The insulin A and B chains contain structural information for the formation of the native molecule. Studies with protein disulphide-isomerase

Structural Lessons From the Mutant Proinsulin Syndrome

Insight into folding mechanisms of proinsulin has been provided by analysis of dominant diabetes-associated mutations in the human insulin gene (INS). Such mutations cause pancreatic β-cell dysfunction due to toxic misfolding of a mutant proinsulin and impairment in trans of wild-type insulin secretion. Anticipated by the "Akita" mouse (a classical model of monogenic diabetes mellitus; DM), this syndrome illustrates the paradigm endoreticulum (ER) stress leading to intracellular proteotoxicity. Diverse clinical mutations directly or indirectly perturb native disulfide pairing leading to protein misfolding and aberrant aggregation. Although most introduce or remove a cysteine (Cys; leading in either case to an unpaired thiol group), non-Cys-related mutations identify key determinants of folding efficiency. Studies of such mutations suggest that the hormone's evolution has been constrained not only by structure-function relationships, but also by the susceptibility of its single-chain precursor to impaired foldability. An intriguing hypothesis posits that INS overexpression in response to peripheral insulin resistance likewise leads to chronic ER stress and β-cell dysfunction in the natural history of non-syndromic Type 2 DM. Cryptic contributions of conserved residues to folding efficiency, as uncovered by rare genetic variants, define molecular links between biophysical principles and the emerging paradigm of Darwinian medicine: Biosynthesis of proinsulin at the edge of non-foldability provides a key determinant of "diabesity" as a pandemic disease of civilization.

Diabetes mellitus due to toxic misfolding of proinsulin variants

BACKGROUND

Dominant mutations in the human insulin gene (INS) lead to pancreatic β-cell dysfunction and diabetes mellitus (DM) due to toxic misfolding of a mutant proinsulin. Analogous to a classical mouse model of monogenic DM ("Akita"), this syndrome highlights the susceptibility of β-cells to endoreticulum (ER) stress due to protein misfolding and aberrant aggregation.

SCOPE OF REVIEW

Diverse clinical mutations directly or indirectly perturb native disulfide pairing. Whereas most introduce or remove a cysteine (Cys; leading in either case to an unpaired thiol group), non-Cys-related mutations identify key determinants of folding efficiency. Studies of such mutations suggest that the hormone's evolution has been constrained not only by structure-function relationships but also by the susceptibility of its single-chain precursor to impaired foldability. An intriguing hypothesis posits that INS overexpression in response to peripheral insulin resistance likewise leads to chronic ER stress and β-cell dysfunction in the natural history of nonsyndromic Type 2 DM.

MAJOR CONCLUSIONS

Cryptic contributions of conserved residues to folding efficiency, as uncovered by rare genetic variants, define molecular links between biophysical principles and the emerging paradigm of Darwinian medicine: Biosynthesis of proinsulin at the edge of nonfoldability provides a key determinant of "diabesity" as a pandemic disease of civilization.

Human αB-crystallin as fusion protein and molecular chaperone increases the expression and folding efficiency of recombinant insulin

Low expression and instability are significant challenges in the recombinant production of therapeutic peptides. The current study introduces a novel expression and purification system for human insulin production using the molecular chaperone αB-crystallin (αB-Cry) as a fusion partner protein. Insulin is composed of A- and B-chain containing three disulfide bonds (one intarchain and two interchains). We have constructed two plasmids harboring the A- or B-chain of insulin joined with human αB-Cry. This system is suitable for cloning of the genes and for directing the synthesis of large amounts of the fusion proteins αB-Cry/A-chain (αB-AC) and αB-Cry/B-chain (αB-BC). The construction of vectors, their efficient expression in Escherichia coli and simple purification of the fusion proteins and two insulin chains are described. A large amount of the recombinant fusion proteins with high purity was obtained by applying a single step anion exchange chromatography or metal chelate affinity. The insulin A- and B-chain were released from the fusion proteins using cyanogen bromide cleavage. The insulin peptides were obtained with an appreciable yield and high purity using one-step gel filtration chromatography. To increase efficiency of chain combination to produce insulin, αB-Cry was used under oxidative conditions. The purification of natively folded insulin was performed by phenyl sepharose hydrophobic interaction chromatography. Finally, using an insulin tolerance test in mice and various biophysical methods, the structure and function of purified human recombinant insulin was compared with authentic insulin, to verify folding of insulin to its native state. Overall, the novel expression system using αB-Cry is highly demanding for producing human insulin and functional protein. The procedure for αB-Cry-mediated insulin folding could be also applicable for the large-scale production of this highly important therapeutic peptide hormone.

Nano-cage-mediated refolding of insulin by PEG-PE micelle

Insulin aggregation has pronounced pharmaceutical implications and biological importance. Deposition of insulin aggregates is associated with type II diabetes and instability of pharmaceutical formulations. We present in this study the renaturation effect of PEG-PE micelle on dithiothreitol (DTT)-denatured insulin revealed by techniques including turbidity assay, circular dichroism (CD), thioflavinT (ThT) binding assay, bis-ANS binding assay, agarose gel electrophoresis and MALDI-TOF MS. The obtained results show that PEG-PE micelle having a hydrophilic nano-cage-like structure in which with a negative charge layer, can capture DTT-induced insulin A and B chains, and block their hydrophobic interaction, thereby preventing aggregation. The reduced insulin A and B chain in the nano-cage are capable of recognizing each other and form the native insulin with yields of ∼30% as measured by hypoglycemic activity analysis in mice. The observed insulin refolding assisted by PEG-PE micelle may be applicable to other proteins.

Cloning, constitutive activity and expression profiling of two receptors related to relaxin receptors in Drosophila melanogaster

Leucine-rich repeat containing G protein-coupled receptors (LGRs) comprise a cluster of transmembrane proteins, characterized by the presence of a large N-terminal extracellular domain. This receptor group can be classified into three subtypes. Belonging to the subtype C LGRs are the mammalian relaxin receptors LGR7 (RXFP1) and LGR8 (RXFP2), which mediate important reproductive and other processes. We identified two related receptors in the genome of the fruit fly and cloned their open reading frames into an expression vector. Interestingly, dLGR3 demonstrated constitutive activity at very low doses of transfected plasmid, whereas dLGR4 did not show any basal activity. Both receptors exhibited a similar expression pattern during development, with relatively high transcript levels during the first larval stage. In addition, both receptors displayed higher expression in male adult flies as compared to female flies. Analysis of the tissue distribution of both receptor transcripts revealed a high expression of dLGR3 in the female fat body, while the expression of dLGR4 peaked in the midgut of both the wandering and adult stage.

Proinsulin entry and transit through the endoplasmic reticulum in pancreatic beta cells

Insulin is an essential hormone for maintaining metabolic homeostasis in the body. To make fully bioactive insulin, pancreatic beta cells initiate synthesis of the insulin precursor, preproinsulin, at the cytosolic side of the endoplasmic reticulum (ER), whereupon it undergoes co- and post-translational translocation across the ER membrane. Preproinsulin is cleaved by signal peptidase to form proinsulin that folds on the luminal side of the ER, forming three evolutionarily conserved disulfide bonds. Properly folded proinsulin forms dimers and exits from the ER, trafficking through Golgi complex into immature secretory granules wherein C-peptide is endoproteolytically excised, allowing fully bioactive two-chain insulin to ultimately be stored in mature granules for insulin secretion. Although insulin biosynthesis has been intensely studied in recent decades, the earliest events, including proinsulin entry and exit from the ER, have been relatively understudied. However, over the past 5 years, more than 20 new insulin gene mutations have been reported to cause a new syndrome termed Mutant INS-gene-induced Diabetes of Youth (MIDY). Although these mutants have not been completely characterized, most of them affect proinsulin entry and exit from the ER. Here, we summarize our current knowledge about the early events of insulin biosynthesis and review recent advances in understanding how defects in these events may lead to pancreatic beta cell failure.

Diabetes mellitus due to the toxic misfolding of proinsulin variants

Dominant mutations in the human insulin gene can lead to pancreatic β-cell dysfunction and diabetes mellitus due to toxic folding of a mutant proinsulin. Analogous to a classical mouse model (the Akita mouse), this monogenic syndrome highlights the susceptibility of human β-cells to endoreticular stress due to protein misfolding and aberrant aggregation. The clinical mutations directly or indirectly perturb native disulfide pairing. Whereas the majority of mutations introduce or remove a cysteine (leading in either case to an unpaired residue), non-cysteine-related mutations identify key determinants of folding efficiency. Studies of such mutations suggest that the evolution of insulin has been constrained not only by its structure and function, but also by the susceptibility of its single-chain precursor to impaired foldability.

Insulin: a small protein with a long journey

Insulin is a hormone that is essential for regulating energy storage and glucose metabolism in the body. Insulin in liver, muscle, and fat tissues stimulates the cell to take up glucose from blood and store it as glycogen in liver and muscle. Failure of insulin control causes diabetes mellitus (DM). Insulin is the unique medicine to treat some forms of DM. The population of diabetics has dramatically increased over the past two decades, due to high absorption of carbohydrates (or fats and proteins), lack of physical exercise, and development of new diagnostic techniques. At present, the two largest developing countries (India and China) and the largest developed country (United States) represent the top three countries in terms of diabetic population. Insulin is a small protein, but contains almost all structural features typical of proteins: α-helix, β-sheet, β-turn, high order assembly, allosteric T®R-transition, and conformational changes in amyloidal fibrillation. More than ten years' efforts on studying insulin disulfide intermediates by NMR have enabled us to decipher the whole picture of insulin folding coupled to disulfide pairing, especially at the initial stage that forms the nascent peptide. Two structural switches are also known to regulate insulin binding to receptors and progress has been made to identify the residues involved in binding. However, resolving the complex structure of insulin and its receptor remains a challenge in insulin research. Nevertheless, the accumulated knowledge of insulin structure has allowed us to specifically design a new ultra-stable and active single-chain insulin analog (SCI-57), and provides a novel way to design super-stable, fast-acting and cheaper insulin formulations for DM patients. Continuing this long journey of insulin study will benefit basic research in proteins and in pharmaceutical therapy.

Conformational dynamics of insulin

We have exploited a prandial insulin analog to elucidate the underlying structure and dynamics of insulin as a monomer in solution. A model was provided by insulin lispro (the active component of Humalog(®); Eli Lilly and Co.). Whereas NMR-based modeling recapitulated structural relationships of insulin crystals (T-state protomers), dynamic anomalies were revealed by amide-proton exchange kinetics in D(2)O. Surprisingly, the majority of hydrogen bonds observed in crystal structures are only transiently maintained in solution, including key T-state-specific inter-chain contacts. Long-lived hydrogen bonds (as defined by global exchange kinetics) exist only at a subset of four α-helical sites (two per chain) flanking an internal disulfide bridge (cystine A20-B19); these sites map within the proposed folding nucleus of proinsulin. The anomalous flexibility of insulin otherwise spans its active surface and may facilitate receptor binding. Because conformational fluctuations promote the degradation of pharmaceutical formulations, we envisage that "dynamic re-engineering" of insulin may enable design of ultra-stable formulations for humanitarian use in the developing world.

Deciphering the hidden informational content of protein sequences: foldability of proinsulin hinges on a flexible arm that is dispensable in the mature hormone

Protein sequences encode both structure and foldability. Whereas the interrelationship of sequence and structure has been extensively investigated, the origins of folding efficiency are enigmatic. We demonstrate that the folding of proinsulin requires a flexible N-terminal hydrophobic residue that is dispensable for the structure, activity, and stability of the mature hormone. This residue (Phe(B1) in placental mammals) is variably positioned within crystal structures and exhibits (1)H NMR motional narrowing in solution. Despite such flexibility, its deletion impaired insulin chain combination and led in cell culture to formation of non-native disulfide isomers with impaired secretion of the variant proinsulin. Cellular folding and secretion were maintained by hydrophobic substitutions at B1 but markedly perturbed by polar or charged side chains. We propose that, during folding, a hydrophobic side chain at B1 anchors transient long-range interactions by a flexible N-terminal arm (residues B1-B8) to mediate kinetic or thermodynamic partitioning among disulfide intermediates. Evidence for the overall contribution of the arm to folding was obtained by alanine scanning mutagenesis. Together, our findings demonstrate that efficient folding of proinsulin requires N-terminal sequences that are dispensable in the native state. Such arm-dependent folding can be abrogated by mutations associated with β-cell dysfunction and neonatal diabetes mellitus.

Crystal structure of a "nonfoldable" insulin: impaired folding efficiency despite native activity

Protein evolution is constrained by folding efficiency ("foldability") and the implicit threat of toxic misfolding. A model is provided by proinsulin, whose misfolding is associated with beta-cell dysfunction and diabetes mellitus. An insulin analogue containing a subtle core substitution (Leu(A16) --> Val) is biologically active, and its crystal structure recapitulates that of the wild-type protein. As a seeming paradox, however, Val(A16) blocks both insulin chain combination and the in vitro refolding of proinsulin. Disulfide pairing in mammalian cell culture is likewise inefficient, leading to misfolding, endoplasmic reticular stress, and proteosome-mediated degradation. Val(A16) destabilizes the native state and so presumably perturbs a partial fold that directs initial disulfide pairing. Substitutions elsewhere in the core similarly destabilize the native state but, unlike Val(A16), preserve folding efficiency. We propose that Leu(A16) stabilizes nonlocal interactions between nascent alpha-helices in the A- and B-domains to facilitate initial pairing of Cys(A20) and Cys(B19), thus surmounting their wide separation in sequence. Although Val(A16) is likely to destabilize this proto-core, its structural effects are mitigated once folding is achieved. Classical studies of insulin chain combination in vitro have illuminated the impact of off-pathway reactions on the efficiency of native disulfide pairing. The capability of a polypeptide sequence to fold within the endoplasmic reticulum may likewise be influenced by kinetic or thermodynamic partitioning among on- and off-pathway disulfide intermediates. The properties of [Val(A16)]insulin and [Val(A16)]proinsulin demonstrate that essential contributions of conserved residues to folding may be inapparent once the native state is achieved.

Enhancing the activity of a protein by stereospecific unfolding: conformational life cycle of insulin and its evolutionary origins

A central tenet of molecular biology holds that the function of a protein is mediated by its structure. An inactive ground-state conformation may nonetheless be enjoined by the interplay of competing biological constraints. A model is provided by insulin, well characterized at atomic resolution by x-ray crystallography. Here, we demonstrate that the activity of the hormone is enhanced by stereospecific unfolding of a conserved structural element. A bifunctional beta-strand mediates both self-assembly (within beta-cell storage vesicles) and receptor binding (in the bloodstream). This strand is anchored by an invariant side chain (Phe(B24)); its substitution by Ala leads to an unstable but native-like analog of low activity. Substitution by d-Ala is equally destabilizing, and yet the protein diastereomer exhibits enhanced activity with segmental unfolding of the beta-strand. Corresponding photoactivable derivatives (containing l- or d-para-azido-Phe) cross-link to the insulin receptor with higher d-specific efficiency. Aberrant exposure of hydrophobic surfaces in the analogs is associated with accelerated fibrillation, a form of aggregation-coupled misfolding associated with cellular toxicity. Conservation of Phe(B24), enforced by its dual role in native self-assembly and induced fit, thus highlights the implicit role of misfolding as an evolutionary constraint. Whereas classical crystal structures of insulin depict its storage form, signaling requires engagement of a detachable arm at an extended receptor interface. Because this active conformation resembles an amyloidogenic intermediate, we envisage that induced fit and self-assembly represent complementary molecular adaptations to potential proteotoxicity. The cryptic threat of misfolding poses a universal constraint in the evolution of polypeptide sequences.

In vitro insulin refolding: Characterization of the intermediates and the putative folding pathway

The in vitro refolding process of the double-chain insulin was studied based on the investigation of in vitro single-chain insulin refolding. Six major folding intermediates, named P1A, P2B, P3A, P4B, P5B, and P6B, were captured during the folding process. The refolding experiments indicate that all of these intermediates are on-pathway. Based on these intermediates and the formation of hypothetic transients, we propose a two-stage folding pathway of insulin. (1) At the early stage of the folding process, the reduced A chain and B chain individually formed the intermediates: two A chain intermediates (P1A and P3A), and four B chain intermediates (P2B, P4B, P5B, and P6B). (2) In the subsequent folding process, transient I was formed from P3A through thiol/disulfide exchange reaction; then, transients II and III, each containing two native disulfides, were formed through the recognition and interaction of transient I with P4B or P6B and the thiol group's oxidation reaction mainly using GSSG as oxidative reagent; finally, transients II and III, through thiol/mixture disulfide exchange reaction, formed the third native disulfide of insulin to complete the folding.

The folding nucleus of the insulin superfamily: a flexible peptide model foreshadows the native state

Oxidative folding of insulin-like growth factor I (IGF-I) and single-chain insulin analogs proceeds via one- and two-disulfide intermediates. A predominant one-disulfide intermediate in each case contains the canonical A20-B19 disulfide bridge (cystines 18-61 in IGF-I and 19-85 in human proinsulin). Here, we describe a disulfide-linked peptide model of this on-pathway intermediate. One peptide fragment (19 amino acids) spans IGF-I residues 7-25 (canonical positions B8-B26 in the insulin superfamily); the other (18 amino acids) spans IGF-I residues 53-70 (positions A12-A21 and D1-D8). Containing only half of the IGF-I sequence, the disulfide-linked polypeptide (designated IGF-p) is not well ordered. Nascent helical elements corresponding to native alpha-helices are nonetheless observed at 4 degrees C. Furthermore, (13)C-edited nuclear Overhauser effects establish transient formation of a native-like partial core; no non-native nuclear Overhauser effects are observed. Together, these observations suggest that early events in the folding of insulin-related polypeptides are nucleated by a native-like molten subdomain containing Cys(A20) and Cys(B19). We propose that nascent interactions within this subdomain orient the A20 and B19 thiolates for disulfide bond formation and stabilize the one-disulfide intermediate once formed. Substitutions in the corresponding region of insulin are associated with inefficient chain combination and impaired biosynthetic expression. The intrinsic conformational propensities of a flexible disulfide-linked peptide thus define a folding nucleus, foreshadowing the structure of the native state.

A conserved histidine in insulin is required for the foldability of human proinsulin: structure and function of an ALAB5 analog

The insulins of eutherian mammals contain histidines at positions B5 and B10. The role of His(B10) is well defined: although not required in the mature hormone for receptor binding, in the islet beta cell this side chain functions in targeting proinsulin to glucose-regulated secretory granules and provides axial zincbinding sites in storage hexamers. In contrast, the role of His(B5) is less well understood. Here, we demonstrate that its substitution with Ala markedly impairs insulin chain combination in vitro and blocks the folding and secretion of human proinsulin in a transfected mammalian cell line. The structure and stability of an Ala(B5)-insulin analog were investigated in an engineered monomer (DKP-insulin). Despite its impaired foldability, the structure of the Ala(B5) analog retains a native-like T-state conformation. At the site of substitution, interchain nuclear Overhauser effects are observed between the methyl resonance of Ala(B5) and side chains in the A chain; these nuclear Overhauser effects resemble those characteristic of His(B5) in native insulin. Substantial receptor binding activity is retained (80 +/- 10% relative to the parent monomer). Although the thermodynamic stability of the Ala(B5) analog is decreased (DeltaDeltaG(u) = 1.7 +/- 0.1 kcal/mol), consistent with loss of His(B5)-related interchain packing and hydrogen bonds, control studies suggest that this decrement cannot account for its impaired foldability. We propose that nascent long-range interactions by His(B5) facilitate alignment of Cys(A7) and Cys(B7) in protein-folding intermediates; its conservation thus reflects mechanisms of oxidative folding rather than structure-function relationships in the native state.

Oral administration of a cholera toxin B subunit–insulin fusion protein produced in silkworm protects against autoimmune diabetes

The oral administration of disease-specific autoantigens can induce oral immune tolerance and prevent or delay the onset of autoimmune disease symptoms. Here, we describe the construction of an edible vaccine consisting of a fusion protein composed of cholera toxin B subunit (CTB) and insulin that is produced in silkworm larvae at levels of up to 0.3 mg/ml of hemolymph. The silkworm bioreactor produced this fusion protein vaccine as the pentameric CTB-insulin form, which retained the GM1-ganglioside binding affinity and the native antigenicity of CTB and insulin. Non-obese diabetic mice fed hemolymph containing microgram quantities of the CTB-insulin fusion protein showed a prominent reduction in pancreatic islet inflammation and a delay in the development of symptoms of clinical diabetes. These results demonstrate that the silkworm bioreactor is a feasible production and delivery system for an oral protein vaccine designed to develop immunological tolerance against T-cell-mediated autoimmune diabetes by regulatory T-cell induction.

Unfolding of human proinsulin. Intermediates and possible role of its C-peptide in folding/unfolding

We have investigated the in vitro refolding process of human proinsulin (HPI) and an artificial mini-C derivative of HPI (porcine insulin precursor, PIP), and found that they have significantly different disulfide-formation pathways. HPI and PIP differ in their amino acid sequences due to the presence of the C-peptide linker found in HPI, therefore suggesting that the C-peptide linker may be responsible for the observed difference in folding behaviour. However, the manner in which the C-peptide contributes to this difference is still unknown. We have used both the disulfide scrambling method and a redox-equilibrium assay to assess the stability of the disulfide bridges. The results show that disulfide reshuffling is easier to induce in HPI than in PIP by the addition of thiol reagent. Thus, the C-peptide may affect the unique folding pathway of HPI by allowing the disulfide bonds of HPI to be easily accessible. The detailed processes of HPI unfolding by reduction of its disulfide bonds and by disulfide scrambling methods were also investigated. In the reductive unfolding process no accumulation of intermediates was detected. In the process of unfolding by disulfide scrambling, HPI gradually rearranged its disulfide bonds to form three major isomers G1, G2 and G3. The most abundant isomer, G1, contains the B7-B19 disulfide bridge. Based on far-UV CD spectra, native gel analysis and cleavage by endoproteinase V8, the G1 isomer has been shown to resemble the intermediate P4 found in the refolding process of HPI. Finally, the major isomer G1 is allowed to refold to native protein HPI by disulfide rearrangement, which indicates that a similar molecular mechanism may exist for the unfolding and refolding process of HPI.

In vitro refolding of human proinsulin. Kinetic intermediates, putative disulfide-forming pathway folding initiation site, and potential role of C-peptide in folding process

Human insulin is a double-chain peptide that is synthesized in vivo as a single-chain human proinsulin (HPI). We have investigated the disulfide-forming pathway of a single-chain porcine insulin precursor (PIP). Here we further studied the folding pathway of HPI in vitro. While the oxidized refolding process of HPI was quenched, four obvious intermediates (namely P1, P2, P3, and P4, respectively) with three disulfide bridges were isolated and characterized. Contrary to the folding pathway of PIP, no intermediates with one- or two-disulfide bonds could be captured under different refolding conditions. CD analysis showed that P1, P2, and P3 retained partially structural conformations, whereas P4 contained little secondary structure. Based on the time-dependent distribution, disulfide pair analysis, and disulfide-reshuffling process of the intermediates, we have proposed that the folding pathway of HPI is significantly different from that of PIP. These differences reveal that the C-peptide not only facilitates the folding of HPI but also governs its kinetic folding pathway of HPI. Detailed analysis of the molecular folding process reveals that there are some similar folding mechanisms between PIP and HPI. These similarities imply that the initiation site for the folding of PIP/HPI may reside in the central alpha-helix of the B-chain. The formation of disulfide A20-B19 may guide the transfer of the folding information from the B-chain template to the unstructured A-chain. Furthermore, the implications of this in vitro refolding study on the in vivo folding process of HPI have been discussed.

Role of the connecting peptide in insulin biosynthesis

In single-chain insulins (SCIs), the C terminus of the insulin B-chain is contiguous with the N terminus of the A-chain, connected by a short bioengineered linker sequence. SCIs have been proposed to offer potential benefit for gene therapy of diabetes (Lee, H. C., Kim, S. J., Kim, K. S., Shin, H. C., and Yoon, J. W. (2000) Nature 408, 483-488) yet relatively little is known about their folding, intracellular transport, or secretion from mammalian cells. Because SCIs can be considered as mutant proinsulin (with selective shortening of the 35-amino acid connecting peptide that normally includes two sets of flanking dibasic residues), they offer insights into understanding the role of the connecting peptide in insulin biosynthesis. Herein we have explored the relationship of the linker sequence to SCI biosynthesis, folding, and intracellular transport in transiently transfected HEK293 or Chinese hamster ovary cells or in stably transfected AtT20 cells. Despite previous reports that direct linkage of B- and A-chains produces a structure isomorphous with authentic two-chain insulin, we find that constructs with short linkers tend to be synthesized at lower levels, with a significant fraction of molecules exhibiting improper disulfide bonding. Nevertheless, disulfide-mispaired isoforms from a number of different SCI constructs are secreted. While this suggests that a novel folded state goes unrecognized by secretory pathway quality control, we find that misfolded SCIs are detected at higher levels in Chinese hamster ovary cells with artificially activated unfolded protein response mediated by inducible overexpression of active ATF-6. Such a maneuver allows analysis of more seriously misfolded mutants with further foreshortening of the linker sequence or loss (by mutation) of the insulin interchain disulfide bonds.

Renaturation of human proinsulin--a study on refolding and conversion to insulin

The production of human proinsulin in Escherichia coli usually leads to the formation of inclusion bodies. As a consequence, the recombinant protein must be isolated, refolded under suitable redox conditions, and enzymatically converted to the biologically active insulin. In this study we describe a detailed in vitro renaturation protocol for human proinsulin that includes native structure formation and the enzymatic conversion to mature insulin. We used a His(8)-Arg-proinsulin that was renatured from the completely reduced and denatured state in the presence of a cysteine/cystine redox couple. The refolding process was completed after 10-30 min and was shown to be strongly dependent on the redox potential and the pH value, but not on the temperature. Refolding yields of 60-70% could be obtained even at high concentrations of denaturant (3M guanidinium-HCl or 4M urea) and protein concentrations of 0.5mg/ml. By stepwise renaturation a concentration of about 6 mg/ml of native proinsulin was achieved. The refolded proinsulin was correctly disulfide-bonded and native and monomeric as shown by RP-HPLC, ELISA, circular dichroism, and analytical gel filtration. Treatment of the renatured proinsulin with trypsin and carboxypeptidase B yielded mature insulin.

Mechanism of insulin chain combination. Asymmetric roles of A-chain alpha-helices in disulfide pairing

The A and B chains of insulin combine to form native disulfide bridges without detectable isomers. The fidelity of chain combination thus recapitulates the folding of proinsulin, a precursor protein in which the two chains are tethered by a disordered connecting peptide. We have recently shown that chain combination is blocked by seemingly conservative substitutions in the C-terminal alpha-helix of the A chain. Such analogs, once formed, nevertheless retain high biological activity. By contrast, we demonstrate here that chain combination is robust to non-conservative substitutions in the N-terminal alpha-helix. Introduction of multiple glycine substitutions into the N-terminal segment of the A chain (residues A1-A5) yields analogs that are less stable than native insulin and essentially without biological activity. (1)H NMR studies of a representative analog lacking invariant side chains Ile(A2) and Val(A3) (A chain sequence GGGEQCCTSICSLYQLENYCN; substitutions are italicized and cysteines are underlined) demonstrate local unfolding of the A1-A5 segment in an otherwise native-like structure. That this and related partial folds retain efficient disulfide pairing suggests that the native N-terminal alpha-helix does not participate in the transition state of the reaction. Implications for the hierarchical folding mechanisms of proinsulin and insulin-like growth factors are discussed.

Intra-A chain disulphide bond forms first during insulin precursor folding

In this study, we investigated the folding pathway of insulin precursor and compared it with that of insulin-like growth factor I (IGF-I). The intra-A chain disulphide bond was found to form early in insulin precursor folding, whereas the corresponding disulphide bond in IGF-I formed late. Intra-A chain disulphide-bond deleted [A6, A11-Ser] proteins, including proinsulin, insulin, and A chain, were employed for this investigation. Under the same conditions the recombination yield of insulin from S-sulphonates of native A and B chains was 22%, while the yield of [A6, A11-Ser] insulin from S-sulphonates of [A6, A11-Ser] A chain and native B chains was only approx. 7%. This indicated that the intra-A chain disulphide bond may serve to stabilize the A chain folding intermediate so as to facilitate the correct recognition and pairing with the B chain. Time courses of oxidation of reduced insulin A chains, reduced A and B chains, and reduced proinsulins showed that the intra-A chain disulphide bond formed first during insulin precursor folding. The formation of intra-A chain disulphide bond further accelerated the formation of the other two inter-chain disulphide bonds. The time course of helix structure formation of insulin A chains also indicated that the intra-A chain disulphide bond formed first, and could stabilize partially folded A chain helix structure. The rate of intra-A chain disulphide bond formation was almost the same as that for both helix structure formation and insulin molecule formation, indicating that the formation of the intra-A chain disulphide bond was the rate limiting step for the folding of insulin precursor.

Characteristic, activity and conformational studies of [A6-Ser, A11-Ser]-insulin

Met-[A6-Ser, A11-Ser]-human proinsulin (Mut-HPI) was prepared in the same way as described previously for Met-human proinsulin (Met-HPI). After trypsin and carboxypeptidase B cleavage and DEAE-Sephadex A25 separation, Met-[A6-Ser, A11-Ser]-human insulin (Mut-HI) and Met-human insulin (Met-HI) were obtained. Their amino-acid compositions are in a good agreement with those expected. Mut-HI keeps full immuno activity, but loses almost all of its receptor binding activity with Met-HI as the control. Some physico-chemical behaviors of Mut-HI have changed to a certain extent. CD studies of mutant insulin demonstrates that the conformation of Met-HI is very similar to that of HI, while that of Mut-HI has altered slightly. The most important observation is that the binding ability of Mut-HI to the receptor has decreased abruptly. These results suggest that though the intra-A chain disulfide bond is deleted, the other two inter-chain disulfide bonds are still correctly paired, and that the intra-A chain disulfide bond is essential for insulin displaying its biological activity.

Structure of a protein in a kinetic trap

We have determined the structure of a metastable disulphide isomer of human insulin. Although not observed for proinsulin folding or insulin-chain recombination, the isomer retains ordered secondary structure and a compact hydrophobic core. Comparison with native insulin reveals a global rearrangement in the orientation of A- and B-chains. One face of the protein's surface is nevertheless in common between native and non-native structures. This face contains receptor-binding determinants, rationalizing the partial biological activity of the isomer. Structures of native and non-native disulphide isomers also define alternative three-dimensional templates. Threading of insulin-like sequences provide an experimental realization of the inverse protein-folding problem.

Comparison of secondary structures of insulin and proinsulin by FTIR

Although the structure of insulin is known in great detail, that of proinsulin has been little investigated, except for a few CD and NMR studies. The secondary structures of human proinsulin are now compared with those of insulin by Fourier Transformed Infrared (FTIR) studies. The deconvolved and second derivative spectra of proinsulin and insulin in the amide I' band region are closely similar with peaks corresponding to alpha-helix, irregular helix, and 3(10) helix at nearly identical positions. For both proteins, the relative contents of the above structures as calculated from the peak areas are in good agreement with the values obtained from the known structure of crystalline porcine insulin. However, compared with insulin, proinsulin has markedly more unordered structures as indicated by the area under the peak at 1643.4 cm-1. In addition, both peak positions and relative areas for turn structure of the prohormone are different from those for insulin. It appears from the above that the A-and B-chain segments of proinsulin and insulin are similar in their secondary structures, especially in helices. The C-chain segment is largely unordered except in a few beta-turns.

Refolding and assembly of penicillin acylase, an enzyme composed of two polypeptide chains that result from proteolytic activation

The in vitro folding and assembly of penicillin acylase (EC 3.5.1.11) (PA) to active enzyme has been studied. PA is a large bacterial protein (Mr = 86,000) comprising two peptides, alpha and beta, produced by proteolytic processing and activation of a 92-kDa precursor. Proteins that result from proteolytic processing are characteristically difficult if not impossible to refold. Different factors that affect folding and assembly of PA, including pH, ionic strength, and temperature, have been studied. Yields of 60% can be obtained, based on recovery of enzyme activity, together with another 20% of folded and associated monomer with conformation closely similar to that of the active enzyme but with the active site not formed. Evidence is presented for in vitro assembly proceeding via initial folding of the N-terminal alpha-peptide with subsequent collapse of the transiently folded beta-chain on to the surface of the former. A slow process of rearrangement follows association in vitro. Competition experiments support the proposal that the linker endopeptide in the precursor serves to increase the probability of productive collision between folded alpha- and beta-peptides. The effect of raised temperature is to interfere with the folding of the alpha-peptide, thus preventing proper folding of the precursor. This finding accounts for the basis of the temperature regulation of PA production in vivo.

The insulin A and B chains contain sufficient structural information to form the native molecule

Refolding studies show that native insulin can be reformed from A and B chains only. This suggests that the A and B chains contain the necessary structural information and that the C peptide is not required for this process.

Oxidative folding of cystine-rich peptides vs regioselective cysteine pairing strategies

The methodology of regioselective cysteine pairings in synthetic multiple-cystine peptides has progressed in the past years to an efficiency that allows for at least three specific inter- and intrachain disulfide bridgings. Conformational studies on various multiple-cystine peptides like hormones, protease inhibitors, and toxins revealed that these bioactive peptides, generated by posttranslational processing of precursor proteins, are folded into miniprotein-like compact globular structures of remarkable stability. This strongly suggests protein domain or subdomain properties of these families of peptides, and thus sufficient sequence-encoded information for correct oxidative refolding under appropriate experimental conditions. From intensive research on the mechanisms and pathways of oxidative refolding of proteins in vivo and in vitro, the efficient methods have emerged for simulating nature in the regeneration of native folds not only for intact proteins, but also for protein domains and subdomains. In fact, the results obtained in the oxidative folding of excised protein fragments and of relatively low mass products of posttranslational processings show that this procedure is indeed a simple way of preparing peptides with several disulfide bonds, if optimization of reaction conditions is performed in terms of redox buffer, temperature, and additives capable of disrupting aggregates and of stabilizing nascent secondary structures. Moreover, with increased knowledge about stable, small natural cystine frameworks, their use instead of artificial templates should facilitate engineering of synthetic miniproteins with specific conformation and tailored functions.