine for tyrosine(2) in the vasopressin moiety of the hormone precursor

The autosomal dominant form of familial neurohypophyseal diabetes insipidus (adFNDI) has been linked to 40 different mutations of the gene encoding the vasopressin-neurophysin II (AVP-NPII) precursor. All of these mutations have been located in either the signal peptide or neurophysin II moiety. We now report a three-generation Turkish kindred in which severe adFNDI cosegregates with a novel missense mutation in the part of the AVP-NPII gene encoding the AVP moiety. This mutation (T-->C at position 285 in the genomic sequence) was found in only one allele and predicts a substitution of histidine for tyrosine at position 2 in AVP. Like other adFNDI mutations, this substitution is expected to impair folding and processing of the precursor, in this case by interfering with normal binding of the AVP and NPII moieties. It is associated clinically with inability to concentrate urine during fluid deprivation, a greater than 80% deficiency of AVP secretion, and absence of the posterior pituitary bright spot on magnetic resonance imaging. These findings are consistent with the hypothesis that mutations in the AVP-NPII gene cause adFNDI by directing the production of a folding incompetent precursor that prevents the expression of the normal allele via a cytotoxic effect on the magnocellular neurons.

Induction of protein aggregation in an early secretory compartment by elevation of expression level

A variety of valuable therapeutic proteins are expressed in mammalian cells. Currently, rate-limiting for secretion of recombinant glycoproteins are activities in the secretory pathway of eukaryotic cells, i.e., folding and glycosylation of the naked polypeptide chain. In this paper we provide evidence that elevation of expression level alone is sufficient to cause intracellular aggregation of a structurally relatively simple glycoprotein, antithrombin III (ATIII). Elevation of expression level by selection for increased drug resistance in Chinese hamster ovary cells stably expressing ATIII resulted in formation of disulfide-bonded aggregates of ATIII. Aggregated ATIII displayed incomplete sialylation and Endo H-sensitivity and located to the endoplasmic reticulum and the cis-Golgi compartment in subcellular fractionations. To explore possible causes for aggregation of ATIII at elevated expression levels we investigated the influence of the two major energy sources of cultured mammalian cells, D-glucose and L-glutamine, on the ATIII-yield. We found that utilization of D-glucose was not limiting for synthesis of ATIII at elevated expression levels. However, the amount of ATIII-synthesized per L-glutamine consumed did not seem to increase steadily with expression level for ATIII, indicating that secretion of ATIII may be limited by the capacity of the cell to utilize L-glutamine.

Yeast expression and NMR analysis of the extracellular domain of muscle nicotinic acetylcholine receptor alpha subunit

The alpha subunit of the nicotinic acetylcholine receptor (AChR) from Torpedo electric organ and mammalian muscle contains high affinity binding sites for alpha-bungarotoxin and for autoimmune antibodies in sera of patients with myasthenia gravis. To obtain sufficient materials for structural studies of the receptor-ligand complexes, we have expressed part of the mouse muscle alpha subunit as a soluble, secretory protein using the yeast Pichia pastoris. By testing a series of truncated fragments of the receptor protein, we show that alpha211, the entire amino-terminal extracellular domain of AChR alpha subunit (amino acids 1-211), is the minimal segment that could fold properly in yeast. The alpha211 protein was secreted into the culture medium at a concentration of >3 mg/liter. It migrated as a 31-kDa polypeptide with N-linked glycosylation on SDS-polyacrylamide gel. The protein was purified to homogeneity by isoelectric focusing electrophoresis (pI 5.8), and it appeared as a 4.5 S monomer on sucrose gradient at concentrations up to 1 mm ( approximately 30 mg/ml). The receptor domain bound monoclonal antibody mAb35, a conformation-specific antibody against the main immunogenic region of the AChR. In addition, it formed a high affinity complex with alpha-bungarotoxin (k(D) 0.2 nm) but showed relatively low affinity to the small cholinergic ligand acetylcholine. Circular dichroism spectroscopy of alpha211 revealed a composition of secondary structure corresponding to a folded protein. Furthermore, the receptor fragment was efficiently (15)N-labeled in P. pastoris, and proton cross-peaks were well dispersed in nuclear Overhauser effect and heteronuclear single quantum coherence spectra as measured by NMR spectroscopy. We conclude that the soluble AChR protein is useful for high resolution structural studies.