A systematic mutational analysis identifies a 5-residue proline tag that enhances the in vivo immunogenicity of a non-immunogenic model protein.

Poor immunogenicity of small proteins is a major hurdle in developing vaccines or producing antibodies for biopharmaceutical usage. Here, we systematically analyzed the effects of 10 solubility controlling peptide tags (SCP-tags) on the immunogenicity of a non-immunogenic model protein, bovine pancreatic trypsin inhibitor (BPTI-19A; 6 kDa). CD, fluorescence, DLS, SLS, and AUC measurements indicated that the SCP-tags did not change the secondary structure content nor the tertiary structures of the protein nor its monomeric state. ELISA results indicated that the 5-proline (C5P) and 5-arginine (C5R) tags unexpectedly increased the IgG level of BPTI-19A by 240- and 73-fold, respectively, suggesting that non-oligomerizing SCP-tags may provide a novel method for increasing the immunogenicity of a protein in a highly specific manner.

Generating quantitative binding landscapes through fractional binding selections combined with deep sequencing and data normalization.

Quantifying the effects of various mutations on binding free energy is crucial for understanding the evolution of protein-protein interactions and would greatly facilitate protein engineering studies. Yet, measuring changes in binding free energy (ΔΔGbind) remains a tedious task that requires expression of each mutant, its purification, and affinity measurements. We developed an attractive approach that allows us to quantify ΔΔGbind for thousands of protein mutants in one experiment. Our protocol combines protein randomization, Yeast Surface Display technology, deep sequencing, and a few experimental ΔΔGbind data points on purified proteins to generate ΔΔGbind values for the remaining numerous mutants of the same protein complex. Using this methodology, we comprehensively map the single-mutant binding landscape of one of the highest-affinity interaction between BPTI and Bovine Trypsin (BT). We show that ΔΔGbind for this interaction could be quantified with high accuracy over the range of 12 kcal mol-1 displayed by various BPTI single mutants.

Disulfide engineering of human Kunitz-type serine protease inhibitors enhances proteolytic stability and target affinity toward mesotrypsin.

Serine protease inhibitors of the Kunitz-bovine pancreatic trypsin inhibitor (BPTI) family are ubiquitous biological regulators of proteolysis. These small proteins are resistant to proteolysis, but can be slowly cleaved within the protease-binding loop by target proteases, thereby compromising their activity. For the human protease mesotrypsin, this cleavage is especially rapid. Here, we aimed to stabilize the Kunitz domain structure against proteolysis through disulfide engineering. Substitution within the Kunitz inhibitor domain of the amyloid precursor protein (APPI) that incorporated a new disulfide bond between residues 17 and 34 reduced proteolysis by mesotrypsin 74-fold. Similar disulfide engineering of tissue factor pathway inhibitor-1 Kunitz domain 1 (KD1TFPI1) and bikunin Kunitz domain 2 (KD2bikunin) likewise stabilized these inhibitors against mesotrypsin proteolysis 17- and 6.6-fold, respectively. Crystal structures of disulfide-engineered APPI and KD1TFPI1 variants in a complex with mesotrypsin at 1.5 and 2.0 Å resolution, respectively, confirmed the formation of well-ordered disulfide bonds positioned to stabilize the binding loop. Long all-atom molecular dynamics simulations of disulfide-engineered Kunitz domains and their complexes with mesotrypsin revealed conformational stabilization of the primed side of the inhibitor-binding loop by the engineered disulfide, along with global suppression of conformational dynamics in the Kunitz domain. Our findings suggest that the Cys-17-Cys-34 disulfide slows proteolysis by dampening conformational fluctuations in the binding loop and minimizing motion at the enzyme-inhibitor interface. The generalizable approach developed here for the stabilization against proteolysis of Kunitz domains, which can serve as important scaffolds for therapeutics, may thus find applications in drug development.

A hetero-micro-seeding strategy for readily crystallizing closely related protein variants.

Protein crystallization remains difficult to rationalize and screening for optimal crystallization conditions is a tedious and time consuming procedure. Here, we report a hetero-micro-seeding strategy for producing high resolution crystals of closely related protein variants, where micro crystals from a readily crystallized variant are used as seeds to develop crystals of other variants less amenable to crystallization. We applied this strategy to Bovine Pancreatic Trypsin Inhibitor (BPTI) variants, which would not crystallize using standard crystallization practice. Out of six variants in our analysis, only one called BPTI-[5,55]A14G formed well behaving crystals; and the remaining five (A14GA38G, A14GA38V, A14GA38L, A14GA38I, and A14GA38K) could be crystallized only using micro-seeds from the BPTI-[5,55]A14G crystal. All hetero-seeded crystals diffracted at high resolution with minimum mosaicity, retaining the same space group and cell dimension. Moreover, hetero-micro-seeding did not introduce any biases into the mutant's structure toward the seed structure, as demonstrated by A14GA38I structures solved using micro-seeds from A14GA38G, A14GA38L and A14GA38I. Though hetero-micro-seeding is a simple and almost naïve strategy, this is the first direct demonstration of its workability. We believe that hetero-micro-seeding, which is contrasting with the popular idea that crystallization requires highly purified proteins, could contribute a new tool for rapidly solving protein structures in mutational analysis studies.

Crystal structures of highly simplified BPTIs provide insights into hydration-driven increase of unfolding enthalpy.

We report a thermodynamic and structural analysis of six extensively simplified bovine pancreatic trypsin inhibitor (BPTI) variants containing 19-24 alanines out of 58 residues. Differential scanning calorimetry indicated a two-state thermal unfolding, typical of a native protein with densely packed interior. Surprisingly, increasing the number of alanines induced enthalpy stabilization, which was however over-compensated by entropy destabilization. X-ray crystallography indicated that the alanine substitutions caused the recruitment of novel water molecules facilitating the formation of protein-water hydrogen bonds and improving the hydration shells around the alanine's methyl groups, both of which presumably contributed to enthalpy stabilization. There was a strong correlation between the number of water molecules and the thermodynamic parameters. Overall, our results demonstrate that, in contrast to our initial expectation, a protein sequence in which over 40% of the residues are alanines can retain a densely packed structure and undergo thermal denaturation with a large enthalpy change, mainly contributed by hydration.

Amorphous protein aggregation monitored using fluorescence self-quenching.

Biophysical understanding of amorphous protein aggregation can significantly impact diverse area of biotechnology. Here, we report the time dependent salt-induced formation of amorphous aggregation as monitored by fluorescence self-quenching and compare the results with conventional methods for detecting protein aggregation [static light scattering (LS) and dynamic light scattering (DLS)]. As a model protein, we used a bovine pancreatic trypsin inhibitor (BPTI) variant extended by two glycines (C2G) at its C terminus, and three variants where three types of Solubility Controlling Peptide tags (SCP tags) made of five serines (C5S), alanines (C5A) or aspartic acids (C5D) were added to the C terminus of C2G. All variants have a native-like BPTI structure and trypsin inhibitory activity, but different solubilities controlled by the SCP tags. The BPTIs were labeled using NHS-Fluorescein (FAM) conjugated to BPTI's lysines, and we measured the changes in fluorescence intensity occurring upon the addition of NaCl. The fluorescence of all FAM-BPTIs decreased almost immediately, albeit to a different extent, upon addition of salt and became constant after 10 min for 24 h or more. On the other hand, LS and DLS signal changes were dependent on the type of tags. Namely, C2G's LS and DLS signals changed immediately, the signals of C5S and C5A tagged FAM-BPTIs increased slowly from 10 min to 24 h, and those of C5D remained constant. These observations indicated the presence of at least one intermediate step, with increased protein-protein interaction yielding a 'molecular condensation' phase. According to this model, C2G would rapidly turn from 'condensates' to aggregates, whereas C5S and C5A tagged FAM-BPTIs would do so slowly, and the soluble C5D tagged variant would remain in the molecular condensation state.

Characterization of a Kunitz-type protease inhibitor (MjKuPI) reveals the involvement of MjKuPI positive hemocytes in the immune responses of kuruma shrimp Marsupenaeus japonicus.

Serine proteases and their inhibitors play vital roles in biological processes. Serine protease inhibitors, including Kunitz-type protease inhibitors play important roles not only in physiological process (i.e. blood clotting and fibrinolysis) but also in immune responses. In this study, we characterized a Kunitz-type protease inhibitor, designated MjKuPI, from kuruma shrimp Marsupenaeus japonicus. An expression profile showed that MjKuPI was mainly expressed in hemocytes. Immunostaining revealed that some hemocytes expressed MjKuPI (MjKuPI(+) hemocytes) and others did not (MjKuPI(-) hemocytes). Injection of shrimp with Vibrio penaeicida and white spot syndrome virus (WSSV) upregulated the mRNA level of MjKuPI, and a flow cytometry analysis revealed that the proportion of MjKuPI(+) hemocytes increased significantly 24 h after injection. Together, these results suggest that MjKuPI and MjKuPI(+) hemocytes have a role in the innate immune system of kuruma shrimp.

A novel uPAg-KPI fusion protein inhibits the growth and invasion of human ovarian cancer cells in vitro.

Urokinase-type plasminogen activator (uPA) acts by breaking down the basement membrane and is involved in cell proliferation, migration and invasion. These actions are mediated by binding to the uPA receptor (uPAR) via its growth factor domain (GFD). The present study evaluated the effects of uPAg-KPI, a fusion protein of uPA-GFD and a kunitz protease inhibitor (KPI) domain that is present in the amyloid ß-protein precursor. Using SKOV-3 cells, an ovarian cancer cell line, we examined cell viability, migration, invasion and also protein expression. Furthermore, we examined wound healing, and migration and invasion using a Transwell assay. Our data showed that uPAg-KPI treatment reduced the viability of ovarian cancer SKOV-3 cells in both a concentration and time-dependent manner by arresting tumor cells at G1/G0 phase of the cell cycle. The IC50 of uPAg-KPI was 0.5 µg/µl after 48 h treatment. At this concentration, uPAg-KPI also inhibited tumor cell colony formation, wound closure, as well as cell migration and invasion capacity. At the protein level, western blot analysis demonstrated that uPAg-KPI exerted no significant effect on the expression of total extracellular signal-regulated kinase (ERK)1/ERK2 and AKT, whereas it suppressed levels of phosphorylated ERK1/ERK2 and AKT. Thus, we suggest that this novel uPAg-KPI fusion protein reduced cell viability, colony formation, wound healing and the invasive ability of human ovarian cancer SKOV-3 cells in vitro by regulating ERK and AKT signaling. Further studies using other cell lines will confirm these findings.

Applications of pressure perturbation calorimetry to study factors contributing to the volume changes upon protein unfolding.

BACKGROUND: Pressure perturbation calorimetry (PPC) is a biophysical method that allows direct determination of the volume changes upon conformational transitions in macromolecules. SCOPE OF THIS REVIEW: This review provides novel details of the use of PPC to analyze unfolding transitions in proteins. The emphasis is made on the data analysis as well as on the validation of different structural factors that define the volume changes upon unfolding. Four case studies are presented that show the application of these concepts to various protein systems. MAJOR CONCLUSIONS: The major conclusions are: 1. Knowledge of the thermodynamic parameters for heat induced unfolding facilitates the analysis of the PPC profiles. 2. The changes in the thermal expansion coefficient upon unfolding appear to be temperature dependent.3.Substitutions on the protein surface have negligible effects on the volume changes upon protein unfolding. 4. Structural plasticity of proteins defines the position dependent effect of amino acid substitutions of the residues buried in the native state. 5. Small proteins have positive volume changes upon unfolding which suggests difference in balance between the cavity/void volume in the native state and the hydration volume changes upon unfolding as compared to the large proteins that have negative volume changes. GENERAL SIGNIFICANCE: The information provided here gives a better understanding and deeper insight into the role played by various factors in defining the volume changes upon protein unfolding.

A novel coagulation inhibitor from Schistosoma japonicum.

Little is known about the molecular mechanisms whereby the human blood fluke Schistosoma japonicum is able to survive in the host venous blood system. Protease inhibitors are likely released by the parasite enabling it to avoid attack by host proteolytic enzymes and coagulation factors. Interrogation of the S. japonicum genomic sequence identified a gene, SjKI-1, homologous to that encoding a single domain Kunitz protein (Sjp_0020270) which we expressed in recombinant form in Escherichia coli and purified. SjKI-1 is highly transcribed in adult worms and eggs but its expression was very low in cercariae and schistosomula. In situ immunolocalization with anti-SjKI-1 rabbit antibodies showed the protein was present in eggs trapped in the infected mouse intestinal wall. In functional assays, SjKI-1 inhibited trypsin in the picomolar range and chymotrypsin, neutrophil elastase, FXa and plasma kallikrein in the nanomolar range. Furthermore, SjKI-1, at a concentration of 7·5 µ m, prolonged 2-fold activated partial thromboplastin time of human blood coagulation. We also demonstrate that SjKI-1 has the ability to bind Ca(++). We present, therefore, characterization of the first Kunitz protein from S. japonicum which we show has an anti-coagulant properties. In addition, its inhibition of neutrophil elastase indicates SjKI-1 have an anti-inflammatory role. Having anti-thrombotic properties, SjKI-1 may point the way towards novel treatment for hemostatic disorders.

Expression and activity analysis of a new fusion protein targeting ovarian cancer cells.

The aim of the present study was to develop a new therapeutic drug to improve the prognosis of ovarian cancer patients. Human urokinase-type plasminogen activator (uPA)17-34-kunitz-type protease inhibitor (KPI) eukaryotic expression vector was constructed and recombinant human uPA17-34-KPI (rhuPA17-34-KPI) in P. pastoris was expressed. In the present study, the DNA sequences that encode uPA 17-34 amino acids were created according to the native amino acids sequence and inserted into the KPI-pPICZαC vector, which was constructed. Then, uPA17­34-KPI-pPICZαC was transformed into P. pastoris X-33, and rhuPA17-34-KPI was expressed by induction of methanol. The bioactivities of a recombinant fusion protein were detected with trypsin inhibition analysis, and the inhibitory effects on the growth of ovarian cancer cells were identified using the TUNEL assay, in vitro wound­healing assay and Matrigel model analysis. The results of the DNA sequence analysis of the recombinant vector uPA17-34-KPI­pPICZα demonstrated that the DNA­encoding human uPA 17-34 amino acids, 285-288 amino acids of amyloid precursor protein (APP) and 1-57 amino acids of KPI were correctly inserted into the pPICZαC vector. Following induction by methonal, the fusion protein with a molecular weight of 8.8 kDa was observed using SDS-PAGE and western blot analysis. RhuPA17-34-KPI was expressed in P. pastoris with a yield of 50 mg/l in a 50-ml tube. The recombinant fusion protein was able to inhibit the activity of trypsin, inhibit growth and induce apoptosis of SKOV3 cells, and inhibit the invasion and metastasis of ovarian cancer cells. By considering uPA17-34 amino acid specific binding uPAR as the targeted part of fusion protein and utilizing the serine protease inhibitor activity of KPI, it was found that the recombinant fusion protein uPA17-34-KPI inhibited the invasion and metastasis of ovarian tumors, and may therefore be regarded as effective in targeted treatment.

Sequence and conformational specificity in substrate recognition: several human Kunitz protease inhibitor domains are specific substrates of mesotrypsin.

Mesotrypsin is an isoform of trypsin that is uniquely resistant to polypeptide trypsin inhibitors and can cleave some inhibitors rapidly. Previous studies have shown that the amyloid precursor protein Kunitz protease inhibitor domain (APPI) is a specific substrate of mesotrypsin and that stabilization of the APPI cleavage site in a canonical conformation contributes to recognition by mesotrypsin. We hypothesized that other proteins possessing potential cleavage sites stabilized in a similar conformation might also be mesotrypsin substrates. Here we evaluated a series of candidate substrates, including human Kunitz protease inhibitor domains from amyloid precursor-like protein 2 (APLP2), bikunin, hepatocyte growth factor activator inhibitor type 2 (HAI2), tissue factor pathway inhibitor-1 (TFPI1), and tissue factor pathway inhibitor-2 (TFPI2), as well as E-selectin, an unrelated protein possessing a potential cleavage site displaying canonical conformation. We find that Kunitz domains within APLP2, bikunin, and HAI2 are cleaved by mesotrypsin with kinetic profiles of specific substrates. TFPI1 and TFPI2 Kunitz domains are cleaved less efficiently by mesotrypsin, and E-selectin is not cleaved at the anticipated site. Cocrystal structures of mesotrypsin with HAI2 and bikunin Kunitz domains reveal the mode of mesotrypsin interaction with its canonical substrates. Our data suggest that major determinants of mesotrypsin substrate specificity include sequence preferences at the P1 and P'2 positions along with conformational stabilization of the cleavage site in the canonical conformation. Mesotrypsin up-regulation has been implicated previously in cancer progression, and proteolytic clearance of Kunitz protease inhibitors offers potential mechanisms by which mesotrypsin may mediate pathological effects in cancer.

A pathogenic nematode targets recognition proteins to avoid insect defenses.

Steinernemacarpocapsae is a nematode pathogenic in a wide variety of insect species. The great pathogenicity of this nematode has been ascribed to its ability to overcome the host immune response; however, little is known about the mechanisms involved in this process. The analysis of an expressed sequence tags (EST) library in the nematode during the infective phase was performed and a highly abundant contig homologous to serine protease inhibitors was identified. In this work, we show that this contig is part of a 641-bp cDNA that encodes a BPTI-Kunitz family inhibitor (Sc-KU-4), which is up-regulated in the parasite during invasion and installation. Recombinant Sc-KU-4 protein was produced in Escherichia coli and shown to inhibit chymotrypsin and elastase activities in a dose-dependent manner by a competitive mechanism with Ki values of 1.8 nM and 2.6 nM, respectively. Sc-KU-4 also inhibited trypsin and thrombin activities to a lesser extent. Studies of the mode of action of Sc-KU-4 and its effects on insect defenses suggest that although Sc-KU-4 did not inhibit the activation of hemocytes or the formation of clotting fibers, it did inhibit hemocyte aggregation and the entrapment of foreign particles by fibers. Moreover, Sc-KU-4 avoided encapsulation and the deposition of clotting materials, which usually occurs in response to foreign particles. We show by protein-protein interaction that Sc-KU-4 targets recognition proteins of insect immune system such as masquerade-like and serine protease-like homologs. The interaction of Sc-KU-4 with these proteins explains the ability of the nematode to overcome host reactions and its large pathogenic spectrum, once these immune proteins are well conserved in insects. The discovery of this inhibitor targeting insect recognition proteins opens new avenues for the development of S. carpocapsae as a biological control agent and provides a new tool to study host-pathogen interactions.

A spider-derived Kunitz-type serine protease inhibitor that acts as a plasmin inhibitor and an elastase inhibitor.

Kunitz-type serine protease inhibitors are involved in various physiological processes, such as ion channel blocking, blood coagulation, fibrinolysis, and inflammation. While spider-derived Kunitz-type proteins show activity in trypsin or chymotrypsin inhibition and K(+) channel blocking, no additional role for these proteins has been elucidated. In this study, we identified the first spider (Araneus ventricosus) Kunitz-type serine protease inhibitor (AvKTI) that acts as a plasmin inhibitor and an elastase inhibitor. AvKTI possesses a Kunitz domain consisting of a 57-amino-acid mature peptide that displays features consistent with Kunitz-type inhibitors, including six conserved cysteine residues and a P1 lysine residue. Recombinant AvKTI, expressed in baculovirus-infected insect cells, showed a dual inhibitory activity against trypsin (K(i) 7.34 nM) and chymotrypsin (K(i) 37.75 nM), defining a role for AvKTI as a spider-derived Kunitz-type serine protease inhibitor. Additionally, AvKTI showed no detectable inhibitory effects on factor Xa, thrombin, or tissue plasminogen activator; however, AvKTI inhibited plasmin (K(i) 4.89 nM) and neutrophil elastase (K(i) 169.07 nM), indicating that it acts as an antifibrinolytic factor and an antielastolytic factor. These findings constitute molecular evidence that AvKTI acts as a plasmin inhibitor and an elastase inhibitor and also provide a novel view of the functions of a spider-derived Kunitz-type serine protease inhibitor.

A novel approach for secretion of heterologous proteins with correct n-terminal processing by using α-factor pre sequence in Pichia pastoris.

Numerous proteins have been secreted in P. pastoris by fusing the target gene with α-factor pre-pro sequence at Kex2 endopeptidase cleavage site. However, in some instances the product cannot be correctly processed due to aberrant cleavage by Kex2 endopeptidase such as aprotinin. In this study, an aprotinin gene was cloned into pPIC9K at the signal peptidase cleavage site through a single NheI restriction site designed at the 3'end of the α-factor signal sequence preregion, and transformed into GS115 host cell. By G418 resistance and ELISA assay, a high-yield recombinant was selected. After fed-batch cultivation in a 7-L bioreactor, the product was efficiently secreted into culture medium and accumulated up to ~4.7 mg L⁻¹. MALDI-TOF/MS and N-terminal analyses confirmed its authenticity. Thus, a novel cloning strategy for secretion of aprotinin with correct N-terminal processing in P. pastoris has been developed which can be potentially applied to other proteins.

Tri-domain bifunctional inhibitor of metallocarboxypeptidases A and serine proteases isolated from marine annelid Sabellastarte magnifica.

This study describes a novel bifunctional metallocarboxypeptidase and serine protease inhibitor (SmCI) isolated from the tentacle crown of the annelid Sabellastarte magnifica. SmCI is a 165-residue glycoprotein with a molecular mass of 19.69 kDa (mass spectrometry) and 18 cysteine residues forming nine disulfide bonds. Its cDNA was cloned and sequenced by RT-PCR and nested PCR using degenerated oligonucleotides. Employing this information along with data derived from automatic Edman degradation of peptide fragments, the SmCI sequence was fully characterized, indicating the presence of three bovine pancreatic trypsin inhibitor/Kunitz domains and its high homology with other Kunitz serine protease inhibitors. Enzyme kinetics and structural analyses revealed SmCI to be an inhibitor of human and bovine pancreatic metallocarboxypeptidases of the A-type (but not B-type), with nanomolar K(i) values. SmCI is also capable of inhibiting bovine pancreatic trypsin, chymotrypsin, and porcine pancreatic elastase in varying measures. When the inhibitor and its nonglycosylated form (SmCI N23A mutant) were overproduced recombinantly in a Pichia pastoris system, they displayed the dual inhibitory properties of the natural form. Similarly, two bi-domain forms of the inhibitor (recombinant rSmCI D1-D2 and rSmCI D2-D3) as well as its C-terminal domain (rSmCI-D3) were also overproduced. Of these fragments, only the rSmCI D1-D2 bi-domain retained inhibition of metallocarboxypeptidase A but only partially, indicating that the whole tri-domain structure is required for such capability in full. SmCI is the first proteinaceous inhibitor of metallocarboxypeptidases able to act as well on another mechanistic class of proteases (serine-type) and is the first of this kind identified in nature.

Evolution, expansion and expression of the Kunitz/BPTI gene family associated with long-term blood feeding in Ixodes Scapularis.

BACKGROUND: Recent studies of the tick saliva transcriptome have revealed the profound role of salivary proteins in blood feeding. Kunitz/BPTI proteins are abundant in the salivary glands of ticks and perform multiple functions in blood feeding, such as inhibiting blood coagulation, regulating host blood supply and disrupting host angiogenesis. However, Kunitz/BPTI proteins in soft and hard ticks have different functions and molecular mechanisms. How these differences emerged and whether they are associated with the evolution of long-term blood feeding in hard ticks remain unknown. RESULTS: In this study, the evolution, expansion and expression of Kunitz/BPTI family in Ixodes scapularis were investigated. Single- and multi-domain Kunitz/BPTI proteins have similar gene structures. Single-domain proteins were classified into three groups (groups I, II and III) based on their cysteine patterns. Group I represents the ancestral branch of the Kunitz/BPTI family, and members of this group function as serine protease inhibitors. The group I domain was used as a module to create multi-domain proteins in hard ticks after the split between hard and soft ticks. However, groups II and III, which evolved from group I, are only present and expanded in the genus Ixodes. These lineage-specific expanded genes exhibit significantly higher expression during long-term blood feeding in Ixodes scapularis. Interestingly, functional site analysis suggested that group II proteins lost the ability to inhibit serine proteases and evolved a new function of modulating ion channels. Finally, evolutionary analyses revealed that the expansion and diversification of the Kunitz/BPTI family in the genus Ixodes were driven by positive selection. CONCLUSIONS: These results suggest that the differences in the Kunitz/BPTI family between soft and hard ticks may be linked to the evolution of long-term blood feeding in hard ticks. In Ixodes, the lineage-specific expanded genes (Group II and III) lost the ancient function of inhibiting serine proteases and evolved new functions to adapt to long-term blood feeding. Therefore, these genes may play a profound role in the long-term blood feeding of hard ticks. Based our analysis, we propose that the six genes identified in our study may be candidate target genes for tick control.

Structural features of the KPI domain control APP dimerization, trafficking, and processing.

The two major isoforms of human APP, APP695 and APP751, differ by the presence of a Kunitz-type protease inhibitor (KPI) domain in the extracellular region. APP processing and function is thought to be regulated by homodimerization. We used bimolecular fluorescence complementation (BiFC) to study dimerization of different APP isoforms and mutants. APP751 was found to form significantly more homodimers than APP695. Mutation of dimerization motifs in the TM domain did not affect fluorescence complementation, but native folding of KPI is critical for APP751 homodimerization. APP751 and APP695 dimers were mostly localized at steady state in the Golgi region, suggesting that most of the APP751 and 695 dimers are in the secretory pathway. Mutation of the KPI led to the retention of the APP homodimers in the endoplasmic reticulum. We finally showed that APP751 is more efficiently processed through the nonamyloidogenic pathway than APP695. These findings provide new insight on the particular role of KPI domain in APP dimerization. The correlation observed between dimerization, subcellular localization, and processing suggests that dimerization acts as an efficient regulator of APP trafficking in the secretory compartments that has major consequences on its processing.

Recombinant expression of ShPI-1A, a non-specific BPTI-Kunitz-type inhibitor, and its protection effect on proteolytic degradation of recombinant human miniproinsulin expressed in Pichia pastoris.

Pichia pastoris is a highly successful system for the large-scale expression of heterologous proteins, with the added capability of performing most eukaryotic post-translational modifications. However, this system has one significant disadvantage - frequent proteolytic degradation by P. pastoris proteases of heterologously expressed proteins. Several methods have been proposed to address this problem, but none has proven fully effective. We tested the effectiveness of a broad specificity protease inhibitor to control proteolysis. A recombinant variant of the BPTI-Kunitz protease inhibitor ShPI-1 isolated from the sea anemone Stichodactyla helianthus, was expressed in P. pastoris. The recombinant inhibitor (rShPI-1A), containing four additional amino acids (EAEA) at the N-terminus, was folded similarly to the natural inhibitor, as assessed by circular dichroism. rShPI-1A had broad protease specificity, inhibiting serine, aspartic, and cysteine proteases similarly to the natural inhibitor. rShPI-1A protected a model protein, recombinant human miniproinsulin (rhMPI), from proteolytic degradation during expression in P. pastoris. The addition of purified rShPI-1A at the beginning of the induction phase significantly protected rhMPI from proteolysis in culture broth. The results suggest that a broad specificity protease inhibitor such as rShPI-1A can be used to improve the yield of recombinant proteins secreted from P. pastoris.

Three genes expressing Kunitz domains in the epididymis are related to genes of WFDC-type protease inhibitors and semen coagulum proteins in spite of lacking similarity between their protein products.

BACKGROUND: We have previously identified a locus on human chromosome 20q13.1, encompassing related genes of postulated WFDC-type protease inhibitors and semen coagulum proteins. Three of the genes with WFDC motif also coded for the Kunitz-type protease inhibitor motif. In this report, we have reinvestigated the locus for homologous genes encoding Kunitz motif only. The identified genes have been analyzed with respect to structure, expression and function. RESULTS: We identified three novel genes; SPINT3, SPINT4 and SPINT5, and the structure of their transcripts were determined by sequencing of DNA generated by rapid amplification of cDNA ends. Each gene encodes a Kunitz domain preceded by a typical signal peptide sequence, which indicates that the proteins of 7.6, 8.7, and 9.7 kDa are secreted. Analysis of transcripts in 26 tissues showed that the genes predominantly are expressed in the epididymis. The recombinantly produced proteins could not inhibit the amidolytic activity of trypsin, chymotrypsin, plasmin, thrombin, coagulation factor Xa, elastase, urokinase and prostate specific antigen, whereas similarly made bovine pancreatic trypsin inhibitor (BPTI) had the same bioactivity as the protein isolated from bovine pancreas. CONCLUSIONS: The similar organization, chromosomal location and site of expression, suggests that the novel genes are homologous with the genes of WFDC-type protease inhibitors and semen coagulum proteins, despite the lack of similarity in primary structure of their protein products. Their restricted expression to the epididymis suggests that they could be important for male reproduction. The recombinantly produced proteins are presumably bioactive, as demonstrated with similarly made BPTI, but may have a narrower spectrum of inhibition, as indicated by the lacking activity against eight proteases with differing specificity. Another possibility is that they have lost the protease inhibiting properties, which is typical of Kunitz domains, in favor of hitherto unknown functions.