Propeptide-mediated folding in subtilisin: the intramolecular chaperone concept

High selectivity of the hyperthermophilic subtilase propeptide domain toward inhibition of its cognate protease

Microbial extracellular subtilases are highly active proteolytic enzymes commonly used in commercial applications. These subtilases are synthesized in their inactive proform, which matures into the active protease under the control of the propeptide domain. In mesophilic bacterial prosubtilases, the propeptide functions as both an obligatory chaperone and an inhibitor of the subtilase catalytic domain. In contrast, the propeptides of hyperthermophilic archaeal prosubtilases act mainly as tight inhibitors and are not essential for subtilase folding. It is unclear whether this stronger inhibitory activity of hyperthermophilic propeptides results in their higher selectivity toward their cognate subtilases, in contrast to promiscuous mesophilic propeptides. Here, we showed that the propeptide of pernisine, a hyperthermostable archaeal subtilase, strongly interacts with and inhibits pernisine, but not the homologous subtilisin Carlsberg and proteinase K. Instead, the pernisine propeptide was readily degraded by subtilisin Carlsberg and proteinase K. In addition, the catalytic domain of unprocessed propernisine was also susceptible to degradation but became proteolytically stable after autoprocessing of propernisine into the inactive, noncovalent complex propeptide:pernisine. This allowed efficient transactivation of the autoprocessed complex propeptide:pernisine through degradation of pernisine propeptide by subtilisin Carlsberg and proteinase K at mesophilic temperature. Moreover, we demonstrated that active pernisine molecules are inhibited by the propeptide that is released after pernisine-catalyzed degradation of the unprocessed propernisine catalytic domain. This highlights the high inhibitory potency of the hyperthermophilic propeptide toward its cognate subtilase and its importance in regulating subtilase maturation, to prevent the degradation of the unprocessed subtilase precursors by the prematurely activated molecules. IMPORTANCE Many microorganisms secrete proteases into their environment to degrade protein substrates for their growth. The important group of these extracellular enzymes are subtilases, which are also widely used in practical applications. These subtilases are inhibited by their propeptide domain, which is degraded during the prosubtilase maturation process. Here, we showed that the propeptide of pernisine, a prion-degrading subtilase from the hyperthermophilic archaeon, strongly inhibits pernisine with extraordinarily high binding affinity. This interaction proved to be highly selective, as pernisine propeptide was rapidly degraded by mesophilic pernisine homologs. This in turn allowed rapid transactivation of propernisine by mesophilic subtilases at lower temperatures, which might simplify the procedures for preparation of active pernisine for commercial use. The results reported in this study suggest that the hyperthermophilic subtilase propeptide evolved to function as tight and selective regulator of maturation of the associated prosubtilase to prevent its premature activation under high temperatures.

Proprotein Convertase Subtilisin/Kexin 6 in Cardiovascular Biology and Disease

Proprotein convertase subtilisin/kexin 6 (PCSK6) is a secreted serine protease expressed in most major organs, where it cleaves a wide range of growth factors, signaling molecules, peptide hormones, proteolytic enzymes, and adhesion proteins. Studies in Pcsk6-deficient mice have demonstrated the importance of Pcsk6 in embryonic development, body axis specification, ovarian function, and extracellular matrix remodeling in articular cartilage. In the cardiovascular system, PCSK6 acts as a key modulator in heart formation, lipoprotein metabolism, body fluid homeostasis, cardiac repair, and vascular remodeling. To date, dysregulated PCSK6 expression or function has been implicated in major cardiovascular diseases, including atrial septal defects, hypertension, atherosclerosis, myocardial infarction, and cardiac aging. In this review, we describe biochemical characteristics and posttranslational modifications of PCSK6. Moreover, we discuss the role of PCSK6 and related molecular mechanisms in cardiovascular biology and disease.

Proprotein Convertases and the Complement System

Proteins destined for secretion - after removal of the signal sequence - often undergo further proteolytic processing by proprotein convertases (PCs). Prohormones are typically processed in the regulated secretory pathway, while most plasma proteins travel though the constitutive pathway. The complement system is a major proteolytic cascade in the blood, serving as a first line of defense against microbes and also contributing to the immune homeostasis. Several complement components, namely C3, C4, C5 and factor I (FI), are multi-chain proteins that are apparently processed by PCs intracellularly. Cleavage occurs at consecutive basic residues and probably also involves the action of carboxypeptidases. The most likely candidate for the intracellular processing of complement proteins is furin, however, because of the overlapping specificities of basic amino acid residue-specific proprotein convertases, other PCs might be involved. To our surprise, we have recently discovered that processing of another complement protein, mannan-binding lectin-associated serine protease-3 (MASP-3) occurs in the blood by PCSK6 (PACE4). A similar mechanism had been described for the membrane protease corin, which is also activated extracellularly by PCSK6. In this review we intend to point out that the proper functioning of the complement system intimately depends on the action of proprotein convertases. In addition to the non-enzymatic components (C3, C4, C5), two constitutively active complement proteases are directly activated by PCs either intracellularly (FI), or extracellularly (MASP-3), moreover indirectly, through the constitutive activation of pro-factor D by MASP-3, the activity of the alternative pathway also depends on a PC present in the blood.

Tobacco phytaspase: Successful expression in a heterologous system

Phytaspase, a plant serine protease, has been demonstrated to play an important role in the programmed cell death of various plants. Phytaspase is synthesized as an inactive proenzyme containing an N-terminal signal peptide followed by a pro-domain and a mature protease catalytic domain. Pre-prophytaspase autocatalytically processes itself into a pro-domain and an active mature phytaspase enzyme. We have recently demonstrated the successful expression of mature phytaspase from tobacco in a bacterial system. Herein, we focus on the expression of pre-prophytaspase as a GST-tag fusion and on its purification by affinity chromatography.

New structural and functional contexts of the Dx[DN]xDG linear motif: insights into evolution of calcium-binding proteins

Binding of calcium ions (Ca²⁺) to proteins can have profound effects on their structure and function. Common roles of calcium binding include structure stabilization and regulation of activity. It is known that diverse families – EF-hands being one of at least twelve – use a Dx[DN]xDG linear motif to bind calcium in near-identical fashion. Here, four novel structural contexts for the motif are described. Existing experimental data for one of them, a thermophilic archaeal subtilisin, demonstrate for the first time a role for Dx[DN]xDG-bound calcium in protein folding. An integrin-like embedding of the motif in the blade of a β-propeller fold – here named the calcium blade – is discovered in structures of bacterial and fungal proteins. Furthermore, sensitive database searches suggest a common origin for the calcium blade in β-propeller structures of different sizes and a pan-kingdom distribution of these proteins. Factors favouring the multiple convergent evolution of the motif appear to include its general Asp-richness, the regular spacing of the Asp residues and the fact that change of Asp into Gly and vice versa can occur though a single nucleotide change. Among the known structural contexts for the Dx[DN]xDG motif, only the calcium blade and the EF-hand are currently found intracellularly in large numbers, perhaps because the higher extracellular concentration of Ca²⁺ allows for easier fixing of newly evolved motifs that have acquired useful functions. The analysis presented here will inform ongoing efforts toward prediction of similar calcium-binding motifs from sequence information alone.

An alternative mature form of subtilisin homologue, Tk-SP, from Thermococcus kodakaraensis identified in the presence of Ca2+

Pro-Tk-SP from Thermococcus kodakaraensis consists of the four domains: N-propeptide, subtilisin (EC 3.4.21.62) domain, β-jelly roll domain and C-propeptide. To analyze the maturation process of this protein, the Pro-Tk-SP derivative with the mutation of the active-site serine residue to Cys (Pro-Tk-S359C), Pro-Tk-S359C derivatives lacking the N-propeptide (ProC-Tk-S359C) and both propeptides (Tk-S359C), and a His-tagged form of the isolated C-propeptide (ProC*) were constructed. Pro-Tk-S359C was purified mostly in an autoprocessed form in which the N-propeptide is autoprocessed but the isolated N-propeptide (ProN) forms a stable complex with ProC-Tk-S359C, indicating that the N-propeptide is autoprocessed first. The subsequent maturation process was analyzed using ProC-Tk-S359C, instead of the ProN:ProC-Tk-S359C complex. The C-propeptide was autoprocessed and degraded when ProC-Tk-S359C was incubated at 80 °C in the absence of Ca(2+). However, it was not autoprocessed in the presence of Ca(2+). Comparison of the susceptibility of ProC* to proteolytic degradation in the presence and absence of Ca(2+) suggests that the C-propeptide becomes highly resistant to proteolytic degradation in the presence of Ca(2+). We propose that Pro-Tk-SP derivative lacking N-propeptide (Val114-Gly640) represents a mature form of Pro-Tk-SP in a natural environment. The enzymatic activity of ProC-Tk-S359C was higher than (but comparable to) that of Tk-S359C, suggesting that the C-propeptide is not important for activity. However, the T(m) value of ProC-Tk-S359C determined by far-UV CD spectroscopy was higher than that of Tk-S359C by 25.9 °C in the absence of Ca(2+) and 7.5 °C in the presence of Ca(2+), indicating that the C-propeptide contributes to the stabilization of ProC-Tk-S359C.

N-terminal deletion of Tk1689, a subtilisin-like serine protease from Thermococcus kodakaraensis, copes with its cytotoxicity in Escherichia coli

Tk1689, a subtilisin-like serine protease from Thermococcus kodakaraensis, was found to be toxic to the host cells when produced in the pro-protein form (Pro-Tk1689) in Eschericia coli. Cytotoxic effect of Pro-Tk1689 was reduced when signal and pro-peptide both were removed and the protein was produced in the mature form (MP-Tk1689). The mature protein was produced in E. coli in the soluble form. Recombinant MP-Tk1689 was catalytically active and exhibited optimum activity at 55°C and pH 7. Specific activity of the enzyme was 700 U/mg. The enzyme displayed a half life of 80 min at 60°C.

Requirement of a unique Ca(2+)-binding loop for folding of Tk-subtilisin from a hyperthermophilic archaeon

Tk-subtilisin from the hyperthermophiolic archaeon Thermococcus kodakaraensis matures from Pro-Tk-subtilisin upon autoprocessing and degradation of Tk-propeptide [Tanaka, S., Saito, K., Chon, H., Matsumura, H., Koga, Y., Takano, K., and Kanaya, S. (2007) J. Biol. Chem. 282, 8246-8255]. It requires Ca(2+) for folding and assumes a molten globule-like structure in the absence of Ca(2+) even in the presence of Tk-propeptide. Tk-subtilisin contains seven Ca(2+)-binding sites. Four of them (Ca2-Ca5) are located within a long loop, which mostly consists of a unique insertion sequence of this protein. To analyze the role of this Ca(2+)-binding loop, three mutant proteins, Deltaloop-Tk-subtilisin, DeltaCa2-Pro-S324A, and DeltaCa3-Pro-S324A, were constructed. These proteins were designed to remove the Ca(2+)-binding loop, Ca2 site, or Ca3 site of Pro-Tk-subtilisin or its active site mutant Pro-S324A. Far-UV CD spectra of these proteins refolded in the absence and presence of Ca(2+) indicated that Deltaloop-Tk-subtilisin completely lost the ability to fold into a native structure. In contrast, two other proteins retained this ability, although their refolding rates were greatly decreased compared to that of Pro-S324A. Determination of the crystal structures of these proteins purified in a Ca(2+)-bound form indicates that the structures of DeltaCa2-Pro-S324A and DeltaCa3-Pro-S324A are virtually identical to that of Pro-S324A, except that they lack the Ca2 and Ca3 sites, respectively, and the structure of the Ca(2+)-binding loop is destabilized. Nevertheless, these proteins were slightly more stable than Pro-S324A. These results suggest that the Ca(2+)-binding loop is required for folding of Tk-subtilisin but does not seriously contribute to the stabilization of Tk-subtilisin in a native structure.

The protease CspB is essential for initiation of cortex hydrolysis and dipicolinic acid (DPA) release during germination of spores of Clostridium perfringens type A food poisoning isolates

The genome of the Clostridium perfringens food poisoning isolate SM101 encodes a subtilisin-like protease, CspB, upstream of the sleC gene encoding the enzyme essential for degradation of the peptidoglycan cortex during spore germination. SleC is an inactive pro-SleC in dormant spores that is converted to active SleC during spore germination and Csp proteases convert pro-SleC to the active enzyme in vitro. In this work, the germination and viability of spores of a cspB deletion mutant of strain SM101, as well as cspB expression, were studied. The cspB gene was expressed only during sporulation, and only in the mother cell compartment. cspB spores were unable to germinate significantly with either a rich nutrient medium, KCl, or a 1 : 1 chelate of Ca(2+) and dipicolinic acid (DPA); the viability of these spores was approximately 10(4)-fold lower than that of wild-type spores, although cspB and wild-type spores had similar viability on plates containing lysozyme, and cspB spores could not process inactive pro-SleC into active SleC during spore germination. Germination of cspB spores was blocked prior to DPA release and cortex hydrolysis, and germination and viability defects in these spores were complemented by an ectopic cspB. These results indicate that Csp proteases are essential to generate active SleC and allow cortex hydrolysis early in C. perfringens spore germination. However, Csp proteases likely play another role in spore germination, since cspB spores did not release DPA upon exposure to germinants, while sleC spores have been shown previously to release DPA, albeit slowly, upon exposure to germinants.

Role of CpSUB1, a subtilisin-like protease, in Cryptosporidium parvum infection in vitro

The apicomplexan parasite Cryptosporidium is a significant cause of diarrheal disease worldwide. Previously, we reported that a Cryptosporidium parvum subtilisin-like serine protease activity with furin-type specificity cleaves gp40/15, a glycoprotein that is proteolytically processed into gp40 and gp15, which are implicated in mediating infection of host cells. Neither the enzyme(s) responsible for the protease activity in C. parvum lysates nor those that process gp40/15 are known. There are no furin or other proprotein convertase genes in the C. parvum genome. However, a gene encoding CpSUB1, a subtilisin-like serine protease, is present. In this study, we cloned the CpSUB1 genomic sequence and expressed and purified the recombinant prodomain. Reverse transcriptase PCR analysis of RNA from C. parvum-infected HCT-8 cells revealed that CpSUB1 is expressed throughout infection in vitro. In immunoblots, antiserum to the recombinant CpSUB1 prodomain revealed two major bands, of approximately 64 kDa and approximately 48 kDa, for C. parvum lysates and proteins "shed" during excystation. In immunofluorescence assays, the antiserum reacted with the apical region of sporozoites and merozoites. The recombinant prodomain inhibited protease activity and processing of recombinant gp40/15 by C. parvum lysates but not by furin. Since prodomains are often selective inhibitors of their cognate enzymes, these results suggest that CpSUB1 may be a likely candidate for the protease activity in C. parvum and for processing of gp40/15. Importantly, the recombinant prodomain inhibited C. parvum infection of HCT-8 cells. These studies indicate that CpSUB1 plays a significant role in infection of host cells by the parasite and suggest that this enzyme may serve as a target for intervention.

Crystal Structure of Unautoprocessed Precursor of Subtilisin from a Hyperthermophilic Archaeon

The crystal structure of an active site mutant of pro-Tk-subtilisin (pro-S324A) from the hyperthermophilic archaeon Thermococcus kodakaraensis was determined at 2.3 A resolution. The overall structure of this protein is similar to those of bacterial subtilisin-propeptide complexes, except that the peptide bond linking the propeptide and mature domain contacts with the active site, and the mature domain contains six Ca2+ binding sites. The Ca-1 site is conserved in bacterial subtilisins but is formed prior to autoprocessing, unlike the corresponding sites of bacterial subtilisins. All other Ca2+-binding sites are unique in the pro-S324A structure and are located at the surface loops. Four of them apparently contribute to the stability of the central alphabetaalpha substructure of the mature domain. The CD spectra, 1-anilino-8-naphthalenesulfonic acid fluorescence spectra, and sensitivities to chymotryptic digestion of this protein indicate that the conformation of pro-S324A is changed from an unstable molten globule-like structure to a stable native one upon Ca2+ binding. Another active site mutant, pro-S324C, was shown to be autoprocessed to form a propeptide-mature domain complex in the presence of Ca2+. The CD spectra of this protein indicate that the structure of pro-S324C is changed upon Ca2+ binding like pro-S324A but is not seriously changed upon subsequent autoprocessing. These results suggest that the maturation process of Tk-subtilisin is different from that of bacterial subtilisins in terms of the requirement of Ca2+ for folding of the mature domain and completion of the folding process prior to autoprocessing.

Autotomic Behavior of the Propeptide in Propeptide-mediated Folding of Prosubtilisin E

The 77 residue propeptide at the N-terminal end of subtilisin E plays an essential role in subtilisin folding as a tailor-made intramolecular chaperone. Upon completion of folding, the propeptide is autoprocessed and removed by subtilisin digestion. This propeptide-mediated protein folding has been used as a paradigm for the study of protein folding. Here, we show by three independent methods, that the propeptide domain and the subtilisin domain show distinctive intrinsic stability that is obligatory for efficient autoprocessing of the propeptide domain. Two tryptophan residues, Trp106 and Trp113, on the surface of subtilisin located on one of the two helices that form the interface between the propeptide and the subtilisin domains play a key role in maintaining the distinctive instability of the propeptide domain, after completion of folding. When either of the Trp residues was substituted with Tyr, the characteristic biphasic heat denaturation profile of two domains unfolding was not observed, resulting in a single transition of denaturation. The results provide evidence that the propeptide not only plays an essential role in subtilisin folding, but upon completion of folding it behaves as an independent domain. Once the propeptide-mediated folding is completed, the propeptide domain is readily eliminated without interference from the subtilisin domain. This "autotomic" behavior of the propeptide may be a prevailing principle in propeptide-mediated protein folding.

Crystallization and preliminary X-ray diffraction study of an active-site mutant of pro-Tk-subtilisin from a hyperthermophilic archaeon

Crystallization of and preliminary crystallographic studies on an active-site mutant of pro-Tk-subtilisin from the hyperthermophilic archaeon Thermococcus kodakaraensis were performed. The crystal was grown at 277 K by the sitting-drop vapour-diffusion method. Native X-ray diffraction data were collected to 2.3 A resolution using synchrotron radiation from station BL41XU at SPring-8. The crystal belongs to the orthorhombic space group I222, with unit-cell parameters a = 92.69, b = 121.78, c = 77.53 A. Assuming the presence of one molecule per asymmetric unit, the Matthews coefficient V(M) was calculated to be 2.6 A(3) Da(-1) and the solvent content was 53.1%.

Ca2+-dependent maturation of subtilisin from a hyperthermophilic archaeon, Thermococcus kodakaraensis: the propeptide is a potent inhibitor of the mature domain but is not required for its folding

Subtilisin from the hyperthermophilic archaeon Thermococcus kodakaraensis KOD1 is a member of the subtilisin family. T. kodakaraensis subtilisin in a proform (T. kodakaraensis pro-subtilisin), as well as its propeptide (T. kodakaraensis propeptide) and mature domain (T. kodakaraensis mat-subtilisin), were independently overproduced in E. coli, purified, and biochemically characterized. T. kodakaraensis pro-subtilisin was inactive in the absence of Ca2+ but was activated upon autoprocessing and degradation of propeptide in the presence of Ca2+ at 80 degrees C. This maturation process was completed within 30 min at 80 degrees C but was bound at an intermediate stage, in which the propeptide is autoprocessed from the mature domain (T. kodakaraensis mat-subtilisin*) but forms an inactive complex with T. kodakaraensis mat-subtilisin*, at lower temperatures. At 80 degrees C, approximately 30% of T. kodakaraensis pro-subtilisin was autoprocessed into T. kodakaraensis propeptide and T. kodakaraensis mat-subtilisin*, and the other 70% was completely degraded to small fragments. Likewise, T. kodakaraensis mat-subtilisin was inactive in the absence of Ca2+ but was activated upon incubation with Ca2+ at 80 degrees C. The kinetic parameters and stability of the resultant activated protein were nearly identical to those of T. kodakaraensis mat-subtilisin*, indicating that T. kodakaraensis mat-subtilisin does not require T. kodakaraensis propeptide for folding. However, only approximately 5% of T. kodakaraensis mat-subtilisin was converted to an active form, and the other part was completely degraded to small fragments. T. kodakaraensis propeptide was shown to be a potent inhibitor of T. kodakaraensis mat-subtilisin* and noncompetitively inhibited its activity with a Ki of 25 +/- 3.0 nM at 20 degrees C. T. kodakaraensis propeptide may be required to prevent the degradation of the T. kodakaraensis mat-subtilisin molecules that are activated later by those that are activated earlier.

Functional characterization of the propeptide of Plasmodium falciparum subtilisin-like protease-1

Erythrocyte invasion by the malaria merozoite is prevented by serine protease inhibitors. Various aspects of the biology of Plasmodium falciparum subtilisin-like protease-1 (PfSUB-1), including the timing of its expression and its apical location in the merozoite, suggest that this enzyme is involved in invasion. Recombinant PfSUB-1 expressed in a baculovirus system is secreted in the p54 form, noncovalently bound to its cognate propeptide, p31. To understand the role of p31 in PfSUB-1 maturation, we examined interactions between p31 and both recombinant and native enzymes. CD analyses revealed that recombinant p31 (rp31) possesses significant secondary structure on its own, comparable with that of folded propeptides of some bacterial subtilisins. Kinetic studies demonstrated that rp31 is a fast binding, high affinity inhibitor of PfSUB-1. Inhibition of two bacterial subtilisins by rp31 was much less effective, with inhibition constants 49-60-fold higher than that for PfSUB-1. Single (at the P4 or P1 position) or double (at P4 and P1 positions) point mutations of residues within the C-terminal region of rp31 had little effect on its inhibitory activity, and truncation of 11 residues from the rp31 C terminus substantially reduced, but did not abolish, inhibition. None of these modifications prevented binding to the PfSUB-1 catalytic domain or rendered the propeptide susceptible to proteolytic digestion by PfSUB-1. These studies provide new insights into the function of the propeptide in PfSUB-1 activation and shed light on the structural requirements for interaction with the catalytic domain.

Trigger factor-mediated prolyl isomerization influences maturation of the Streptococcus pyogenes cysteine protease

Trigger factor, a ribosome-associated chaperone and peptidyl-prolyl cis-trans isomerase (PPIase), is essential for the secretion and maturation of the cysteine protease of the pathogenic gram-positive bacterium Streptococcus pyogenes. In the absence of trigger factor, the nascent protease polypeptide is not targeted to the secretory pathway. Some partial-function mutations restore targeting. However, the secreted protease does not efficiently mature into an enzymatically active form, suggesting that trigger factor has an additional role in protease biogenesis. Here, we show that, while not required for targeting, the PPIase activity of trigger factor is essential for maturation of the protease following its secretion from the bacterial cell. Site-specific mutations introduced into ropA, the gene which encodes trigger factor in S. pyogenes, produced mutant proteins deficient in PPIase activity. When these mutant alleles were used to replace the wild-type gene on the streptococcal chromosome, analysis of protease biogenesis revealed that, although the protease was secreted normally, it did not efficiently mature to an active form. Furthermore, mutation of a single proline residue in the protease prodomain suppressed the requirement for PPIase activity, suggesting that this residue is the target of trigger factor. These data support a model in which trigger factor-mediated prolyl isomerization influences the conformation of the prodomain, which in turn directs the protease into one of several alternative folding pathways.

Active subtilisin-like protease from a hyperthermophilic archaeon in a form with a putative prosequence

The gene encoding subtilisin-like protease T. kodakaraensis subtilisin was cloned from a hyperthermophilic archaeon Thermococcus kodakaraensis KOD1. T. kodakaraensis subtilisin is a member of the subtilisin family and composed of 422 amino acid residues with a molecular weight of 43,783. It consists of a putative presequence, prosequence, and catalytic domain. Like bacterial subtilisins, T. kodakaraensis subtilisin was overproduced in Escherichia coli in a form with a putative prosequence in inclusion bodies, solubilized in the presence of 8 M urea, and refolded and converted to an active molecule. However, unlike bacterial subtilisins, in which the prosequence was removed from the catalytic domain by autoprocessing upon refolding, T. kodakaraensis subtilisin was refolded in a form with a putative prosequence. This refolded protein of recombinant T. kodakaraensis subtilisin which is composed of 398 amino acid residues (Gly(-82) to Gly(316)), was purified to give a single band on a sodium dodecyl sulfate (SDS)-polyacrylamide gel and characterized for biochemical and enzymatic properties. The good agreement of the molecular weights estimated by SDS-polyacrylamide gel electrophoresis (44,000) and gel filtration (40,000) suggests that T. kodakaraensis subtilisin exists in a monomeric form. T. kodakaraensis subtilisin hydrolyzed the synthetic substrate N-succinyl-Ala-Ala-Pro-Phe-p-nitroanilide only in the presence of the Ca(2+) ion with an optimal pH and temperature of pH 9.5 and 80 degrees C. Like bacterial subtilisins, it showed a broad substrate specificity, with a preference for aromatic or large nonpolar P1 substrate residues. However, it was much more stable than bacterial subtilisins against heat inactivation and lost activity with half-lives of >60 min at 80 degrees C, 20 min at 90 degrees C, and 7 min at 100 degrees C.

Neuroendocrine secretory protein 7B2: structure, expression and functions

7B2 is an acidic protein residing in the secretory granules of neuroendocrine cells. Its sequence has been elucidated in many phyla and species. It shows high similarity among mammals. A Pro-Pro-Asn-Pro-Cys-Pro polyproline motif is its most conserved feature, being carried by both vertebrate and invertebrate sequences. It is biosynthesized as a precursor protein that is cleaved into an N-terminal fragment and a C-terminal peptide. In neuroendocrine cells, 7B2 functions as a specific chaperone for the proprotein convertase (PC) 2. Through the sequence around its Pro-Pro-Asn-Pro-Cys-Pro motif, it binds to an inactive proPC2 and facilitates its transport from the endoplasmic reticulum to later compartments of the secretory pathway where the zymogen is proteolytically matured and activated. Its C-terminal peptide can inhibit PC2 in vitro and may contribute to keep the enzyme transiently inactive in vivo. The PC2-7B2 model defines a new neuroendocrine paradigm whereby proteolytic activation of prohormones and proneuropeptides in the secretory pathway is spatially and temporally regulated by the dynamics of interactions between converting enzymes and their binding proteins. Interestingly, unlike PC2-null mice, which are viable, 7B2-null mutants die early in life from Cushing's disease due to corticotropin ('ACTH') hypersecretion by the neurointermediate lobe, suggesting a possible involvement of 7B2 in secretory granule formation and in secretion regulation. The mechanism of this regulation is yet to be elucidated. 7B2 has been shown to be a good marker of several neuroendocrine cell dysfunctions in humans. The possibility that anomalies in its structure and expression could be aetiological causes of some of these dysfunctions warrants investigation.

Protein engineering of subtilisin

The serine protease subtilisin is an important industrial enzyme as well as a model for understanding the enormous rate enhancements affected by enzymes. For these reasons along with the timely cloning of the gene, ease of expression and purification and availability of atomic resolution structures, subtilisin became a model system for protein engineering studies in the 1980s. Fifteen years later, mutations in well over 50% of the 275 amino acids of subtilisin have been reported in the scientific literature. Most subtilisin engineering has involved catalytic amino acids, substrate binding regions and stabilizing mutations. Stability has been the property of subtilisin which has been most amenable to enhancement, yet perhaps least understood. This review will give a brief overview of the subtilisin engineering field, critically review what has been learned about subtilisin stability from protein engineering experiments and conclude with some speculation about the prospects for future subtilisin engineering.

Von Willebrand factor storage and multimerization: 2 independent intracellular processes

The von Willebrand factor propeptide, vW AgII, has been shown to be required for the formation of vWF multimers and sorting of vWF to storage granules; whether these 2 processes are independent events has been unclear. Chimeric constructs of human and canine vWF were developed to further define these processes and to determine whether they are independent intracellular events. Cells expressing only mature vWF (Δpro) produced vWF dimers that were not stored in AtT-20 cells; whereas the expression of vW AgII alone resulted in vW AgII granular storage. Expression of vW AgII in trans with Δpro resulted in the multimerization of vWF and colocalized storage of vW AgII and vWF. Expression of canine vW AgII in trans orcis with human Δpro resulted in the multimerization of human vWF, with no storage of human vWF but with normal storage of canine vW AgII. This dissociation of functions indicates that the signals for multimerization of vWF are different from the signals for trafficking of vWF to storage and demonstrates that vWF storage and multimerization are 2 independent intracellular processes. vW AgII contains the signal(s) required for trafficking to storage, and only through interaction with vW AgII is vWF chaperoned into granules.

Von Willebrand factor storage and multimerization: 2 independent intracellular processes

The von Willebrand factor propeptide, vW AgII, has been shown to be required for the formation of vWF multimers and sorting of vWF to storage granules; whether these 2 processes are independent events has been unclear. Chimeric constructs of human and canine vWF were developed to further define these processes and to determine whether they are independent intracellular events. Cells expressing only mature vWF (Δpro) produced vWF dimers that were not stored in AtT-20 cells; whereas the expression of vW AgII alone resulted in vW AgII granular storage. Expression of vW AgII in trans with Δpro resulted in the multimerization of vWF and colocalized storage of vW AgII and vWF. Expression of canine vW AgII in trans orcis with human Δpro resulted in the multimerization of human vWF, with no storage of human vWF but with normal storage of canine vW AgII. This dissociation of functions indicates that the signals for multimerization of vWF are different from the signals for trafficking of vWF to storage and demonstrates that vWF storage and multimerization are 2 independent intracellular processes. vW AgII contains the signal(s) required for trafficking to storage, and only through interaction with vW AgII is vWF chaperoned into granules.

Display of active subtilisin 309 on phage: analysis of parameters influencing the selection of subtilisin variants with changed substrate specificity from libraries using phosphonylating inhibitors

Many attempts have been made to endow enzymes with new catalytic activities. One general strategy involves the creation of random combinatorial libraries of mutants associated with an efficient screening or selection scheme. Phage display has been shown to greatly facilitate the selection of polypeptides with desired properties by establishing a close link between the polypeptide and the gene that encodes it. Selection of phage displayed enzymes for new catalytic activities remains a challenge. The aim of this study was to display the serine protease subtilisin 309 (savinase) from Bacillus lentus on the surface of filamentous fd phage and to develop selection schemes that allow the extraction of subtilisin variants with a changed substrate specificity from libraries. Subtilisins are produced as secreted preproenzyme that mature in active enzyme autocatalytically. They have a broad substrate specificity but exhibit a significant preference for hydrophobic residues and very limited reactivity toward charged residues at the P4 site in the substrate. Here, we show that savinase can be functionally displayed on phage in the presence of the proteic inhibitor CI2. The free enzyme is released from its complex with CI2 upon addition of the anionic detergent LAS. The phage-enzyme can be panned on streptavidin beads after labelling by reaction with (biotin-N-epsilon-aminocaproyl-cystamine-N'-glutaryl)-l-Ala-l-Ala-l-P ro-Phe(P)-diphenyl ester. Reactions of libraries, in which residues 104 and 107 forming part of the S4 pocket have been randomised, with (biotin-N-epsilon-aminocaproyl-cystamine-N'-glutaryl)-alpha-l-Lys-l-A la-l-Pro-Phe(P)-diphenylester allowed us to select enzymes with increased specific activity for a substrate containing a lysine in P4. Parameters influencing the selection as for instance the efficiency of maturation of mutant enzymes in libraries have been investigated.

The cell biology of the prohormone convertases PC1 and PC2

Mature peptide hormones and neuropeptides are typically synthesized from much larger precursors and require several posttranslational processing steps--including proteolytic cleavage--for the formation of the bioactive species. The subtilisin-related proteolytic enzymes that accomplish neuroendocrine-specific cleavages are known as prohormone convertases 1 and 2 (PC1 and PC2). The cell biology of these proteases within the regulated secretory pathway of neuroendocrine cells is complex, and they are themselves initially synthesized as inactive precursor molecules. ProPC1 propeptide cleavage occurs rapidly in the endoplasmic reticulum, yet its major site of action on prohormones takes place later in the secretory pathway. PC1 undergoes an interesting carboxyl terminal processing event whose function appears to be to activate the enzyme. ProPC2, on the other hand, exhibits comparatively long initial folding times and exits the endoplasmic reticulum without propeptide cleavage, in association with the neuroendocrine-specific protein 7B2. Once the proPC2/7B2 complex arrives at the trans-Golgi network, 7B2 is internally cleaved into two domains, the 21-kDa fragment and a carboxy-terminal 31 residue peptide. PC2 propeptide removal occurs in the maturing secretory granule, most likely through autocatalysis, and 7B2 association does not appear to be directly required for this cleavage event. However, if proPC2 has not encountered 7B2 intracellularly, it cannot generate a catalytically active mature species. The molecular mechanism behind the intriguing intracellular association of 7B2 and proPC2 is still unknown, but may involve conformational rearrangement or stabilization of a proPC2 conformer mediated by a 36-residue internal segment of 21-kDa 7B2.

The prosequence of thermolysin acts as an intramolecular chaperone when expressed in trans with the mature sequence in Escherichia coli

The zinc metalloendopeptidase, thermolysin (EC 3.4.24.27) produced by Bacillus thermoproteolyticus serves as a model of important physiological enzymes such as neprilysin, angiotensin converting enzyme and endothelin converting enzyme. Thermolysin is synthesised as a pre-proenzyme, with an N-terminal prosequence of 204 residues and a mature sequence of 316 residues. The prosequence facilitates the folding of the denatured mature sequence in vitro and the cleavage of the peptide bond linking the pro and mature sequences occurs by an autocatalytic, intramolecular process. With the aim to study the role of the prosequence in vivo and to produce active mutants for structural studies, the mature sequence of thermolysin has now been expressed in Escherichia coli, either alone or with the prosequence as an independent polypeptide, i.e. in trans form. In addition, the mature sequence of an inactive mutant in which Glu143 involved in the catalytic process was replaced by Ala has also been expressed in trans with the prosequence. The results show that the pro-sequence is required to obtain active thermolysin and that a covalent link with the mature sequence is not necessary for the correct folding of the protease in vivo. Moreover, when expressed in E. coli (in trans with the prosequence), the yield of correctly folded E143A mutant was similar to that of the wild-type protease, whereas no mature enzyme was detected when it was expressed as a pre-proenzyme in Bacillus subtilis. These results demonstrate that the thermolysin prosequence acts as an intramolecular chaperone in vivo and open the way to structural studies of catalytic site mutants produced in large quantities in E. coli.

Bi-cycling the furin pathway: from TGN localization to pathogen activation and embryogenesis

Furin is a secretory pathway endoprotease that catalyses the maturation of a strikingly diverse group of proprotein substrates, ranging from growth factors and receptors to pathogen proteins, in multiple compartments within the trans-Golgi network (TGN)/endosomal system. This review focuses on recent developments in the biochemistry and cell biology of the endoprotease, including the mechanism of TGN localization, phosphorylation-dependent regulation of protein traffic, and novel insights into early embryogenesis, extracellular matrix formation and pathogen virulence.

Advances in refolding of proteins produced in E. coli

Inclusion body production is a common theme in recombinant protein technology. Hence, renaturation of these inclusion body proteins is a field of increasing interest for gaining large amounts of proteins. Recent developments of renaturation procedures include the inhibition of aggregation during refolding by the application of low molecular weight additives and matrix-bound renaturation techniques.