The native metastable fold of C1-inhibitor is stabilized by disulfide bonds

Altered native stability is the dominant basis for susceptibility of α1-antitrypsin mutants to polymerization

Serpins are protease inhibitors whose most stable state is achieved upon transition of a central 5-stranded β-sheet to a 6-stranded form. Mutations, low pH, denaturants and elevated temperatures promote this transition, which can result in a growing polymer chain of inactive molecules. Different types of polymer are possible, but, experimentally only heat has been shown to generate polymers in vitro consistent with ex vivo pathological specimens. Many mutations that alter the rate of heat-induced polymerization have been described, but interpretation is problematic because discrimination is lacking between the effect of global changes in native stability and specific effects on structural mechanism. We show that the temperature midpoint (Tm) of thermal denaturation reflects the transition of α1-antitrypsin to the polymerization intermediate, and determine the relationship with fixed-temperature polymerization half-times (t0.5) in the presence of stabilizing additives [TMAO (trimethylamine N-oxide), sucrose and sodium sulfate], point mutations and disulfide bonds. Combined with a retrospective analysis of 31 mutants characterized in the literature, the results of the present study show that global changes to native state stability are the predominant basis for the effects of mutations and osmolytes on heat-induced polymerization, summarized by the equation: ln(t0.5,mutant/t0.5,wild-type)=0.34×ΔTm. It is deviations from this relationship that hold key information about the polymerization process.

Arabidopsis AtSerpin1, crystal structure and in vivo interaction with its target protease RESPONSIVE TO DESICCATION-21 (RD21)

In animals, protease inhibitors of the serpin family are associated with many physiological processes, including blood coagulation and innate immunity. Serpins feature a reactive center loop (RCL), which displays a protease target sequence as a bait. RCL cleavage results in an irreversible, covalent serpin-protease complex. AtSerpin1 is an Arabidopsis protease inhibitor that is expressed ubiquitously throughout the plant. The x-ray crystal structure of recombinant AtSerpin1 in its native stressed conformation was determined at 2.2 A. The electrostatic surface potential below the RCL was found to be highly positive, whereas the breach region critical for RCL insertion is an unusually open structure. AtSerpin1 accumulates in plants as a full-length and a cleaved form. Fractionation of seedling extracts by nonreducing SDS-PAGE revealed the presence of an additional slower migrating complex that was absent when leaves were treated with the specific cysteine protease inhibitor L-trans-epoxysuccinyl-L-leucylamido (4-guanidino)butane. Significantly, RESPONSIVE TO DESICCATION-21 (RD21) was the major protease labeled with the L-trans-epoxysuccinyl-L-leucylamido (4-guanidino)butane derivative DCG-04 in wild type extracts but not in extracts of mutant plants constitutively overexpressing AtSerpin1, indicating competition. Fractionation by nonreducing SDS-PAGE followed by immunoblotting with RD21-specific antibody revealed that the protease accumulated both as a free enzyme and in a complex with AtSerpin1. Importantly, both RD21 and AtSerpin1 knock-out mutants lacked the serpin-protease complex. The results establish that the major Arabidopsis plant serpin interacts with RD21. This is the first report of the structure and in vivo interaction of a plant serpin with its target protease.

Protein expression and refolding--a practical guide to getting the most out of inclusion bodies

The release of sequence data, particularly from a number of medically and biotechnologically important genomes, is increasing in an exponential fashion. In light of this, elucidating the structure and function of proteins, particularly in a "high throughput" manner, is an important quest. The production of recombinant proteins however is not always straightforward, with a number of proteins falling prey to low expression problems, a high susceptibility to proteolysis and the often despised production of inclusion bodies. Whilst expression as inclusion bodies can often be advantageous, their solubilization and renaturation is often a time consuming and empirical process. In this review, we aim to outline some of the more common approaches that have been applied to a variety of proteins and address issues associated with their handling.

The functional integrity of the serpin domain of C1-inhibitor depends on the unique N-terminal domain, as revealed by a pathological mutant

C1-inhibitor (C1-Inh) is a serine protease inhibitor (serpin) with a unique, non-conserved N-terminal domain of unknown function. Genetic deficiency of C1-Inh causes hereditary angioedema. A novel type of mutation (Delta 3) in exon 3 of the C1-Inh gene, resulting in deletion of Asp62-Thr116 in this unique domain, was encountered in a hereditary angioedema pedigree. Because the domain is supposedly not essential for inhibitory activity, the unexpected loss-of-function of this deletion mutant was further investigated. The Delta 3 mutant and three additional mutants starting at Pro76, Gly98, and Ser115, lacking increasing parts of the N-terminal domain, were produced recombinantly. C1-Inh76 and C1-Inh98 retained normal conformation and interaction kinetics with target proteases. In contrast, C1-Inh115 and Delta 3, which both lack the connection between the serpin and the non-serpin domain via two disulfide bridges, were completely non-functional because of a complex-like and multimeric conformation, as demonstrated by several criteria. The Delta 3 mutant also circulated in multimeric form in plasma from affected family members. The C1-Inh mutant reported here is unique in that deletion of an entire amino acid stretch from a domain not shared by other serpins leads to a loss-of-function. The deletion in the unique N-terminal domain results in a "multimerization phenotype" of C1-Inh, because of diminished stability of the central beta-sheet. This phenotype, as well as the location of the disulfide bridges between the serpin and the non-serpin domain of C1-Inh, suggests that the function of the N-terminal region may be similar to one of the effects of heparin in antithrombin III, maintenance of the metastable serpin conformation.

A redox-sensitive loop regulates plasminogen activator inhibitor type 2 (PAI-2) polymerization

Plasminogen activator inhibitor type 2 (PAI-2) is the only wild-type serpin that polymerizes spontaneously under physiological conditions. We show that PAI-2 loses its ability to polymerize following reduction of thiol groups, suggesting that an intramolecular disulfide bond is essential for the polymerization. A novel disulfide bond was identified between C79 (in the CD-loop) and C161 (at the bottom of helix F). Substitution mutants in which this disulfide bond was broken did not polymerize. Reactive center loop peptide insertion experiments and binding of bis-ANS to hydrophobic cavities indicate that the C79-C161 disulfide bond stabilizes PAI-2 in a polymerogenic conformation with an open A-beta-sheet. Elimination of this disulfide bond causes A-beta-sheet closure and abrogates the polymerization. The finding that cytosolic PAI-2 is mostly monomeric, whereas PAI-2 in the secretory pathway is prone to polymerize, suggests that the redox status of the cell could regulate PAI-2 polymerization. Taken together, our data suggest that the CD-loop functions as a redox-sensitive switch that converts PAI-2 between an active stable monomeric and a polymerogenic conformation, which is prone to form inactive polymers.

Effect of various forms of the C1 esterase inhibitor (C1-INH) and DAF on complement mediated xenogeneic cell lysis

The purpose of the present study was to assess the effect of various forms of the surface-bound form of the C1 esterase inhibitor (C1-INH-PI) and decay accelerating factor (DAF) on xenogenic cells. cDNAs of various deletion mutants of the C1-INH-PI, such as delta-1-99 amino acid (AA), delta-108-183AA loop, delta-whole loop, delta-exon5, delta-exon6 + 7, and delta-exon5 + 6 + 7, and that of DAF, the delta-short consensus repeat (SCR) 1-DAF were established. While all deletion mutants of C1-INH-PI except the delta-1-99AA were expressed in the cytoplasm but not on the cell surface, the delta-1-99AA was clearly expressed on the xenogeneic cell surface. Amelioration of complement-mediated xenogeneic cell lysis by delta-1-99AA was next tested, and compared with delta-SCR1 DAF. Both molecules blocked human complement-mediated cell lysis by approximately 57 to 90 and 93 to 98%, respectively, in Chinese hamster ovarian tumor (CHO) cells and pig endothelial cells (PECs). The CHO cell transfectants were incubated with 20% normal human serum, and the amounts of C4 and C3 deposition on the cell surface were analysed by flow cytometry. The DAF transfectant showed a large amount of C4-deposition and much less C3-deposition than the controls (approximately 85% suppression), whereas the delta-1-99AA showed approximately a 40% suppression in both C4- and C3-deposition. Consequently, both the delta-1-99AA C1-INH-PI and delta-SCR1 DAF molecules are quite effective in down-regulating the xenogeneic cell lysis, but accomplished this in different manners.

Expression of active human C1 inhibitor serpin domain in Escherichia coli

Human C1 inhibitor is a highly glycosylated serine protease inhibitor of the serpin family. The protein contains two disulfide bonds. In this study, an N-terminally truncated form of recombinant C1 inhibitor was overexpressed in Escherichia coli strains BL21(DE3) and AD494(DE3), the latter enabling the formation of disulfide bonds within the cytoplasm. With both strains, a major fraction of the recombinant protein produced appeared to be insoluble. However, the soluble fraction of lysates from strain AD494(DE3) inhibited the C1s target protease in functional assays. Recombinant C1 inhibitor produced in this strain also displayed the ability to complex with C1s in vitro. In contrast, lysates from strain BL21(DE3) displayed no C1 inhibitor activity. These data support the notion that glycosylation is not important, whereas disulfide bond formation appears to be essential for the production of an active recombinant C1 inhibitor. Thus, bacterial strains that permit the formation of disulfide bonds may represent a reliable system for the production of recombinant C1 inhibitor. However, a major obstacle to large-scale production will be to produce the protein in a soluble form. Attempts to increase the yield of soluble protein by coexpression of the GroEL/ES chaperonins resulted in an increase in solubility.