Post-translational changes in tertiary and quaternary structure of the insulin proreceptor. Correlation with acquisition of function

Transcriptional Regulation of INSR, the Insulin Receptor Gene

The insulin receptor gene encodes an evolutionarily conserved signaling protein with a wide spectrum of functions in metazoan development. The insulin signaling pathway plays key roles in processes such as metabolic regulation, growth control, and neuronal function. Misregulation of the pathway features in diabetes, cancer, and neurodegenerative diseases, making it an important target for clinical interventions. While much attention has been focused on differential pathway activation through ligand availability, sensitization of overall signaling may also be mediated by differential expression of the insulin receptor itself. Although first characterized as a “housekeeping” gene with stable expression, comparative studies have shown that expression levels of the human INSR mRNA differ by tissue and in response to environmental signals. Our recent analysis of the transcriptional controls affecting expression of the Drosophila insulin receptor gene indicates that a remarkable amount of DNA is dedicated to encoding sophisticated feedback and feed forward signals. The human INSR gene is likely to contain a similar level of transcriptional complexity; here, we summarize over three decades of molecular biology and genetic research that points to a still incompletely understood regulatory control system. Further elucidation of transcriptional controls of INSR will provide the basis for understanding human genetic variation that underlies population-level physiological differences and disease.

Structural Basis and Genotype-Phenotype Correlations of INSR Mutations Causing Severe Insulin Resistance

The insulin receptor (INSR) gene was analyzed in four patients with severe insulin resistance, revealing five novel mutations and a deletion that removed exon 2. A patient with Donohue syndrome (DS) had a novel p.V657F mutation in the second fibronectin type III domain (FnIII-2), which contains the α-β cleavage site and part of the insulin-binding site. The mutant INSR was expressed in Chinese hamster ovary cells, revealing that it reduced insulin proreceptor processing and impaired activation of downstream signaling cascades. Using online databases, we analyzed 82 INSR missense mutations and demonstrated that mutations causing DS were more frequently located in the FnIII domains than those causing the milder type A insulin resistance (P = 0.016). In silico structural analysis revealed that missense mutations predicted to severely impair hydrophobic core formation and stability of the FnIII domains all caused DS, whereas those predicted to produce localized destabilization and to not affect folding of the FnIII domains all caused the less severe Rabson-Mendenhall syndrome. These results suggest the importance of the FnIII domains, provide insight into the molecular mechanism of severe insulin resistance, will aid early diagnosis, and will provide potential novel targets for treating extreme insulin resistance.

The Roles of Insulin-Like Growth Factors in Mesenchymal Stem Cell Niche

Many tissues contain adult mesenchymal stem cells (MSCs), which may be used in tissue regeneration therapies. However, the MSC availability in most tissues is limited which demands expansion in vitro following isolation. Like many developing cells, the state of MSCs is affected by the surrounding microenvironment, and mimicking this natural microenvironment that supports multipotent or differentiated state in vivo is essential to understand for the successful use of MSC in regenerative therapies. Many researchers are, therefore, optimizing cell culture conditions in vitro by altering growth factors, extracellular matrices, chemicals, oxygen tension, and surrounding pH to enhance stem cells self-renewal or differentiation. Insulin-like growth factors (IGFs) system has been demonstrated to play an important role in stem cell biology to either promote proliferation and self-renewal or enhance differentiation onset and outcome, depending on the cell culture conditions. In this review, we will describe the importance of IGFs, IGF-1 and IGF-2, in development and in the MSC niche and how they affect the pluripotency or differentiation towards multiple lineages of the three germ layers.

The Role of Insulin Receptor Isoforms in Diabetes and Its Metabolic and Vascular Complications

The insulin receptor (IR) presents by alternative splicing two isoforms: IRA and IRB. The differential physiological and pathological role of both isoforms is not completely known, and it is determinant the different binding affinity for insulin-like growth factor. IRB is more abundant in adult tissues and it exerts mainly the metabolic actions of insulin, whereas IRA is mainly expressed in fetal and prenatal period and exerts mitogenic actions. However, the change in the expression profile of both IR isoforms and its dysregulation are associated with the development of different pathologies, such as cancer, insulin resistance, diabetes, obesity, and atherosclerosis. In some of them, there is a significant increase of IRA/IRB ratio conferring a proliferative and migratory advantage to different cell types and favouring IGF-II actions with a sustained detriment in the metabolic effects of insulin. This review discussed specifically the role of IR isoforms as well as IGF-IR in diabetes and its associated complications as obesity and atherosclerosis. Future research with new IR modulators might be considered as possible targets to improve the treatment of diabetes and its associated complications.

Structural Dynamics of Insulin Receptor and Transmembrane Signaling

The insulin receptor (IR) is a (αβ)2-type transmembrane tyrosine kinase that plays a central role in cell metabolism. Each αβ heterodimer consists of an extracellular ligand-binding α-subunit and a membrane-spanning β-subunit that comprises the cytoplasmic tyrosine kinase (TK) domain and the phosphorylation sites. The α- and β-subunits are linked via a single disulfide bridge, and the (αβ)2 tetramer is formed by disulfide bonds between the α-chains. Insulin binding induces conformational changes in IR that reach the intracellular β-subunit followed by a protein phosphorylation and activation cascade. Defects in this signaling process, including IR dysfunction caused by mutations, result in type 2 diabetes. Rational drug design aimed at treatment of diabetes relies on knowledge of the detailed structure of IR and the dynamic structural transformations during transmembrane signaling. Recent X-ray crystallographic studies have provided important clues about the mode of binding of insulin to IR, the resulting structural changes and their transmission to the TK domain, but a complete understanding of the structural basis underlying insulin signaling has not been achieved. This review presents a critical analysis of the current status of the structure-function relationship of IR, with a comparative assessment of the other IR family receptors, and discusses potential advancements that may provide insight into the molecular mechanism of insulin signaling.

Heterodimerization of glycosylated insulin-like growth factor-1 receptors and insulin receptors in cancer cells sensitive to anti-IGF1R antibody

BACKGROUND

Identification of predictive biomarkers is essential for the successful development of targeted therapy. Insulin-like growth factor 1 receptor (IGF1R) has been examined as a potential therapeutic target for various cancers. However, recent clinical trials showed that anti-IGF1R antibody and chemotherapy are not effective for treating lung cancer.

METHODOLOGY/PRINCIPAL FINDINGS

In order to define biomarkers for predicting successful IGF1R targeted therapy, we evaluated the anti-proliferation effect of figitumumab (CP-751,871), a humanized anti-IGF1R antibody, against nine gastric and eight hepatocellular cancer cell lines. Out of 17 cancer cell lines, figitumumab effectively inhibited the growth of three cell lines (SNU719, HepG2, and SNU368), decreased p-AKT and p-STAT3 levels, and induced G 1 arrest in a dose-dependent manner. Interestingly, these cells showed co-overexpression and altered mobility of the IGF1R and insulin receptor (IR). Immunoprecipitaion (IP) assays and ELISA confirmed the presence of IGF1R/IR heterodimeric receptors in figitumumab-sensitive cells. Treatment with figitumumab led to the dissociation of IGF1-dependent heterodimeric receptors and inhibited tumor growth with decreased levels of heterodimeric receptors in a mouse xenograft model. We next found that both IGF1R and IR were N-linked glyosylated in figitumumab-sensitive cells. In particular, mass spectrometry showed that IGF1R had N-linked glycans at N913 in three figitumumab-sensitive cell lines. We observed that an absence of N-linked glycosylation at N913 led to a lack of membranous localization of IGF1R and figitumumab insensitivity.

CONCLUSION AND SIGNIFICANCE

The data suggest that the level of N-linked glycosylated IGF1R/IR heterodimeric receptor is highly associated with sensitivity to anti-IGF1R antibody in cancer cells.

A heterozygous mutation of the insulin-like growth factor-I receptor causes retention of the nascent protein in the endoplasmic reticulum and results in intrauterine and postnatal growth retardation

BACKGROUND

Mutations in the IGF-I receptor (IGF1R) gene can be responsible for intrauterine and postnatal growth disorders.

OBJECTIVE

Here we report on a novel mutation in the IGF1R gene in a female patient. The aim of our study was to analyze the functional impact of this mutation.

PATIENT

At birth, the girl's length was 47 cm [-1.82 sd score (SDS)], and her weight was 2250 g (-2.26 SDS). Clinical examination revealed microcephaly and retarded cognitive development. She showed no postnatal catch-up growth but had relatively high IGF-I levels (+1.83 to +2.17 SDS).

RESULTS

Denaturing HPLC screening and direct DNA sequencing disclosed a heterozygous missense mutation resulting in an amino acid exchange from valine to glutamic acid at position 599 (V599E-IGF1R). Using various cell systems, we found that the V599E-IGF1R mutant was not tyrosine phosphorylated and had an impaired downstream signaling in the presence of IGF-I. Flow cytometry and live cell confocal laser scanning microscopy revealed a lack of cell surface expression due to an extensive retention of V599E-IGF1R proteins within the endoplasmic reticulum.

CONCLUSION

The V599E-IGF1R mutation interferes with the receptor's trafficking path, thereby abrogating proreceptor processing and plasma membrane localization. Diminished cell surface receptor density solely expressed from the patient's wild-type allele is supposed to lead to insufficient IGF-I signaling. We hypothesize that this mechanism results in intrauterine and postnatal growth retardation of the affected patient. The reported retention of the nascent IGF1R in the endoplasmic reticulum presents a novel mechanism of IGF-I resistance.

Insulin Receptor and its Relationship with Different Forms of Insulin Resistance

Insulin plays an important role in maintaining the whole organism’s homeostasis. The presence of insulin receptors in all vertebrates and invertebrates cells reflects the diversity of regulatory processes in which this hormone is involved. Furthermore, many different factors may influence the level of insulin receptor expression. These factors include e.g. the sole insulin or stage of development. Mutations in the receptor may lead to the development of insulin resistance. These mutations differ in the level of severity and are frequently associated with diabetes mellitus, hypertension, cardiovascular disorders, heart failure, metabolic syndrome and infertility in women. More than 50 mutations in insulin receptor gene have already been characterized. These mutations are associated with rare forms of insulin resistance like leprechaunism, insulin resistance type A or Rabson-Mendenhall syndrome. Molecular analysis of insulin receptor gene may lead to a better understanding of molecular mechanisms underlying various types of insulin resistance and help to develop more efficient treatment.

Insulin receptor isoforms and insulin receptor/insulin-like growth factor receptor hybrids in physiology and disease

In mammals, the insulin receptor (IR) gene has acquired an additional exon, exon 11. This exon may be skipped in a developmental and tissue-specific manner. The IR, therefore, occurs in two isoforms (exon 11 minus IR-A and exon 11 plus IR-B). The most relevant functional difference between these two isoforms is the high affinity of IR-A for IGF-II. IR-A is predominantly expressed during prenatal life. It enhances the effects of IGF-II during embryogenesis and fetal development. It is also significantly expressed in adult tissues, especially in the brain. Conversely, IR-B is predominantly expressed in adult, well-differentiated tissues, including the liver, where it enhances the metabolic effects of insulin. Dysregulation of IR splicing in insulin target tissues may occur in patients with insulin resistance; however, its role in type 2 diabetes is unclear. IR-A is often aberrantly expressed in cancer cells, thus increasing their responsiveness to IGF-II and to insulin and explaining the cancer-promoting effect of hyperinsulinemia observed in obese and type 2 diabetic patients. Aberrant IR-A expression may favor cancer resistance to both conventional and targeted therapies by a variety of mechanisms. Finally, IR isoforms form heterodimers, IR-A/IR-B, and hybrid IR/IGF-IR receptors (HR-A and HR-B). The functional characteristics of such hybrid receptors and their role in physiology, in diabetes, and in malignant cells are not yet fully understood. These receptors seem to enhance cell responsiveness to IGFs.

Autophagy-mediated insulin receptor down-regulation contributes to endoplasmic reticulum stress-induced insulin resistance

Endoplasmic reticulum (ER) stress is associated with obesity-induced insulin resistance, yet the underlying mechanisms remain to be fully elucidated. Here we show that ER stress-induced insulin receptor (IR) down-regulation may play a critical role in obesity-induced insulin resistance. The expression levels of IR are negatively associated with the ER stress marker C/EBP homologous protein (CHOP) in insulin target tissues of db/db mice and mice fed a high-fat diet. Significant IR down-regulation was also observed in fat tissue of obese human subjects and in 3T3-L1 adipocytes treated with ER stress inducers. ER stress had little effect on IR tyrosine phosphorylation per se but greatly reduced IR downstream signaling. The ER stress-induced reduction in IR cellular levels was greatly alleviated by the autophagy inhibitor 3-methyladenine but not by the proteasome inhibitor N-benzoyloxycarbonyl (Z)-Leu-Leu-leucinal (MG132). Inhibition of autophagy prevented IR degradation but did not rescue IR downstream signaling, consistent with an adaptive role of autophagy in response to ER stress-induced insulin resistance. Finally, chemical chaperone treatment protects cells from ER stress-induced IR degradation in vitro and obesity-induced down-regulation of IR and insulin action in vivo. Our results uncover a new mechanism underlying obesity-induced insulin resistance and shed light on potential targets for the prevention and treatment of obesity-induced insulin resistance and type 2 diabetes.

Insulin signaling in mouse oocytes

Continuous exposure of follicles/oocytes to elevated levels of insulin compromises embryonic developmental competence, although the underlying cellular mechanisms are unknown. The objectives of the present study were to determine whether mouse oocytes have insulin receptors and a functional insulin signaling cascade, and whether insulin exposure during oocyte growth or maturation influences meiotic progression and chromatin remodeling. Immunoblot and immunocytochemical analyses of germinal vesicle-intact (GVI) oocytes demonstrated the presence of insulin receptor-beta. Insulin receptor expression in oocytes was increased by gonadotropin stimulation, and remained elevated throughout meiotic maturation. Fully grown GVI oocytes contained 3-phosphoinositide-dependent protein kinase-1 (PDPK1), thymoma viral proto-oncogene 1 (AKT1), and glycogen synthase kinase 3 (GSK3). In vitro maturation of GVI oocytes in 5 microg/ml insulin had no influence on meiotic progression or the incidence of normal metaphase II (MII) chromosome condensation. Treatment of oocytes during maturation had no effect on GSK3A/B protein expression or phosphorylation of S21/9. However, the culturing of preantral follicles for 10 days with 5 microg/ml insulin increased the phosphorylation of oocyte GSK3B, indicating GSK3 inactivation. The rates of development to metaphase I (MI) were similar for oocytes obtained from insulin-treated follicles and controls, whereas the incidence of abnormal MI chromatin condensation was significantly higher in oocytes obtained from follicles cultured with insulin compared to those cultured without insulin. These results demonstrate that oocytes contain a functional insulin signaling pathway, and that insulin exposure during oocyte growth results in chromatin remodeling aberrations. These findings begin to elucidate the mechanisms by which chronic elevated insulin influences oocyte meiosis, chromatin remodeling, and embryonic developmental competence.

RILM: a web-based resource to aid comparative and functional analysis of the insulin and IGF-1 receptor family

The metazoan receptors for insulin (INSR), insulin-like growth factor 1 (IGF1R), and other insulin-like molecules are transmembrane tyrosine kinases involved in the regulation of cell size, cell proliferation, development, signaling of nutritional and environmental conditions, and aging. Historically, mutations in the human insulin receptor have been studied because such changes often lead to severe insulin resistance. More recently, amino acid sequence alterations in the insulin receptor-like receptors of Drosophila melanogaster and Caenorhabditis elegans, as well as in the mouse insulin receptor have been the focus of attention. These modifications can have profound effects on growth, body size, metabolism, and aging. To integrate the many findings on insulin/IGF1 receptor structure and function across species we have created "Receptors for Insulin and Insulin-like Molecules" (RILM), a curated computer-based resource that displays residue-by-residue information on sequence homology, three-dimensional structure, structure/function annotation, and documented mutations. The resource includes data obtained from sequence and structure analysis tools, primary database resources, and published reports. The information is integrated via a structure-based multiple sequence alignment of diverse members of the family. RILM was designed to provide easy access to multiple data types that could prove useful in the analysis of the effect of mutations on protein structure and ligand binding within this receptor family. RILM is available at www.biochem.ucl.ac.uk/RILM.

Calreticulin and Hsp90 stabilize the human insulin receptor and promote its mobility in the endoplasmic reticulum

Elimination of misfolded membrane proteins in the endoplasmic reticulum (ER) affects cell survival and growth and can be triggered by either local physiologic events or disease-associated mutations. Regulation of signaling receptor degradation involves both cytosolic and ER luminal molecular chaperones, but the mechanisms and timing of this process remain uncertain. Here we report that calreticulin (CRT) and Hsp90 exert distinct effects on the stability and cell surface levels of native and misfolded forms of the human insulin receptor (hIR) and a human variant found in type A insulin resistance. CRT was unique in stabilizing the disease variant and in augmenting hIR expression when glycolysis was abrogated. Effects of Hsp90 were independent of receptor tyrosine phosphorylation and did not change levels of downstream signaling kinases. Live cell imaging revealed that movement of the hIR through the ER was accelerated by misfolding or by overexpression of either CRT or Hsp90. Together, our results indicate that both CRT and Hsp90 control expression of hIR at its earliest maturation stages and modulate its movement within the ER before either degradation or cell surface expression.

Characterization of insulin/IGF hybrid receptors: contributions of the insulin receptor L2 and Fn1 domains and the alternatively spliced exon 11 sequence to ligand binding and receptor activation

The IR (insulin receptor) and IGFR (type I insulin-like growth factor receptor) are found as homodimers, but the respective pro-receptors can also heterodimerize to form insulin-IGF hybrid receptors. There are conflicting data on the ligand affinity of hybrids, and especially on the influence of different IR isoforms. To investigate further the contribution of individual ligand binding epitopes to affinity and specificity in the IR/IGFR family, we generated hybrids incorporating both IR isoforms (A and B) and IR/IGFR domain-swap chimaeras, by ectopic co-expression of receptor constructs in Chinese hamster ovary cells, and studied ligand binding using both radioligand competition and bioluminescence resonance energy transfer assays. We found that IR-A-IGFR and IR-B-IGFR hybrids bound insulin with similar relatively low affinity, which was intermediate between that of homodimeric IR and homodimeric IGFR. However, both IR-A-IGFR and IR-B-IGFR hybrids bound IGF-I and IGF-II with high affinity, at a level comparable with homodimeric IGFR. Incorporation of a significant fraction of either IR-A or IR-B into hybrids resulted in abrogation of insulin- but not IGF-I-stimulated autophosphorylation. We conclude that the sequence of 12 amino acids encoded by exon 11 of the IR gene has little or no effect on ligand binding and activation of IR-IGFR hybrids, and that hybrid receptors bind IGFs but not insulin at physiological concentrations regardless of the IR isoform they contained. To reconstitute high affinity insulin binding within a hybrid receptor, chimaeras in which the IGFR L1 or L2 domains had been replaced by equivalent IR domains were co-expressed with full-length IR-A or IR-B. In the context of an IR-A-IGFR hybrid, replacement of IR residues 325-524 (containing the L2 domain and part of the first fibronectin domain) with the corresponding IGFR sequence increased the affinity for insulin by 20-fold. We conclude that the L2 and/or first fibronectin domains of IR contribute in trans with the L1 domain to create a high affinity insulin-binding site within a dimeric receptor.

Interaction of PTPB with the insulin receptor precursor during its biosynthesis in the endoplasmic reticulum

PTP1B is a protein tyrosine-phosphatase predominantly located on the cystosolic surface of the endoplasmic reticulum. This tyrosine-phosphatase plays a major role in the regulation of the activity of the insulin receptor (IR). We have studied the interaction of the IR with PTP1B in living cells using bioluminescence resonance energy transfer (BRET). The IR was fused to Renilla luciferase and a substrate-trapping mutant of PTP1B was fused to the yellow variant of the green fluorescent protein (YFP). When the two partners interacted, an energy transfer occurred between the luciferase and the YFP, and a fluorescent signal, emitted by the YFP, could be detected. The interaction of the IR with PTP1B could be monitored in real time for more than 30 min. Insulin rapidly and dose-dependently stimulated this interaction. The basal (insulin-independent) interaction of IR with PTP1B was much lower with a soluble form than with the endoplasmic reticulum-targeted form of PTP1B, indicating that this basal interaction mainly occurred in the endoplasmic reticulum. In the basal state, PTP1B and the IR indeed co-localized in the endoplasmic reticulum, as demonstrated by confocal microscopy and cell fractionation experiments. Moreover, inhibition of IR processing with tunicamycin indicated that the basal interaction of PTP1B with IR occurred during biosynthesis of the IR precursor in the endoplasmic reticulum. These results strongly suggest that PTP1B not only dephosphorylates the insulin receptor that has been activated by insulin, but also regulates the insulin receptor precursor during its biosynthesis. Localisation of PTP1B to the endoplasmic reticulum may be important to prevent insulin-independent autonomous activity of the immature insulin receptor precursor.

The role of glycosylation in the function of a 48-kDa glycoprotein from carrot

Carrot extracellular dermal glycoprotein (EDGP) may play an important role in plant defense systems and in signal transduction. Our experiments show that differences in pI values of EDGP isoforms are caused by differences in amino acid sequence and not by heterogeneity in phosphorylation. The binding affinity of native EDGP for a 4-kDa hormone-like peptide from soybean was approximately one-third that of deglycosylated EDGP, and deglycosylation of EDGP caused complete loss of its ability to inhibit xyloglucan-specific endo-beta-1,4-glucanase. Experiments using tunicamycin-treated carrot cell cultures showed that glycosylation is essential for correct EDGP folding and secretion, and that tunicamycin does not affect EDGP gene transcription.

IRS-1 and Vascular Complications in Diabetes Mellitus

The expected explosive increase in the number of patients with diabetes mellitus will increase the stress on health care. Treatment is focused on preventing vascular complications associated with the disorder. In order to develop better treatment regimens, the field of research has made a great effort in understanding this disorder. This chapter summarizes the current views on the insulin signaling pathway with emphasis on intracellular signaling events associated with insulin resistance, which lead to the prothrombotic condition in the vasculature of patience with diabetes mellitus.

Proreceptor Dimerization Is Required for Insulin Receptor Post-translational Processing

The insulin receptor is a transmembrane protein dimer composed of two alphabeta monomers held together by inter-alpha-chain disulfide bonds. In a previous report we described a monomeric insulin receptor obtained by replacing Cys-524, -682, -683, and -685 with serine. The membrane-bound monomeric insulin receptors could be cross-linked to dimers in the presence of insulin, indicating that although covalent interactions had been abolished, noncovalent dimerization could still occur in the membrane. To eliminate noncovalent dimerization, we replaced all or some of Cys-524, -682, -683, and -685 with arginine or aspartic acid with the expectation that the electrostatic repulsion at these contact sites would prevent noncovalent dimerization. The results indicate that mutant insulin receptors that are able to form covalent dimers are expressed at the wild type level; mutants that can form noncovalent dimers are expressed at half the level of the wild type receptor, whereas insulin receptor mutants that cannot dimerize are expressed at less than 10% of the wild type level. To elucidate the mechanism of the decrease in expression of the mutant insulin receptors, we examined their subcellular localization and biosynthesis. The results suggest that the extent of expression of these mutant receptors is related to their ability to form covalent or noncovalent dimers at the proreceptor stage.

Protein tyrosine phosphatase-1B dephosphorylation of the insulin receptor occurs in a perinuclear endosome compartment in human embryonic kidney 293 cells

Protein tyrosine phosphatase-1B (PTP-1B) is a negative regulator of insulin signaling. It is thought to carry out this role by interacting with and dephosphorylating the activated insulin receptor (IR). However, little is known regarding the nature of the cellular interaction between these proteins, especially because the IR is localized to the plasma membrane and PTP-1B to the endoplasmic reticulum. Using confocal microscopy and fluorescence resonance energy transfer (FRET), the interaction between PTP-1B and the IR was examined in co-transfected human embryonic kidney 293 cells. Biological activities were not significantly affected for either PTP-1B or the IR with the fusion of W1B-green fluorescent protein (GFP) to the N terminus of PTP-1B (W1B-PTP-1B) or the fusion of Topaz-GFP to the C terminus of the IR (Topaz-IR). FRET between W1B and Topaz was monitored in cells transfected with either wild type PTP-1B (W1B-PTP-1B) or the substrate-trapping form PTP-1B(D181A) (W1B-PTP-1B(D181A)) and Topaz-IR. Co-expression of W1B-PTP-1B with Topaz-IR resulted in distribution of Topaz-IR to the plasma membrane, but no FRET was obtained upon insulin treatment. In contrast, co-expression of W1B-PTP-1B(D181A) with Topaz-IR caused an increase in cytosolic Topaz-IR fluorescence and, in some cells, a significant basal FRET signal, suggesting that PTP-1B is interacting with the IR during its synthesis. Stimulation of these cells with insulin resulted in a rapid induction of FRET that increased over time and was localized to a perinuclear spot. Co-expression of Topaz-IR with a GFP-labeled RhoB endosomal marker and treatment of the cells with insulin identified a perinuclear endosome compartment as the site of localization. Furthermore, the insulin-induced FRET could be prevented by the treatment of the cells with a specific PTP-1B inhibitor. These results suggest that PTP-1B appears not only to interact with and dephosphorylate the insulin-stimulated IR in a perinuclear endosome compartment but is also involved in maintaining the IR in a dephosphorylated state during its biosynthesis.

Regulation of nicotinic acetylcholine receptor assembly

The four muscle-type nicotinic acetylcholine receptor (AChR) subunits, alpha, beta, gamma, and delta, assemble into functional alpha(2)betagammadelta pentamers in the endoplasmic reticulum (ER) through a series of interdependent folding and oligomerization events. The first stable assembly intermediate is a trimer composed of alpha, beta, and gamma subunits. The formation of alphabetagamma trimers initiates a series of subunit folding and processing events that allow addition of delta subunits to form alphabetagammadelta tetramers. Subunit folding and processing continue with formation of the ligand-binding sites on the alpha subunit of alphabetagammadelta tetramers and the second alpha subunit added to assemble alpha(2)betagammadelta pentamers. AChR assembly is inefficient. Only 20-30% of synthesized subunits assemble into mature receptors in the ER, while the remaining unassembled subunits are degraded. However, the efficiency of subunit assembly can be regulated under certain conditions leading to higher AChR expression. Increased intracellular cAMP levels cause a 2- to 3-fold increase in AChR assembly efficiency and a comparable increase in surface expression. Additionally, block of ubiquitin-proteasome degradation appears to enhance AChR assembly and expression. Thus, the regulation of AChR assembly through posttranslational mechanisms is a potential therapeutic target for increasing AChR expression in diseases in which expression is compromised.

Dynamics of the interaction between the insulin receptor and protein tyrosine-phosphatase 1B in living cells

The dynamics of the interaction of the insulin receptor with a substrate-trapping mutant of protein-tyrosine phosphatase 1B (PTP1B) were monitored in living human embryonic kidney cells using bioluminescence resonance energy transfer (BRET). Insulin dose-dependently stimulates this interaction, which could be followed in real time for more than 30 minutes. The effect of insulin on the BRET signal could be detected at early time-points (30 seconds), suggesting that in intact cells the tyrosine-kinase activity of the insulin receptor is tightly controlled by PTP1B. Interestingly, the basal (insulin-independent) interaction of the insulin receptor with PTP1B was much weaker with a soluble form of the tyrosine-phosphatase than with the endoplasmic reticulum (ER)-targeted form. Inhibition of insulin-receptor processing using tunicamycin suggests that the basal interaction occurs during insulin-receptor biosynthesis in the ER. Therefore, localization of PTP1B in this compartment might be important for the regulation of insulin receptors during their biosynthesis.

Down-regulation of cell surface insulin receptor and insulin receptor substrate-1 phosphorylation by inhibitor of 90-kDa heat-shock protein family: endoplasmic reticulum retention of monomeric insulin receptor precursor with calnexin in adrenal chromaffin cells

Treatment (>/=6 h) of cultured bovine adrenal chromaffin cells with geldanamycin (GA) or herbimycin A (HA), an inhibitor of the 90-kDa heat-shock protein (Hsp90) family, decreased cell surface (125)I-insulin binding. The effect of GA was concentration (EC(50) = 84 nM)- and time (t(1/2) = 8.5 h)-dependent; GA (1 microM for 24 h) lowered the B(max) value of (125)I-insulin binding by 80%, without changing the K(d) value. Western blot analysis showed that GA (>/=3 h) lowered insulin receptor (IR) level by 83% (t(1/2) = 7.4 h; EC(50) = 74 nM), while raising IR precursor level by 100% (t(1/2) = 7.9 h; EC(50) = 300 nM). Pulse-label followed by reducing and nonreducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed that monomeric IR precursor (~190 kDa) developed into the homodimeric IR precursor (approximately 380 kDa) and the mature alpha(2)beta(2) IR (~410 kDa) in nontreated cells, but not in GA-treated cells; in GA-treated cells, the homodimerization-incompetent form of monomeric IR precursor was degraded via endoplasmic reticulum (ER)-associated protein degradation. Immunoprecipitation followed by immunoblot analysis showed that IR precursor was associated with calnexin (CNX) to a greater extent in GA-treated cells, compared with nontreated cells. GA had no effect on IR mRNA levels and internalization rate of cell surface IRs. In GA-treated cells, insulin-induced tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1) was attenuated by 77%, with no change in IRS-1 level. Thus, inhibition of the Hsp90 family by GA or HA interrupts homodimerization of monomeric IR precursor in the ER and increases retention of monomeric IR precursor with CNX; this event retards cell surface expression of IR and attenuates insulin-induced activation of IRS-1.

Subcellular localization and oligomerization of the Arabidopsis thaliana somatic embryogenesis receptor kinase 1 protein

The Arabidopsis thaliana somatic embryogenesis receptor kinase 1 (AtSERK1) gene is expressed in developing ovules and early embryos. AtSERK1 is also transiently expressed during somatic embryogenesis. The predicted AtSERK1 protein contains an extracellular domain with a leucine zipper motif followed by five leucine-rich repeats, a proline-rich region, a single transmembrane region and an intracellular kinase domain. The AtSERK1 cDNA was fused to two different variants of green fluorescent protein (GFP), a yellow-emitting GFP (YFP) and a cyan-emitting GFP (CFP), and transiently expressed in both plant protoplasts and insect cells. Using confocal laser scanning microscopy it was determined that the AtSERK1-YFP fusion protein is targeted to plasma membranes in both plant and animal cells. The extracellular leucine-rich repeats, and in particular the N-linked oligosaccharides that are present on them appear to be essential for correct localization of the AtSERK1-YFP protein. The potential for dimerization of the AtSERK1 protein was investigated by measuring the YFP/CFP fluorescence emission ratio using fluorescence spectral imaging microscopy. This ratio will increase due to fluorescence resonance energy transfer if the AtSERK1-CFP and AtSERK1-YFP fusion proteins interact. In 15 % of the cells the YFP/CFP emission ratio for plasma membrane localized AtSERK1 proteins was enhanced. Yeast-protein interaction experiments confirmed the possibility for AtSERK1 homodimerization. Elimination of the extracellular leucine zipper domain reduced the YFP/CFP emission ratio to control levels indicating that without the leucine zipper domain AtSERK1 is monomeric.

Identification and characterization of SNX15, a novel sorting nexin involved in protein trafficking

Sorting nexins are a family of phox homology domain containing proteins that are homologous to yeast proteins involved in protein trafficking. We have identified a novel 342-amino acid residue sorting nexin, SNX15, and a 252-amino acid splice variant, SNX15A. Unlike many sorting nexins, a SNX15 ortholog has not been identified in yeast or Caenorhabditis elegans. By Northern blot analysis, SNX15 mRNA is widely expressed. Although predicted to be a soluble protein, both endogenous and overexpressed SNX15 are found on membranes and in the cytosol. The phox homology domain of SNX15 is required for its membrane association and for association with the platelet-derived growth factor receptor. We did not detect association of SNX15 with receptors for epidermal growth factor or insulin. However, overexpression of SNX15 led to a decrease in the processing of insulin and hepatocyte growth factor receptors to their mature subunits. Immunofluorescence studies showed that SNX15 overexpression resulted in mislocalization of furin, the endoprotease responsible for cleavage of insulin and hepatocyte growth factor receptors. Based on our data and the existing findings with yeast orthologs of other sorting nexins, we propose that overexpression of SNX15 disrupts the normal trafficking of proteins from the plasma membrane to recycling endosomes or the trans-Golgi network.

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.

Furin-mediated processing in the early secretory pathway: sequential cleavage and degradation of misfolded insulin receptors

Improperly folded membrane proteins are retained in the endoplasmic reticulum and then diverted to a degradative pathway by a network of molecular chaperones and intracellular proteases. Here we report that mutant insulin proreceptors (Pro(62)) retained in the early secretory pathway undergo proteolytic cleavage at a tetrabasic concensus site for the subtilisin-like protease furin (SPC 1), generating two unstable proteolytic intermediates of 80/120 kDa corresponding to alpha (135 kDa) and beta (90 kDa) subunits. These are degraded more rapidly than the uncleaved proreceptor protein. Site-directed mutagenesis of the normal RKRR processing site prevented cleavage. Use of inhibitors and furin-deficient cell lines confirmed that furin is responsible for proreceptor cleavage; furin overexpression increased the degradation of mutant but not wild-type receptors. Together, these results suggest that processing and degradation occur sequentially for mutant proreceptors.

The in vitro synthesized and processed human insulin receptor precursor binds insulin

The cell-free examination of the human insulin receptor during biogenesis may provide a greater understanding of the elements that contribute to the acquisition of receptor function. The insulin receptor precursor components were produced in a cell-free system and the insulin binding ability of the [35S]methionine-labeled translation products was determined. The processed proreceptor represented by a 190 kDa band was retained on insulin-linked biotin-streptavidin agarose or an insulin column. The insulin binding 190 kDa band migrated slower than the non-binding 190 kDa band on SDS-PAGE which suggests that covalent modifications account for these differences. The trypsin-digested product of the 190 kDa proreceptor was also retained on insulin-linked biotin-streptavidin agarose, however the alpha-subunit precursor was retained on insulin agarose to a much lesser degree. We conclude that a significant fraction of the processed, in vitro translated insulin proreceptor acquires insulin binding ability.

Congenital insulin resistance associated with a conformational alteration in a conserved beta-sheet in the insulin receptor L1 domain

The hormone binding site of members of the insulin receptor family is contained within a highly conserved extracellular region of the receptor. Recent crystallization of the N-terminal region of the binding site revealed two large domains (L1, L2), each organized as a single-stranded right-handed beta-helix, connected by a rod-shaped cysteine-rich domain. Here, we analyze two new naturally occurring mutations in a single beta-sheet within L1, D59G and L62P, that we previously identified in a young woman with classic congenital insulin resistance (type A). Substitution of D59G, a beta-sheet connecting loop residue, caused decreased hormone binding but did not disrupt overall folding, assembly, or movement to the cell surface. In contrast, replacement of the adjacent residue L62P, which is located within the beta-sheet, and positioned in a hormone binding surface, completely disrupted intracellular folding, oligomerization, and trafficking and resulted in aberrant proteolytic degradation. Immunohistochemistry in combination with biosynthetic studies showed that misfolded receptors were retained in an incorrect cellular location and that they colocalized with the resident endoplasmic reticulum chaperone calnexin. This study, together with other mutagenesis data, shows that formation of beta-sheet elements within the L1 beta-helix are critical for the folding of the entire extracellular domain of the receptor and that the hormone contact site is composed in part by residues in this domain.

Insulin/insulin-like growth factor-I hybrid receptors with high affinity for insulin are developmentally regulated during neurogenesis

The extensive colocalization of insulin receptor (IR) and insulin-like growth factor-I receptor (IGFR) messenger RNAs during central nervous system development, together with the effects of insulin and IGF-I in neurogenesis, raises the question of how stage- and factor-specific signaling occurs. Thus, it is necessary to characterize the receptor proteins present in vivo to start addressing this issue. Here we have studied the chick embryonic neuroretina at day 6 (E6), when it is predominantly proliferative, and at E12, when neuronal differentiation is advanced. Developmentally regulated high-affinity binding sites for both insulin and IGF-I were detected at E6 and E12. In proliferative neuroretina, typical IGFR with the highest affinity for IGF-I coexisted with separate atypical insulin binding sites, which had similar high affinity for insulin and IGF-I. Immunoprecipitation of ligand-cross-linked receptors with specific antibodies for the IR alpha-subunit, the IR beta-subunit, or the IGFR beta-subunit demonstrated the presence of IR/IGFR hybrids. They were more abundant in E6 than in E12 retina. These hybrid receptors bound most of radiolabeled insulin, but little radiolabeled IGF-I, at tracer concentrations. At E12, the specificity of the insulin binding sites changed, and it was closer to that found with IR in liver, where hybrids were undetectable. The basal autophosphorylation level of these atypical hybrid receptors was high, although insulin and, even more so, IGF-I modestly increased the phosphorylation of two IR beta-subunits of 95 and 105 kDa. The high-affinity/low-discriminative IR/IGFR hybrids predominantly found in a proliferative stage of neurogenesis can mediate the effects of proinsulin and insulin, previously demonstrated in organoculture at this stage. More importantly, this hybrid receptor may be physiologically relevant for the action of the locally produced proinsulin found in early neurogenesis.

Folding of insulin receptor monomers is facilitated by the molecular chaperones calnexin and calreticulin and impaired by rapid dimerization

Many complex membrane proteins undergo subunit folding and assembly in the ER before transport to the cell surface. Receptors for insulin and insulin-like growth factor I, both integral membrane proteins and members of the family of receptor tyrosine kinases (RTKs), are unusual in that they require homodimerization before export from the ER. To better understand chaperone mechanisms in endogenous membrane protein assembly in living cells, we have examined the folding, assembly, and transport of the human insulin receptor (HIR), a dimeric RTK. Using pulse-chase labeling and nonreducing SDS-PAGE analysis, we have explored the molecular basis of several sequential maturation steps during receptor biosynthesis. Under normal growth conditions, newly synthesized receptor monomers undergo disulfide bond formation while associated with the homologous chaperones calnexin (Cnx) and calreticulin (Crt). An inhibitor of glucose trimming, castanospermine (CST), abolished binding to Cnx/Crt but also unexpectedly accelerated receptor homodimerization resulting in misfolded oligomeric proreceptors whose processing was delayed and cell surface expression was also decreased by approximately 30%. Prematurely-dimerized receptors were retained in the ER and more avidly associated with the heat shock protein of 70 kD homologue binding protein. In CST-treated cells, receptor misfolding followed disordered oligomerization. Together, these studies demonstrate a chaperone function for Cnx/Crt in HIR folding in vivo and also provide evidence that folding efficiency and homodimerization are counterbalanced.

Fusion of insulin receptor ectodomains to immunoglobulin constant domains reproduces high-affinity insulin binding in vitro

A unique feature of the insulin receptor is that it is dimeric in the absence of ligand. Dimerization of two adjacent transmembrane domain (TMD) alpha helices has been shown to be critical in receptor kinase activation. Moreover, previous work has suggested that the TMD is involved in stabilizing the high-affinity binding site; soluble receptors expressed after simple truncation at the ectodomain-TMD junction have reduced affinity for insulin. To further examine this issue, we have replaced the TMD and intracellular domain of the soluble human insulin receptor (HIRs) with constant domains from immunoglobulin Fc and lambda subunits (HIRs-Fc and HIRs-lambda). Studies of receptor biosynthesis and binding characteristics were performed following transient transfection of receptor cDNAs into human embryonal kidney 293 cells. Each hybrid receptor was initially synthesized as a single chain proreceptor, followed by cleavage into alpha- and beta-Fc or beta-lambda subunits. The majority of secreted protein appeared in the cell medium as fully processed heterotetramer. Fc fragments released from HIRs-Fc by papain digestion and analyzed by nonreducing SDS-polyacrylamide gel electrophoresis were dimeric. Furthermore, dissociation constants for both chimeras were similar to those for the full-length holoreceptor (wild-type receptor, Kd1 = 200 pM and Kd2 = 2 nM; HIRs-Fc, Kd1 = 200 pM and Kd2 = 40 nM; and HIRs-lambda, Kd1 = 200 pM and Kd2 = 5 nM). These results extend previous observations that dimerization of the membrane-proximal ectodomain is necessary to maintain an intact high-affinity insulin-binding site.

Expression of the insulin receptor with a recombinant vaccinia virus. Biochemical evidence that the insulin receptor has intrinsic serine kinase activity

We have previously reported the tight association of a serine kinase activity with the human insulin receptor (Lewis, R. E., Wu, G. P., MacDonald, R. G., and Czech, M. P. (1990) J. Biol. Chem. 265, 947-954). We tested the possibility that the associated serine kinase activity was intrinsic to the receptor catalytic domain. The ratio of phosphoserine to phosphotyrosine on insulin receptors phosphorylated in vitro was used as an index of the associated serine kinase activity. Phosphorylation and phosphoamino acid analysis of insulin proreceptors revealed associated serine kinase activity early in receptor synthesis. Insulin receptors were expressed in HeLa cells using a recombinant vaccinia virus. The ratio of phosphoserine to phosphotyrosine on insulin receptors expressed by the recombinant vaccinia virus was determined relative to endogenous insulin receptors in cells treated with alpha-amanitin to block host cell mRNA synthesis. alpha-Amanitin treatment had no effect on the ratio of phosphoserine to phosphotyrosine on insulin receptors expressed from the recombinant virus even though they were present in a 4000-fold excess above endogenous receptors. We conclude that the serine kinase activity associated with the insulin receptor is intrinsic to the receptor catalytic domain. Receptor-catalyzed autophosphorylation of serine may play an important role in modulating insulin receptor signaling.

Postoligomerization folding of human cytomegalovirus glycoprotein B: identification of folding intermediates and importance of disulfide bonding

Human cytomegalovirus glycoprotein B (gB or UL55) has been demonstrated to be a disulfide-linked homodimer within the envelope of mature virions. Previously, it has been shown that gB undergoes a rapid dimerization nearly coincident with its synthesis. Following dimerization, the molecule slowly folds into a form which can be transported from the endoplasmic reticulum. In this study we have examined the prolonged folding of gB by using a set of defined gB-reactive murine monoclonal antibodies and gB expressed as a recombinant protein in the absence of other human cytomegalovirus proteins. Our results have documented a folding pathway consistent with the relatively rapid dimerization of the translation product followed by delayed conversion into a fully folded molecule. Assembly of the dominant antigenic domain of gB, AD-1, preceded dimerization and folding of the molecule. The fully folded dimer was heat stable, but its conformation was altered by treatment with 2% sodium dodecyl sulfate (SDS), whereas an oligomeric folding intermediate was both heat and SDS stable. Postoligomerization disulfide bond formation could be demonstrated during folding of gB, suggesting that the formation of these covalent bonds could contribute to the prolonged folding of this glycoprotein.

Posttranslational folding of alpha 1-inhibitor 3. Evidence for a compaction process

alpha 1-inhibitor 3 (alpha 1 I3) is a rodent-specific proteinase inhibitor of about 190 kDa belonging to the alpha 2-macroglobulin family. It consists of five globular domains, three of which are connected by disulfide bridges, and contains an intramolecular thiol ester which can react with attacking proteinases. To explore the folding of newly synthesized alpha 1 I3, we have used rat hepatocytes and pulsechase experiments. In one of the analyses, the radiolabeled protein was isolated from cell lysates by immunoprecipitation and its Asp-Pro bonds cleaved by treatment with formic acid. The size of the major fragment, as assessed by electrophoresis under nonreducing conditions, was found to increase from 100 to 150 kDa upon the chasing. This result, together with knowledge of the positions of the cleavage sites and the disulfide arrangement, indicates that one of the interdomain disulfide bonds is formed after the synthesis of the polypeptide. Analysis of the same material by limited proteolysis and by velocity centrifugation showed that the folded regions became larger and that the protein became more compact; the thiol ester was found to be formed after these conformational changes. These results suggest that the domains of alpha 1 I3 are only partially developed directly after the synthesis of the polypeptide and that they acquire their final structure as the protein condenses and the domains interact with one another.

Replacements of leucine 87 in human insulin receptor alter affinity for insulin

In a previous analysis, we identified a point mutation that substituted Pro (CCG) for Leu (CTG) at amino acid 87 in the alpha-subunit of the insulin receptor (IR) in a Japanese patient with leprechaunism. In the present study, we transfected either the wild type (Leu-87) or the mutant (Pro-87) IR cDNA into NIH3T3 cells. Pulse-chase in nonreducing conditions revealed that the dimerization of Pro-87 IR was slightly impaired. However, cell surface biotinylation showed that Pro-87 IR was transported to the cell surface. The Pro-87 IR reduced the insulin binding affinity to about 15% of Leu-87 IR, and the dissociation of insulin in Pro-87 IR was more rapid than in Leu-87 IR. The autophosphorylation of Pro-87 IR was less sensitive to insulin than that of Leu-87 IR, suggesting the reduced insulin binding affinity. Site-directed mutagenesis at amino acid 87 was performed to substitute Ile or Ala for Leu. Both mutant IRs were transported to the cell surface and labeled by cell surface biotinylation. The Ile-87 IR enhanced the insulin binding affinity about 4-fold. The insulin binding affinity of Ala-87 IR was reduced by 85% relative to that of Leu-87 IR. In addition, the dissociation of insulin in Ile-87 IR was slower than in Leu-87 IR, but in Ala-87 IR it was more rapid. These results provide the first direct evidence for a critical role of Leu-87 in binding insulin.

Up-regulation of insulin binding in fish skeletal muscle by high insulin levels

The effects of insulin titres on regulation of receptor binding were studied in several fish species. Insulin receptors were semi-purified by affinity chromatography (WGA-agarose) from skeletal muscle of carp, brown trout and rainbow trout that had been subjected to increases in insulinemia produced either by arginine injection, food administration, or adaptation to an experimental diet (extruded diet with high-digestibility carbohydrates). Arginine injection provoked acute hyperinsulinemia in both carp and trout. Specific binding of insulin to the skeletal muscle was significantly increased 3 h after injection (from 5.8 +/- 0.3 to 9.6 +/- 0.9%/10 micrograms protein in carp and from 0.8 +/- 0.2 to 1.5 +/- 0.4%/10 micrograms in trout). The same effect was observed in carp liver preparations (from 6.0 +/- 0.75 to 9.9 +/- 1.25%/10 micrograms). No alterations in tyrosine kinase activity of the receptors were detected in either carp or trout preparations: basal activities of the receptors were maintained (3100 +/- 200 fmol P/fmol receptors/30 min and 3700 +/- 400 fmol P/fmol receptors/30 min, in carp and trout, respectively), as were the percentage of stimulation over basal levels obtained by incubation with insulin (227 +/- 25% and 160 +/- 10% respectively). Food ingestion raised plasma insulin levels more steadily. Specific binding also increased in skeletal muscle preparations, especially in carp (from 5.7 +/- 0.3 to 11 +/- 1.7%/10 micrograms at 4 h and 10 +/- 0.7%/10 micrograms at 8 h). Tyrosine kinase activity was maintained without significant changes. Rainbow trout adapted for 2 months to an extruded diet presented higher insulin titres and higher glycogen reserves in liver and muscle. Insulin binding to skeletal muscle preparations was also significantly increased (from 0.36 +/- 0.02 to 0.77 +/- 0.1%/10 micrograms), as was tyrosine kinase activity (from 132 +/- 4% to 156 +/- 6%, without alterations in the basal activity). Results showed that fish can respond to both acute and maintained increases in insulinemia by increasing the number of insulin receptors. Tyrosine kinase activity, in contrast, is only modified after long-term adaptation.

Formation and rearrangement of disulfide bonds during maturation of the Sindbis virus E1 glycoprotein

The rigidly ordered icosahedral lattice of the Sindbis virus envelope is composed of a host-derived membrane bilayer in which the viral glycoproteins E1 and E2 reside. E1-E1 interactions stabilized by intramolecular disulfide bridges play a significant role in maintaining the envelope's structural integrity (R. P. Anthony and D. T. Brown, J. Virol. 65:1187-1194, 1991; R. P. Anthony, A. M. Paredes, and D. T. Brown, Virology 190:330-336, 1992). We have examined the acquisition of disulfide bridges within E1 during its maturation. Prior to exit from the endoplasmic reticulum, E1 folds via at least three intermediates, differing in the number and/or arrangement of their disulfides, into a single, compact form. This E1 species remains stable with respect to its disulfides until late in the secretory pathway, when E1 attains a metastable conformation. At this point, when appropriately triggered, intramolecular thiol-disulfide exchange reactions within E1 can occur, resulting in the generation of alternative E1 species. This metastable nature of mature E1 may have important implications for the mechanism of virus disassembly during the initial stages of the infection process (B. Abell and D. T. Brown, J. Virol. 67:5496-5501, 1993).

Deletion of C-terminal 113 amino acids impairs processing and internalization of human insulin receptor: comparison of receptors expressed in CHO and NIH-3T3 cells

We have studied the structure and the function of a truncated human insulin receptor in which 113 amino acids (aa 1231-1343) at the C-terminus of the beta-subunit were deleted. In this study, wild-type and truncated insulin receptors were expressed by stable transfection in NIH-3T3 cells and CHO cells. The mutation impairs post-translational processing of the insulin receptor; proteolytic cleavage is retarded, and degradation of the truncated receptor is accelerated. Furthermore, insulin-stimulated autophosphorylation of the mutant insulin receptor is impaired. This is associated with a defect in insulin-stimulated endocytosis. Finally, in NIH-3T3 cells, the mutant insulin receptor failed to mediate the mitogenic effects of insulin. In CHO cells, transfection of insulin receptor cDNA (either wild-type or mutant) did not alter mitogenic response to insulin. It has previously been shown that deletion of 43 amino acids at the C-terminus of the beta-subunit did not affect insulin receptor tyrosine kinase activity. Our data suggest that the structural domain located 43-113 amino acids from the C-terminus appears to have several functional roles. First, the domain appears to promote folding of receptor into the optimal conformation for post-translational processing. Second, the presence of this domain appears to promote the stability of the receptor beta-subunit in intact cells. Finally, perhaps as a consequence of the effects upon the stability of the receptor, this domain is required in intact cells for insulin-stimulated autophosphorylation and signal transmission.

A putative lectin-binding receptor mediates cadmium-evoked calcium release

Nanomolar concentrations of cadmium (Cd2+) produce an immediate rise in free Ca2+ in human dermal fibroblasts, which is mostly caused by the release of stored Ca2+ via inositol trisphosphate. Here we have used lectins to evaluate the hypothesis that a cell surface glycoprotein mediates the response to Cd2+. A prior incubation with wheat germ agglutinin (WGA) or certain other lectins inhibited calcium release evoked by Cd2+. WGA reversibly inhibited Cd(2+)-evoked calcium release as indicated by measurements of cytosolic free Ca2+ and 45Ca2+ efflux. WGA half-maximally inhibited Ca2+ release at 1.2 x 10(-7) M. The Kd for the binding of fluoresceinylated WGA was 2.8 x 10(-7) M. Chitotriose dissociated fluoresceinylated WGA from the cells and restored cadmium responsiveness. WGA inhibited Cd(2+)-evoked 45Ca2+ efflux similarly at 18 and 37 degrees C. A brief incubation with chitotriose at 18 or 10 degrees C reversed the inhibition by WGA. WGA neither bound 109Cd2+ nor affected 109Cd2+ uptake by the cells. Succinylated WGA, which binds N-acetylglucosamine but not N-acetylneuraminic acid, failed to inhibit Ca2+ release evoked by Cd2+. WGA probably inhibits Ca2+ release produced by Cd2+ by binding to N-acetylneuraminic acid in the external domain of a plasma membrane receptor.