Do-it-yourself histidine-tagged bovine enterokinase: a handy member of the protein engineer's toolbox

Efficient production of fluorophore-labeled CC chemokines for biophysical studies using recombinant enterokinase and recombinant sortase

Chemokines are important immune system proteins, many of which mediate inflammation due to their function to activate and cause chemotaxis of leukocytes. An important anti-inflammatory strategy is therefore to bind and inhibit chemokines, which leads to the need for biophysical studies of chemokines as they bind various possible partners. Because a successful anti-chemokine drug should bind at low concentrations, techniques such as fluorescence anisotropy that can provide nanomolar signal detection are required. To allow fluorescence experiments to be carried out on chemokines, a method is described for the production of fluorescently labeled chemokines. First, a fusion-tagged chemokine is produced in Escherichia coli, then efficient cleavage of the N-terminal fusion partner is carried out with lab-produced enterokinase, followed by covalent modification with a fluorophore, mediated by the lab-produced sortase enzyme. This overall process reduces the need for expensive commercial enzymatic reagents. Finally, we utilize the product, vMIP-fluor, in binding studies with the chemokine binding protein vCCI, which has great potential as an anti-inflammatory therapeutic, showing a binding constant for vCCI:vMIP-fluor of 0.37 ± 0.006 nM. We also show how a single modified chemokine homolog (vMIP-fluor) can be used in competition assays with other chemokines and we report a Kd for vCCI:CCL17 of 14 μM. This work demonstrates an efficient method of production and fluorescent labeling of chemokines for study across a broad range of concentrations.

Molecular mechanisms controlling the biogenesis of the TGF-β signal Vg1

The TGF-beta signals Vg1 (Dvr1/Gdf3) and Nodal form heterodimers to induce vertebrate mesendoderm. The Vg1 proprotein is a monomer retained in the endoplasmic reticulum (ER) and is processed and secreted upon heterodimerization with Nodal, but the mechanisms underlying Vg1 biogenesis are largely elusive. Here, we clarify the mechanisms underlying Vg1 retention, processing, secretion, and signaling and introduce a Synthetic Processing (SynPro) system that enables the programmed cleavage of ER-resident and extracellular proteins. First, we find that Vg1 can be processed by intra- or extracellular proteases. Second, Vg1 can be processed without Nodal but requires Nodal for secretion and signaling. Third, Vg1-Nodal signaling activity requires Vg1 processing, whereas Nodal can remain unprocessed. Fourth, Vg1 employs exposed cysteines, glycosylated asparagines, and BiP chaperone-binding motifs for monomer retention in the ER. These observations suggest two mechanisms for rapid mesendoderm induction: Chaperone-binding motifs help store Vg1 as an inactive but ready-to-heterodimerize monomer in the ER, and the flexibility of Vg1 processing location allows efficient generation of active heterodimers both intra- and extracellularly. These results establish SynPro as an in vivo processing system and define molecular mechanisms and motifs that facilitate the generation of active TGF-beta heterodimers.

Nuclear Magnetic Resonance-Guided Structural Analysis of Moderate-Affinity Protein Complexes with Intrinsically Disordered Polypeptides

Binding affinity of an individual binding site of an intrinsically disordered protein for its folded partner may be moderate. In such cases, a straightforward determination of the structure of the binding interface is difficult. We offer a hybrid protocol combining NMR chemical shift information, NMR spectral data on amino acid residue sequence substitution effects, residual dipolar coupling, and molecular dynamics simulation that allowed us to determine the structure of a complex between the intrinsically disordered tropomyosin-binding site of leiomodin and a coiled-coil peptide modeling the N-terminal fragment of tropomyosin. The protocol can be used for other moderate-affinity complexes composed of an intrinsically disordered peptide bound to a structured protein partner.

A simple and efficient method for cytoplasmic production of human enterokinase light chain in E. coli

Human enterokinase light chain (hEKL) cDNA sequence was designed with the help of codon optimization towards Escherichia coli codon preference and ribosome binding site design and artificially synthesized with a thioredoxin fusion tag at the N-terminal and a five his-tag peptide at the C-terminal. The synthetic hEKL gene was cloned into the pET-15 expression vector and transferred into the three different expression strains of E. coli BL21(DE3), NiCo21, and SHuffle T7 Express. Different growth and induction conditions were studied using a statistical response surface methodology (RSM). Recombinant hEKL protein was expressed at high levels in soluble form with 0.71 mM IPTG after 4 h of induction at 25 °C. Autocatalytic process cleaved TRX tag with enterokinase recognition site by the impure hEKL and yielded the mature enzyme. The target protein was then purified to homogeneity (> 95%) by affinity chromatography. The activity of hEKL was comparable to the commercial enzyme. From 1 L culture, 80 mg pure active hEKL was obtained with the specific activity of 6.25 × 10² U/mg. Three main parameters that help us to produce the enzyme in the folded and active form are the type of strain, SHuffle T7 strain, TRX and histidine fusion tags, and growth conditions including the increase of OD of induction and IPTG concentration and the decrease of induction temperature.

Construction, Investigation and Application of TEV Protease Variants with Improved Oxidative Stability

Tobacco etch virus protease (TEVp) is a useful tool for removing fusion tags, but wild-type TEVp is less stable under oxidized redox state. In this work, we introduced and combined C19S, C110S and C130S into TEVp variants containing T17S, L56V, N68D, I77V and S135G to improve protein solubility, and S219V to inhibit self-proteolysis. The solubility and cleavage activity of the constructed variants in Escherichia coli strains including BL21(DE3), BL21(DE3)pLys, Rossetta(DE3) and Origami(DE3) under the same induction conditions were analyzed and compared. The desirable soluble amounts, activity, and oxidative stability were identified to be reluctantly favored in the TEVp. Unlike C19S, C110S and C130S hardly impacted on decreasing protein solubility in the BL21(DE3), but they contributed to improved tolerance to the oxidative redox state in vivo and in vitro. After two fusion proteins were cleaved by purified TEVp protein containing double mutations under the oxidized redox state, the refolded disulfide-rich bovine enterokinase catalytic domain or maize peroxidase with enhanced yields were released from the regenerated amorphous cellulose via affinity absorption of the cellulose-binding module as the affinity tag.

Recombinant Production of Bovine Enteropeptidase Light Chain in SHuffle® T7 Express and Optimization of Induction Parameters

Enteropeptidase is a duodenum serine protease that triggers the activation of pancreatic enzymes by remarkably specific cleavages after lysine residues of peptidyl substrate (Asp)₄-Lys. This high specific cleavage makes the enzyme a widely used biotechnological tool in laboratory researches and industrial scale. Previous studies both in small and large scales were showed low expression and miss-folding of the expressed protein. In this study, the DNA sequence encoding the light chain (catalytic subunit) of bovine enteropeptidase (EP_L) was subcloned into plasmid pET-32b, downstream to the DNA encoding the fusion partner thioredoxin immediately after the EP_L cleavage site. SHuffle® T7 Express was selected as an expression host due to the ability to promote proper folding and correction of the mis-oxidized bonds. Expression and purification of protein was performed, and the result of biological activity confirmed that the active EP_L was obtained. Optimization of protein expression conditions was accomplished by response surface methodology for significant factors including induction temperature, duration of induction, inducer concentration and OD₆₀₀ of induction. The best conditions were achieved in 1.05 mM IPTG at OD₆₀₀ of 0.6 for seven h incubation at 26.5 °C, and a high level of protein expression was obtained in the optimized condition.

Antimicrobial Peptides from Plants: A cDNA-Library Based Isolation, Purification, Characterization Approach and Elucidating Their Modes of Action

Even in a natural ecosystem, plants are continuously threatened by various microbial diseases. To save themselves from these diverse infections, plants build a robust, multilayered immune system through their natural chemical compounds. Among the several crucial bioactive compounds possessed by plants’ immune systems, antimicrobial peptides (AMPs) rank in the first tier. These AMPs are environmentally friendly, anti-pathogenic, and do not bring harm to humans. Antimicrobial peptides can be isolated in several ways, but recombinant protein production has become increasingly popular in recent years, with the Escherichia coli expression system being the most widely used. However, the efficacy of this expression system is compromised due to the difficulty of removing endotoxin from its system. Therefore, this review suggests a high-throughput cDNA library-based plant-derived AMP isolation technique using the Bacillus subtilis expression system. This method can be performed for large-scale screening of plant sources to classify unique or homologous AMPs for the agronomic and applied field of plant studies. Furthermore, this review also focuses on the efficacy of plant AMPs, which are dependent on their numerous modes of action and exceptional structural stability to function against a wide range of invaders. To conclude, the findings from this study will be useful in investigating how novel AMPs are distributed among plants and provide detailed guidelines for an effective screening strategy of AMPs.

Expression and purification of soluble and active human enterokinase light chain in Escherichia coli

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Recombinant production of soluble, active enterokinase (EK) is challenging.

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Maltose binding protein-fusion improves EK solubility but reduces activity.

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GroEL/ES and Erv2/PDI induces correct refolding and improves EK activity.

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Replacing free cysteine with serine dramatically improves EK activity.

Human enterokinase light chain (hEK_L) specifically cleaves the sequence (Asp)₄-Lys↓X (D₄K), making this a frequently used enzyme for site-specific cleavage of recombinant fusion proteins. However, hEK_L production from Escherichia coli is limited due to intramolecular disulphide bonds. Here, we present strategies to obtain soluble and active hEK_L from E. coli by expressing the hEK_L variant C112S fused with maltose-binding protein (MBP) through D₄K and molecular chaperons including GroEL/ES. The fusion protein self-cleaved in vivo, thereby removing the MBP in the E. coli cells. Thus, the self-cleaved hEK_L variant was released into the culture medium. One-step purification using HisTrap™ chromatography purified the hEK_L variant exhibiting an enzymatic activity of 3.1 × 10³ U/mL (9.934 × 10⁵ U/mg). The approaches presented here greatly simplify the purification of hEK_L from E. coli without requiring refolding processes.

Selective targeting of the androgen receptor-DNA binding domain by the novel antiandrogen SBF-1 and inhibition of the growth of prostate cancer cells

Prostate cancers are reliant on androgens for growth and survival. Clinicians and researchers are looking for potent treatments for the resistant forms of prostate cancer; however, a handful of small molecules used in the treatment of castration-resistant prostate cancer have not shown potent effects owing to the mutations in the AR (Androgen Receptor). We used SBF-1, a well-characterized antitumor agent with potent cytotoxic effects against different kinds of cancers and investigated its effect on human prostate cancer. SBF-1 substantially inhibited the proliferation, induced apoptosis, and caused cell cycle arrest in LNCaP and PC3/AR⁺ prostate cancer cell lines. SBF-1 inhibited the activation of the IGF-1-PNCA pathway, as demonstrated by decreased expression of IGF-1 (insulin-like growth factor 1), proliferating cell nuclear antigen (PCNA), and its downstream Bcl-2 protein. Using microscale thermophoresis (MST) and isothermal titration calorimetry (ITC) assays, we observed a direct binding of SBF-1 to the AR. SBF-1 binds to the AR-DBD (DNA-binding domain) and blocks the transcription of its target gene. SBF-1 demonstrated a potent antitumor effect in vivo; it inhibited AR signaling and suppressed tumor growth in animals. Our study suggests that SBF-1 is an inhibitor of the AR and might be used in the treatment of prostate cancer.

ITIH4 acts as a protease inhibitor by a novel inhibitory mechanism

Inter-α-inhibitor heavy chain 4 (ITIH4) is a poorly characterized plasma protein that is proteolytically processed in multiple pathological conditions. However, no biological function of ITIH4 has been identified. Here, we show that ITIH4 is cleaved by several human proteases within a protease-susceptible region, enabling ITIH4 to function as a protease inhibitor. This is exemplified by its inhibition of mannan-binding lectin-associated serine protease-1 (MASP-1), MASP-2, and plasma kallikrein, which are key proteases for intravascular host defense. Mechanistically, ITIH4 acts as bait that, upon cleavage, forms a noncovalent, inhibitory complex with the executing protease that depends on the ITIH4 von Willebrand factor A domain. ITIH4 inhibits the MASPs by sterically preventing larger protein substrates from accessing their active sites, which remain accessible and fully functional toward small substrates. Thus, we demonstrate that ITIH4 functions as a protease inhibitor by a previously undescribed inhibitory mechanism.

The adhesive PitA pilus protein from the early dental plaque colonizer Streptococcus oralis: expression, purification, crystallization and X-ray diffraction analysis

PitA is the putative tip adhesin of the pilus islet 2 (PI-2)-encoded sortase-dependent pilus in the Gram-positive Streptococcus oralis, an opportunistic pathogen that often flourishes within the diseased human oral cavity. Early colonization by S. oralis and its interaction with Actinomyces oris seeds the development of oral biofilm or dental plaque. Here, the PI-2 pilus plays a vital role in mediating adherence to host surfaces and other bacteria. A recombinant form of the PitA adhesin has now been produced and crystallized. Owing to the large size (∼100 kDa), flexibility and complicated folding of PitA, obtaining diffraction-quality crystals has been a challenge. However, by the use of limited proteolysis with α-chymotrypsin, the diffraction quality of the PitA crystals was considerably enhanced to 2.16 Å resolution. These crystals belonged to space group P1, with unit-cell parameters a = 61.48, b = 70.87, c = 82.46 Å, α = 80.08, β = 87.02, γ = 87.70°. The anomalous signal from the terbium derivative of α-chymotrypsin-treated PitA crystals prepared with terbium crystallophore (Tb-Xo4) was sufficient to obtain an interpretable electron-density map via terbium SAD phasing.

Purification and immobilization of α-amylase in one step by gram-positive enhancer matrix (GEM) particles from the soluble protein and the inclusion body

Immobilization of the enzyme benefits the catalytic industry a lot. The gram-positive enhancer matrix (GEM) particles could purify and immobilize the recombinant α-amylase in one step without changing the enzymatic character. The enzyme immobilized by GEM particles exhibited good reusability and storage stability. The denaturants dissolved some of the GEM particles and a part of the GEM particles could bear the denaturants. The GEM particles had strong binding ability to the recombination protein with the AcmA tag even when the denaturants existed. The inclusion body was dissolved by urea and then bound by the GEM particles. The GEM particles binding the recombination protein were separated by centrifugation and resuspended in the renaturation solution. GEM particles were recycled by repeating the boiling procedure used in preparing them. The recombination α-amylase without any tag was obtained by digestion and separated via centrifugation. Altogether, our findings suggest that GEM particles have the potential to function as both immobilization and purification materials to bind the soluble recombinant protein with the AcmA tag and the inclusion body dissolved in the denaturants.

Production of recombinant proteins through sequestration in chloroplasts: a strategy based on nuclear transformation and post-translational protein import

Recently, plants have emerged as a lucrative alternative system for the production of recombinant proteins, as recombinant proteins produced in plants are safer and cheaper than those produced in bacteria and animal cell-based production systems. To obtain high yields in plants, recombinant proteins are produced in chloroplasts using different strategies. The first strategy is based on chloroplast transformation, followed by gene expression and translation in chloroplasts. This has proven to be a powerful approach for the production of proteins at high levels. The second approach is based on nuclear transformation, followed by post-translational import of proteins from the cytosol into chloroplasts. In the nuclear transformation approach, foreign genes are stably integrated into the nuclear genome or transiently expressed in the nucleus by non-integrating T-DNA. Although this approach also has great potential for protein production at high levels, it has not been thoroughly investigated. In this review, we focus on nuclear transformation-based protein expression and its subsequent sequestration in chloroplasts, and summarize the different strategies used for high-level production of recombinant proteins. We also discuss future directions for further improvements in protein production in chloroplasts through nuclear transformation-based gene expression.

Site-specific, covalent immobilization of an engineered enterokinase onto magnetic nanoparticles through transglutaminase-catalyzed bioconjugation

Enterokinase (EK) is one of the most popular enzymes for the in vitro cleavage of fusion proteins due to its high degree of specificity for the amino-acid sequence (Asp)₄-Lys. Enzyme reusability is desirable for reducing operating costs and facilitating the industrial application of EK. In this work, we report the controlled, site-specific and covalent cross-linking of an engineered EK_LC on amine-modified magnetic nanoparticles (NH₂-MNPs) via microbial transglutaminase-catalyzed bioconjugation for the development of the oriented-immobilized enzyme, namely, EK_LC@NH₂-MNP biocatalyst. Upon the site-specific immobilization, approximately 90% EK_LC enzymatic activity was retained, and the biocatalyst exhibited more than 85% of initial enzymatic activity regardless of storage or reusable stability over a month. The EK_LC@NH₂-MNP biocatalyst was further applied to remove the His tag-(Asp)₄-Lys fusion partner from the His tag-(Asp)₄-Lys-(GLP-1)₃ substrate fusion protein, result suggested the EK_LC@NH₂-MNP possessed remarkable reusability, without a significant decrease of enzymatic activity over 10 cycles (P >  0.05). Supported by the unique properties of MNPs, the proposed EK_LC@NH₂-MNP biocatalyst is expected to promote the economical utilization of enterokinase in fusion protein cleavage.

A Novel Method of Affinity Tag Cleavage in the Purification of a Recombinant Thermostable Lipase from Aneurinibacillus thermoaerophilus Strain HZ

The development of an efficient and economical purification method is required to obtain a pure and mature recombinant protein in a simple process with high efficiency. Hence, a new technique was invented to cleave the tags from the N-terminal region of recombinant fusion HZ lipase in the absence of protease treatment. The recombinant pET32b/rHZ lipase was overexpressed into E. coli BL21 (DE3). Affinity chromatography was performed as the first step of purification. The stability of the protein was then tested in different temperatures. The fused Trx-His-S-tags to the rHZ lipase was cleaved by treatment of the fusion protein at 20 °C in 100 mM Tris-HCl buffer, pH 8.0. The precipitated tag was removed, and the mature recombinant enzyme was further characterized to specify its properties. A purification yield of 78.9% with 1.3-fold and 21.8 mg total purified mature protein was obtained from 50 mL starting a bacterial culture. N-terminal sequencing of purified recombinant HZ lipase confirmed the elimination of the 17.4 kDa tag from one amino acid before the native start codon (Methionine) of the protein. The mature rHZ lipase was highly active at 65 °C and a pH of 7.0, with a half-life of 2 h 15 min at 55 °C and 45 min at 60 °C. The rHZ lipase showed a preference for the hydrolysis of natural oil with a long carbon chain (C18) and medium-size acyl chain p-nitrophenyl esters (C10). The enzyme remained stable in the presence of nonpolar organic solvents, and its activity was increased by polar organic solvents. This study thus demonstrates a simple and convenient purification method which resulted in the high yield of mature enzyme along with unique and detailed biochemical characterization of rHZ lipase, making the enzyme favorable in various industrial applications.

Conformational Dynamics Govern the Free-Energy Landscape of a Membrane-Interacting Protein

The equilibrium stabilities and the folding rates of membrane-bound proteins are determined by hydrophobic and polar intermolecular contacts with their environment as well as by intramolecular packing and conformational dynamics. The contributions of these factors, however, remain elusive and might vary considerably among proteins. Mistic from Bacillus subtilis is a particularly intriguing example of an α-helical protein that associates with membranes in spite of its unusual hydrophilicity. In micelles, Mistic is stabilized by hydrophobic and polar interactions with detergents, but it is unclear whether and how these intermolecular contacts are coupled to structural and dynamic adaptations of the protein itself. Here, we investigated the packing and the conformational dynamics of Mistic as functions of detergent headgroup chemistry and chain length, employing single-molecule Förster resonance energy transfer spectroscopy and time-resolved intrinsic tryptophan fluorescence spectroscopy. Surprisingly, in nonionic detergents, more effective hydrophobic burial and, thus, greater protein stability with increasing hydrophobic micellar thickness were accompanied by a gradual loosening of the helical bundle. By contrast, Mistic was found to assume a stable, compact fold in zwitterionic detergents that allowed faster dynamics on the nanosecond timescale. Thus, intramolecular packing per se is insufficient for conferring high protein stability; instead, enhanced nanosecond dynamics and, consequently, greater conformational entropy in the compact folded state account for Mistic's high equilibrium stability and fast folding rates in zwitterionic micelles even at the expense of less effective hydrophobic burial.

Activation and activity of glycosylated KLKs 3, 4 and 11

Human kallikrein-related peptidases 3, 4, 11, and KLK2, the activator of KLK3/PSA, belong to the prostatic group of the KLKs, whose major physiological function is semen liquefaction during the fertilization process. Notably, these KLKs are upregulated in prostate cancer and are used as clinical biomarkers or have been proposed as therapeutic targets. However, this potential awaits a detailed characterization of these proteases. In order to study glycosylated prostatic KLKs resembling the natural proteases, we used Leishmania (LEXSY) and HEK293 cells for secretory expression. Both systems allowed the subsequent purification of soluble pro-KLK zymogens with correct propeptides and of the mature forms. Periodic acid-Schiff reaction, enzymatic deglycosylation assays, and mass spectrometry confirmed the glycosylation of these KLKs. Activation of glycosylated pro-KLKs 4 and 11 turned out to be most efficient by glycosylated KLK2 and KLK4, respectively. By comparing the glycosylated prostatic KLKs with their non-glycosylated counterparts from Escherichia coli, it was observed that the N-glycans stabilize the KLK proteases and change their activation profiles and their enzymatic activity to some extent. The functional role of glycosylation in prostate-specific KLKs could pave the way to a deeper understanding of their biology and to medical applications.

Large-Scale Generation of Recombinant Granulin Peptides in E. coli

Generating milligram quantities of correctly folded granulin molecules with properly formed disulfide bonds and biologically relevant activities may represent a considerable challenge. Here I describe a protocol for obtaining well-folded human granulins A, C, and F by expressing them as thioredoxin fusion proteins in Origami (DE3) Escherichia coli cells promoting disulfide bond formation in the cytoplasm environment. The thioredoxin tag is removed by proteolytic cleavage with enterokinase and granulins which are purified by reversed-phase HPLC. Well-folded disulfide species display lower retention time than misfolded species and therefore can be readily purified.

Optimization of the fermentation and downstream processes for human enterokinase production in Pichia pastoris

Enterokinase is one of the most frequently used enzymes for the removal of affinity tags from target recombinant proteins. In this study, several fermentation strategies were assayed for the production of human enterokinase in Pichia pastoris under constitutive GAP promoter. Two of them with controlled specific growth rate during whole cultivation showed a very low enterokinase activity, under 1 U/ml, of the fermentation medium. On the contrary, the combined fermentation with a maximum specific growth rate at the initial phase of the fermentation and stationary-like phase during the rest of the fermentation showed a significant accumulation of the enterokinase in the medium, which counted up to 1400 U/ml. Lower cultivation temperature had a negative impact on the enzyme accumulation during this fermentation strategy. Downstream processes were focused on buffer environment optimization directly after cultivation, as at this time, the most amount of the activity is eliminated by endogenous proteases. Slightly positive effect on enzyme activity in the medium had an addition of liquid storage solution of EDTA and KOH to adjust pH to 8 and molarity of the EDTA to 50 mM. During the purification process, a significant amount of the enzyme was detected to be lost, which counted up to 90%. The purified enzyme, enterokinase, kept quality standard of the published enzymes.

The N-terminal tropomyosin- and actin-binding sites are important for leiomodin 2's function

Leiomodin is a potent actin nucleator related to tropomodulin, a capping protein localized at the pointed end of the thin filaments. Mutations in leiomodin-3 are associated with lethal nemaline myopathy in humans, and leiomodin-2-knockout mice present with dilated cardiomyopathy. The arrangement of the N-terminal actin- and tropomyosin-binding sites in leiomodin is contradictory and functionally not well understood. Using one-dimensional nuclear magnetic resonance and the pointed-end actin polymerization assay, we find that leiomodin-2, a major cardiac isoform, has an N-terminal actin-binding site located within residues 43-90. Moreover, for the first time, we obtain evidence that there are additional interactions with actin within residues 124-201. Here we establish that leiomodin interacts with only one tropomyosin molecule, and this is the only site of interaction between leiomodin and tropomyosin. Introduction of mutations in both actin- and tropomyosin-binding sites of leiomodin affected its localization at the pointed ends of the thin filaments in cardiomyocytes. On the basis of our new findings, we propose a model in which leiomodin regulates actin poly-merization dynamics in myocytes by acting as a leaky cap at thin filament pointed ends.

Localization of the binding interface between leiomodin-2 and α-tropomyosin

The development of some familial dilated cardiomyopathies (DCM) correlates with the presence of mutations in proteins that regulate the organization and function of thin filaments in cardiac muscle cells. Harmful effects of some mutations might be caused by disruption of yet uncharacterized protein-protein interactions. We used nuclear magnetic resonance spectroscopy to localize the region of striated muscle α-tropomyosin (Tpm1.1) that interacts with leiomodin-2 (Lmod2), a member of tropomodulin (Tmod) family of actin-binding proteins. We found that 21 N-terminal residues of Tpm1.1 are involved in interactions with residues 7-41 of Lmod2. The K15N mutation in Tpm1.1, known to be associated with familial DCM, is located within the newly identified Lmod2 binding site of Tpm1.1. We studied the effect of this mutation on binding Lmod2 and Tmod1. The mutation reduced binding affinity for both Lmod2 and Tmod1, which are responsible for correct lengths of thin filaments. The effect of the K15N mutation on Tpm1.1 binding to Lmod2 and Tmod1 provides a molecular rationale for the development of familial DCM.

Structure-function analyses of human kallikrein-related peptidase 2 establish the 99-loop as master regulator of activity

Human kallikrein-related peptidase 2 (KLK2) is a tryptic serine protease predominantly expressed in prostatic tissue and secreted into prostatic fluid, a major component of seminal fluid. Most likely it activates and complements chymotryptic KLK3 (prostate-specific antigen) in cleaving seminal clotting proteins, resulting in sperm liquefaction. KLK2 belongs to the “classical” KLKs 1–3, which share an extended 99- or kallikrein loop near their non-primed substrate binding site. Here, we report the 1.9 Å crystal structures of two KLK2-small molecule inhibitor complexes. In both structures discontinuous electron density for the 99-loop indicates that this loop is largely disordered. We provide evidence that the 99-loop is responsible for two biochemical peculiarities of KLK2, i.e. reversible inhibition by micromolar Zn²⁺ concentrations and permanent inactivation by autocatalytic cleavage. Indeed, several 99-loop mutants of KLK2 displayed an altered susceptibility to Zn²⁺, which located the Zn²⁺ binding site at the 99-loop/active site interface. In addition, we identified an autolysis site between residues 95e and 95f in the 99-loop, whose elimination prevented the mature enzyme from limited autolysis and irreversible inactivation. An exhaustive comparison of KLK2 with related structures revealed that in the KLK family the 99-, 148-, and 220-loop exist in open and closed conformations, allowing or preventing substrate access, which extends the concept of conformational selection in trypsin-related proteases. Taken together, our novel biochemical and structural data on KLK2 identify its 99-loop as a key player in activity regulation.