Substitution of arginine for histidine at position 209 in the alpha-subunit of the human insulin receptor. A mutation that impairs receptor dimerization and transport of receptors to the cell surface.

Receptors for insulin and epidermal growth factor contain cysteine-rich domains in the extracellular portion of the molecule. His209 (insulin receptor numbering system) is 1 of 2 amino acid residues that are identically conserved in the cysteine-rich domains of insulin receptors, epidermal growth factor receptors, and other homologous receptors. Previously, we have identified a patient with leprechaunism who is homozygous for a mutation substituting Arg for His209 in the insulin receptor gene (Kadowaki, T., Kadowaki, H., Rechler, M. M., Serrano-Rios, M., Roth, J., Gorden, P., and Taylor, S. I. (1990) J. Clin. Invest. 86, 254-264). In this investigation, the Arg209 mutant receptor was expressed by transfection of mutant cDNA into NIH-3T3 cells. The mutation impairs several steps in the post-translational processing of the insulin receptor:dimerization of 190-kDa proreceptors into a disulfide linked species, proteolytic cleavage of the proreceptor into alpha- and beta-subunits, and terminal processing of the high mannose form of N-linked oligosaccharide into complex carbohydrate. In addition, the defects in post-translational processing within the endoplasmic reticulum and Golgi apparatus are associated with a marked inhibition in transport of receptors to the plasma membrane. Nevertheless, a small number (approximately 10%) of the receptors are transported to the cell surface. These receptors on the cell surface bind insulin with normal affinity and have normal tyrosine kinase activity.     

Posttranslational association of immunoglobulin heavy chain binding protein with nascent heavy chains in nonsecreting and secreting hybridomas

A rat monoclonal antibody specific for immunoglobulin (Ig) heavy chain binding protein (BiP) has allowed the examination of the association of BiP with assembling Ig precursors in mouse B lymphocyte-derived cell lines. The anti-BiP monoclonal antibody immunoprecipitates BiP along with noncovalently associated Ig heavy chains. BiP is a component of the endoplasmic reticulum and binds free intracellular heavy chains in nonsecreting pre-B (mu+, L-) cell lines or incompletely assembled Ig precursors in (H+, L+) secreting hybridomas and myelomas. In the absence of light chain synthesis, heavy chains remain associated with BiP and are not secreted. The association of BiP with assembling Ig molecules in secreting hybridomas is transient and is restricted to the incompletely assembled molecules which are found in the endoplasmic reticulum. BiP loses affinity and disassociates with Ig molecules when polymerization with light chain is complete. We propose that the association of BiP with Ig heavy chain precursors is a novel posttranslational processing event occurring in the endoplasmic reticulum. The Ig heavy chains associated with BiP are not efficiently transported from the endoplasmic reticulum to the Golgi apparatus. Therefore, BiP may prevent the premature escape and eventual secretion of incompletely assembled Ig molecules.     

Postbinding characterization of five naturally occurring mutations in the human insulin receptor gene: impaired insulin-stimulated c-jun expression and thymidine incorporation despite normal receptor autophosphorylation.

Some patients with extreme insulin resistance have mutations in their insulin receptor gene. We previously identified five such mutations located in the extracellular domain of the insulin receptor (Asn-->Lys15, His-->Arg209, Phe-->Val382, Lys-->Glu460, and Asn-->Ser462) and studied the effects of these mutations upon posttranslational processing, insulin binding, and tyrosine autophosphorylation. We now characterize the ability of these mutant receptors to mediate biological actions of insulin in transfected NIH-3T3 fibroblasts. All cell lines expressing mutant receptors showed marked impairment in insulin-stimulated c-jun expression and thymidine incorporation when compared with cells expressing wild-type human insulin receptors. The most severe impairment was seen in cells expressing the Val382 mutant (a mutation which causes an intrinsic defect in receptor autophosphorylation). These cells had insulin responses similar to the untransfected cells (used as a negative control). In contrast, cells expressing the Lys15 mutant have the ability to achieve a normal level of maximal autophosphorylation but require an abnormally high concentration of insulin to do so (as the result of decreased insulin binding affinity). These cells show a higher basal rate and much lower insulin stimulation of both c-jun expression and thymidine incorporation when compared with the cells expressing the wild-type human insulin receptors. This pattern is also seen in the cells expressing the other mutants with normal autophosphorylation (Arg209, Glu460, and Ser462). Although the most severe defects in insulin action are seen with the mutation which has an intrinsic defect in receptor autophosphorylation, the ability to undergo normal autophosphorylation does not seem to preclude mutations from impairing the ability of receptors to mediate some of the actions of insulin.     

Substitution of lysine for asparagine at position 15 in the alpha-subunit of the human insulin receptor. A mutation that impairs transport of receptors to the cell surface and decreases the affinity of insulin binding

Mutations in the insulin receptor gene can compromise the ability of the receptor to mediate insulin action. Previously, in investigations of a patient with a genetic form of insulin resistance, we have identified a mutant allele encoding an insulin receptor in which lysine is substituted for asparagine at position 15 of the alpha-subunit. In the present study, we have characterized the Lys15-mutant receptor expressed by transfection of mutant cDNA into NIH-3T3 cells. The Lys15-mutation causes at least two defects in insulin receptor function. First, the mutation retards the post-translational processing of the receptor and impairs transport of the receptor to the plasma membrane, thereby reducing the number of receptors on the cell surface. Second, the mutation causes a 5-fold reduction in the affinity of the receptor to bind insulin. These two defects combine to render the target cell resistant to normal physiological concentrations of insulin. It seems likely that both functional defects associated with the Lys15-mutation can be explained by assuming that the mutation distorts the three-dimensional structure of the receptor. Presumably, the abnormal conformation interferes with the transport of the receptor through the endoplasmic reticulum and Golgi, and also inhibits the binding of insulin to its binding site on the receptor.     

Heavy chain binding protein recognizes incompletely disulfide-bonded forms of vesicular stomatitis virus G protein

To investigate the function of heavy chain binding protein (BiP, GRP 78) in the endoplasmic reticulum, we have characterized its interaction with a model plasma membrane glycoprotein, the G protein of vesicular stomatitis virus. We used a panel of well characterized mutant G proteins and immunoprecipitation with anti-BiP antibodies to determine if BiP interacted with newly synthesized G protein and/or mutant G proteins retained in the endoplasmic reticulum. We made three major observations: 1) BiP bound transiently to folding intermediates of wild-type G protein which were incompletely disulfide-bonded; 2) BiP did not bind stably to all mutant G proteins which remain in the endoplasmic reticulum; and 3) BiP bound stably only to mutant G proteins which do not form correct intrachain disulfide bonds.     

Binding of BiP to an assembly-defective protein in plant cells

The binding protein (BiP) has been implicated as a mediator of protein folding and assembly in the endoplasmic reticulum of mammalian cells and has often been found in stable association with structurally defective proteins. To acquire information on the activity of BiP in plant cells, we have expressed in tobacco protoplasts the wild type form and an assembly-defective form of bean phaseolin. Phaseolin (PHSL) is a soluble, trimeric, storage glycoprotein co-translationally inserted into the lumen of the endoplasmic reticulum and then transported along the secretory pathway to the protein storage vacuoles. We have previously shown that a PHSL mutant in which the last 59 amino acids have been deleted (Δ363PHSL) is unable to form trimers and is retained in a pre-Golgi compartment when synthesized in Xenopus oocytes. When transiently expressed in tobacco leaf protoplasts, wild-type PHSL is correctly glycosylated and assembles efficiently and rapidly into trimers. Δ363PHSL is also correctly glycosylated but does not trimerize. Tobacco BiP and Δ363PHSL are co-immunoselected using either anti-PHSL or anti-BiP antibodies. Under the same conditions, co-immunoselection of BiP with wild-type PHSL is not detectable. The BiP bound to Δ363PHSL can be released by treatment of the complex with ATP, indicating that the binding is related to the proposed function of BiP in protein folding and assembly in the endoplasmic reticulum. These data indicate that BiP stably binds structurally defective proteins in plant cells.     

Distribution of insulin receptors among mouse liver endomembranes.

Specific binding of insulin to highly purified preparations of rough endoplasmic reticulum, Golgi apparatus, and plasma membrane of mouse liver was determined. 125I-labeled insulin bound maximally to the plasma membrane in radio-receptor assays. Golgi apparatus fractions exhibited binding 10--20% that of plasma membrane and rough endoplasmic reticulum exhibited only 1--2% of plasma membrane binding. Binding was proportional to membrane concentration and dose vs. response curves were very similar for the different fractions. Scatchard analysis of the insulin binding data for the plasma membrane and Golgi apparatus fractions showed curvilinear plots yielding similar apparent binding affinities (0.9 and 3.0-10(8) M-1, respectively). Purity of the isolated endomembranes was analyzed by morphometry and (Na+ + K+ + Mg2+)-ATPase and these preparations displayed less than 1% contamination by plasma membrane. These findings provide important confirmation of the presence of insulin receptors in Golgi apparatus membranes comparable to those located on the plasma membrane. Finally, the present study did not allow us to verify the existence of insulin receptors in the endoplasmic reticulum.     

Golgi-mediated vacuolar sorting of the endoplasmic reticulum chaperone BiP may play an active role in quality control within the secretory pathway

Quality control in the endoplasmic reticulum (ER) prevents the arrival of incorrectly or incompletely folded proteins at their final destinations and targets permanently misfolded proteins for degradation. Such proteins have a high affinity for the ER chaperone BiP and are finally degraded via retrograde translocation from the ER lumen back to the cytosol. This ER-associated protein degradation (ERAD) is currently thought to constitute the main disposal route, but there is growing evidence for a vacuolar role in quality control. We show that BiP is transported to the vacuole in a wortmannin-sensitive manner in tobacco (Nicotiana tabacum) and that it could play an active role in this second disposal route. ER export of BiP occurs via COPII-dependent transport to the Golgi apparatus, where it competes with other HDEL receptor ligands. When HDEL-mediated retrieval from the Golgi fails, BiP is transported to the lytic vacuole via multivesicular bodies, which represent the plant prevacuolar compartment. We also demonstrate that a subset of BiP-ligand complexes is destined to the vacuole and differs from those likely to be disposed of via the ERAD pathway. Vacuolar disposal could act in addition to ERAD to maximize the efficiency of quality control in the secretory pathway.     

In vivo expression of mammalian BiP ATPase mutants causes disruption of the endoplasmic reticulum.

BiP possesses ATP binding/hydrolysis activities that are thought to be essential for its ability to chaperone protein folding and assembly in the endoplasmic reticulum (ER). We have produced a series of point mutations in a hamster BiP clone that inhibit ATPase activity and have generated a species-specific anti-BiP antibody to monitor the effects of mutant hamster BiP expression in COS monkey cells. The enzymatic inactivation of BiP did not interfere with its ability to bind to Ig heavy chains in vivo but did inhibit ATP-mediated release of heavy chains in vitro. Immunofluorescence staining and electron microscopy revealed vesiculation of the ER membranes in COS cells expressing BiP ATPase mutants. ER disruption was not observed when a "44K" fragment of BiP that did not include the protein binding domain was similarly mutated but was observed when the protein binding region of BiP was expressed without an ATP binding domain. This suggests that BiP binding to target proteins as an inactive chaperone is responsible for the ER disruption. This is the first report on the in vivo expression of mammalian BiP mutants and is demonstration that in vitro-identified ATPase mutants behave as dominant negative mutants when expressed in vivo.     

Transport and secretion of truncated T cell receptor beta-chain occurs in the absence of association with CD3

The T cell receptor (TCR) beta-chain is produced in the endoplasmic reticulum where it associates with the TCR alpha-chain and the members of the CD3 complex to form the complete receptor. When the other chains of the complex are not available, the beta-chain is rapidly degraded within the endoplasmic reticulum. When incomplete TCR.CD3 complexes are formed, they are transported through the Golgi apparatus and degraded in lysosomes. In this study, a truncated form of the TCR beta-chain has been made by removal of the transmembrane and cytoplasmic segments. Unlike the normal beta-chain, the truncated molecule is stable and is transported through the Golgi apparatus and secreted. This process occurs at a similar rate in both T and B cells, indicating that it is not affected by the presence or absence of CD3 components. These data suggest that an element in the transmembrane or cytoplasmic region of the beta-chain confers sensitivity to the degradative control mechanisms that regulate TCR expression.     

Mutant vasopressin precursors that cause autosomal dominant neurohypophyseal diabetes insipidus retain dimerization and impair the secretion of wild-type proteins

Autosomal dominant familial neurohypophyseal diabetes insipidus is caused by mutations in the arginine vasopressin (AVP) gene. We demonstrated recently that mutant AVP precursors accumulate within the endoplasmic reticulum of neuronal cells, leading to cellular toxicity. In this study, the possibility that mutant AVP precursors interact with wild-type (WT) proteins to alter their processing and function was explored. WT and mutant precursors were epitope-tagged to allow them to be distinguished in transfected cells. An in vivo cross-linking reaction revealed homo- and heterodimer formation between WT and mutant precursors. Mutant precursors were also shown to impair intracellular trafficking of WT precursors from the endoplasmic reticulum to the Golgi apparatus. In addition to the cytotoxicity caused by mutant AVP precursors, the interaction between the WT and mutant precursors suggests that a dominant-negative mechanism may also contribute to the pathogenesis of familial neurohypophyseal diabetes insipidus.     

How a cytokine is chaperoned through the secretory pathway by complexing with its own receptor: lessons from interleukin-15 (IL-15)/IL-15 receptor alpha

While it is well appreciated that receptors for secreted cytokines transmit ligand-induced signals, little is known about additional roles for cytokine receptor components in the control of ligand transport and secretion. Here, we show that interleukin-15 (IL-15) translocation into the endoplasmic reticulum occurs independently of the presence of IL-15 receptor α (IL-15Rα). Subsequently, however, IL-15 is transported through the Golgi apparatus only in association with IL-15Rα and then is secreted. This intracellular IL-15/IL-15Rα complex already is formed in the endoplasmic reticulum and, thus, enables the further trafficking of complexed IL-15 through the secretory pathway. Just transfecting IL-15Rα in cells, which transcribe but normally do not secrete IL-15, suffices to induce IL-15 secretion. Thus, we provide the first evidence of how a cytokine is chaperoned through the secretory pathway by complexing with its own high-affinity receptor and show that IL-15/IL-15Rα offers an excellent model system for the further exploration of this novel mechanism for the control of cytokine secretion.     

Distribution of insulin receptors among mouse liver endomembranes

Specific binding of insulin to highly purified preparations of rough endoplasmic reticulum, Golgi apparatus, and plasma membrane of mouse liver was determined. ¹²⁵I-labeled insulin bound maximally to the plasma membrane in radio-receptor assays. Golgi apparatus fractions exhibited binding 10–20% that of plasma membrane and rough endoplasmic reticulum exhibited only 1–2% of plasma membrane binding. Binding was proportional to membrane concentration and dose vs. response curves were very similar for the different fractions. Scatchard analysis of the insulin binding data for the plasma membrane and Golgi apparatus fractions showed curvilinear plots yielding similar apparent binding affinities (0.9 and 3.0 · 10⁸ M^?1, respectively). Purity of the isolated endomembranes was analyzed by morphometry and (Na⁺ + K⁺ + Mg²⁺)-ATPase and these preparations displayed less than 1% contamination by plasma membrane. These findings provide important confirmation of the presence of insulin receptors in Golgi apparatus membranes comparable to those located on the plasma membrane. Finally, the present study did not allow us to verify the existence of insulin receptors in the endoplasmic reticulum.     

78 kDa receptor for Man6P-independent lysosomal enzyme targeting: biosynthetic transport from endoplasmic reticulum to "high-density vesicles"

Recent work has shown that the cation-independent mannose 6-phosphate and the 78 kDa receptors for lysosomal enzyme targeting are located in different cell compartments. While the mannose 6-phosphate receptor is enriched in the Percoll fractions that contain Golgi apparatus, most of the 78 kDa receptor is localized in a heavy fraction at the bottom of the Percoll gradient. This report presents the biosynthetic transport of the 78 kDa receptor. Newly synthesized 78 kDa receptor was transported to Golgi from endoplasmic reticulum with a half life of 5 min. From the Golgi apparatus, the receptor takes two routes; about 15-25% is transported to the plasma membrane, and the rest migrates to late endosomes, subsequently to prelysosomes and finally to the dense vesicles. The 78 kDa receptor starts appearing at the dense vesicles 120 min after biosynthesis and reaches a maximum of 40-50% of the total receptor. Treatment of cells with NH4Cl causes depletion of the receptor from the dense vesicles and prelysosomes and corresponding augmentation in endosomes and plasma membrane. These results suggest that the 78 kDa receptor cycles between compartments and that the dense vesicles seem to represent the most distal compartment in the biosynthetic pathway of this receptor.     

Retrieval of HDEL proteins is required for growth of yeast cells

The ERD2 gene of Saccharomyces cerevisiae encodes the receptor which retrieves HDEL-containing containing ER proteins from the Golgi apparatus. Viable erd2 mutants have been isolated that show no obvious HDEL-dependent retention of the luminal ER protein BiP, suggesting that retrieval of HDEL proteins is not essential for growth. However, cells that lack Erd2p completely have a defective Golgi apparatus and cannot grow. This observation led to the suggestion that the receptor had a second function, possibly related to its ability to recycle from Golgi to ER. In this paper we investigate the requirements for Erd2p to support growth. We show that mutations that block its recycling also prevent growth. In addition, we show that all mutant receptors that can support growth have a residual ability to retrieve BiP, which is detectable when they are overexpressed. Mere recycling of an inactive form of the receptor, mediated by a cytoplasmic KKXX sequence, is not sufficient for growth. Furthermore, saturation of the receptor by expression of an HDEL-tagged version of pro-alpha factor inhibits growth, even of strains that do not show obvious BiP retention. We conclude that growth requires the HDEL-dependent retrieval of one or more proteins, and that these proteins can be recognized even under conditions where BiP is secreted. Genetic screens have failed to identify any one protein whose loss could account for the Erd2p requirement. Therefore, a growth may require the retention of multiple HDEL proteins in the ER, or alternatively the removal of such proteins from the Golgi apparatus.     

The coatomer-interacting protein Dsl1p is required for Golgi-to-endoplasmic reticulum retrieval in yeast

Sec22p is an endoplasmic reticulum (ER)-Golgi v-SNARE protein whose retrieval from the Golgi compartment to the endoplasmic reticulum (ER) is mediated by COPI vesicles. Whether Sec22p exhibits its primary role at the ER or the Golgi apparatus is still a matter of debate. To determine the role of Sec22p in intracellular transport more precisely, we performed a synthetic lethality screen. We isolated mutant yeast strains in which SEC22 gene function, which in a wild type strain background is non-essential for cell viability, has become essential. In this way a novel temperature-sensitive mutant allele, dsl1-22, of the essential gene DSL1 was obtained. The dsl1-22 mutation causes severe defects in Golgi-to-ER retrieval of ER-resident SNARE proteins and integral membrane proteins harboring a C-terminal KKXX retrieval motif, as well as of the soluble ER protein BiP/Kar2p, which utilizes the HDEL receptor, Erd2p, for its recycling to the ER. DSL1 interacts genetically with mutations that affect components of the Golgi-to-ER recycling machinery, namely sec20-1, tip20-5, and COPI-encoding genes. Furthermore, we demonstrate that Dsl1p is a peripheral membrane protein, which in vitro specifically binds to coatomer, the major component of the protein coat of COPI vesicles.     

Inheritance of the endoplasmic reticulum and Golgi apparatus

Yeast and mammalian cells use a variety of different mechanisms to ensure that the endoplasmic reticulum and Golgi apparatus are inherited by both daughter cells on cell division. In yeast, endoplasmic reticulum inheritance involves both active microtubule and passive actin-based mechanisms, while the Golgi is transported into the forming daughter cell by an active actin-based mechanism. Animal cells actively partition the endoplasmic reticulum and Golgi apparatus, but association with the mitotic spindle-rather than the actin cytoskeleton-appears to be the mechanism     

Cys 786 and Cys 776 in the posttranslational processing of the insulin and IGF-I receptors

The extracellular regions of insulin and IGF-I receptors (IR and IGF-IR) contain fibronectin type III repeats with cysteine residues potentially involved in S==S bond. In this report we show that Cys 786 in the IR and the corresponding Cys 776 in the IGF-IR regulate proreceptor dimerization with high specificity. Both C786S insulin and C776S IGF-I proreceptors reach the monomeric 210-kDa step, but do not proceed further. Mature IR(C786S) and IGF-IR(C776S) expression on plasmamembrane is abolished. No retention of C786S IR precursor was detected in the endoplasmic reticulum, which is degraded by a nonlysosomal mechanism. The rearrangement of the remaining cysteines in the insulin receptor beta subunit ectodomain does not rescue dimerization of C786S insulin proreceptor. As observed in other transmembrane receptors, iuxtamembrane cysteines, specifically Cys 786 in the IR and Cys 776 in the IGF-IR, are critical for correct processing of proreceptors.     

The small GTPase Rab5a is essential for intracellular transport of proglutelin from the Golgi apparatus to the protein storage vacuole and endosomal membrane organization in developing rice endosperm

Rice (Oryza sativa) glutelins are synthesized on the endoplasmic reticulum as larger precursors, which are then transported via the Golgi to the protein storage vacuole (PSV), where they are processed into acidic and basic subunits. Three independent glutelin precursor mutant4 (glup4) rice lines, which accumulated elevated levels of proglutelin over the wild type, were identified as loss-of-function mutants of Rab5a, the small GTPase involved in vesicular membrane transport. In addition to the plasma membrane, Rab5a colocalizes with glutelins on the Golgi apparatus, Golgi-derived dense vesicles, and the PSV, suggesting that Rab5a participates in the transport of the proglutelin from the Golgi to the PSV. This spatial distribution pattern was dramatically altered in the glup4 mutants. Numerous smaller protein bodies containing glutelin and α-globulin were evident, and the proteins were secreted extracellularly. Moreover, all three independent glup4 allelic lines displayed the novel appearance of a large dilated, structurally complex paramural body containing proglutelins, α-globulins, membrane biomarkers for the Golgi apparatus, prevacuolar compartment, PSV, and the endoplasmic reticulum luminal chaperones BiP and protein disulfide isomerase as well as β-glucan. These results indicate that the formation of the paramural bodies in glup4 endosperm was due to a significant disruption of endocytosis and membrane vesicular transport by Rab5a loss of function. Overall, Rab5a is required not only for the intracellular transport of proglutelins from the Golgi to the PSV in rice endosperm but also in the maintenance of the general structural organization of the endomembrane system in developing rice seeds.