Role of the prosequence of guanylin

Characterisation of proguanylin expressing cells in the intestine - evidence for constitutive luminal secretion

Guanylin, a peptide implicated in regulation of intestinal fluid secretion, is expressed in the mucosa, but the exact cellular origin remains controversial. In a new transgenic mouse model fluorescent reporter protein expression driven by the proguanylin promoter was observed throughout the small intestine and colon in goblet and Paneth(-like) cells and, except in duodenum, in mature enterocytes. In Ussing chamber experiments employing both human and mouse intestinal tissue, proguanylin was released predominantly in the luminal direction. Measurements of proguanylin expression and secretion in cell lines and organoids indicated that secretion is largely constitutive and requires ER to Golgi transport but was not acutely regulated by salt or other stimuli. Using a newly-developed proguanylin assay, we found plasma levels to be raised in humans after total gastrectomy or intestinal transplantation, but largely unresponsive to nutrient ingestion. By LC-MS/MS we identified processed forms in tissue and luminal extracts, but in plasma we only detected full-length proguanylin. Our transgenic approach provides information about the cellular origins of proguanylin, complementing previous immunohistochemical and in-situ hybridisation results. The identification of processed forms of proguanylin in the intestinal lumen but not in plasma supports the notion that the primary site of action is the gut itself.

Simple MD-based model for oxidative folding of peptides and proteins

Significant strides have been recently made to fold peptides and small proteins in silico using MD simulations. However, facilities are currently lacking to include disulfide bonding in the MD models of protein folding. To address this problem, we have developed a simple empirical protocol to model formation of disulfides, which is perturbation-free, retains the same speed as conventional MD simulations and allows one to control the reaction rate. The new protocol has been tested on 15-aminoacid peptide guanylin containing four cysteine residues; the net simulation time using Amber ff14SB force field was 61 μs. The resulting isomer distribution is in qualitative agreement with experiment, suggesting that oxidative folding of guanylin in vitro occurs under kinetic control. The highly stable conformation of the so-called isomer 2(B) has been obtained for full-length guanylin, which is significantly different from the poorly ordered structure of the truncated peptide PDB ID 1GNB. In addition, we have simulated oxidative folding of guanylin within the 94-aminoacid prohormone proguanylin. The obtained structure is in good agreement with the NMR coordinates 1O8R. The proposed modeling strategy can help to explore certain fundamental aspects of protein folding and is potentially relevant for manufacturing of synthetic peptides and recombinant proteins.

Crystallization and preliminary X-ray structural studies of human prouroguanylin

Uroguanylin, which serves as an endogenous ligand of guanylyl cyclase C, is initially secreted in the form of a precursor, prouroguanylin. The N-terminal region of prouroguanylin interacts with the mature portion of prouroguanylin during the folding pathway. Here, a preliminary X-ray crystallographic study of prouroguanylin is presented. Prouroguanylin was refolded, purified and crystallized using the hanging-drop vapour-diffusion method. Prouroguanylin crystals were cryocooled and used for data collection. The diffraction data showed that the crystals belonged to space group P6(1)22, with unit-cell parameters a = b = 55.6, c = 157.7 A, and diffracted to 2.5 A resolution. The structure is currently being analyzed.

The two cysteine-rich head domains of minicollagen from Hydra nematocysts differ in their cystine framework and overall fold despite an identical cysteine sequence pattern

Synthetic replicates of naturally occurring cysteine-rich peptides such as hormones, neurotransmitters, growth factors, enzyme inhibitors, defensins and toxins often can be oxidatively folded in high yields to their native structure in simple redox buffers. Thereby, identical cysteine patterns in the sequence were found to generate identical disulfide connectivities and homologous spatial structures despite significant variability in the non-cysteine positions. Minicollagen-1 from the nematocysts of Hydra is a trimeric protein that contains cysteine-rich domains at the N and C termini, which are involved in the assembly of an intermolecular disulfide network. Determination of the three-dimensional structures of peptides corresponding to the N-terminal and C-terminal domains by NMR spectroscopy revealed a remarkable exception from the general rule. Despite an identical cysteine pattern, the two domains of minicollagen-1 form different disulfide bridges and exhibit distinctly different folds, both of which are not found in the current structural databases. To our knowledge, this is the first case where two relatively short peptides with the abundant cysteine residues in identical sequence positions fold uniquely and with high yields into defined, but differing, structures. Therefore, the cysteine-rich domains of minicollagen constitute ideal model systems for studies of the interplay between folding and oxidation in proteins.

Recognition and signal transduction mechanism of Escherichia coli heat-stable enterotoxin and its receptor, guanylate cyclase C

Guanylate cyclase C (GC-C), a member of the membrane-bound GC family, consists of an extracellular domain (ECD) and an intracellular domain, which are connected by a single-transmembrane region. GC-C is a receptor protein, i.e. specifically stimulated by the endogenous peptides guanylin, uroguanylin, lymphoguanylin, and the exogenous peptide heat-stable enterotoxin (ST(a)), secreted by pathogenic Escherichia coli and acting on the intestinal brush border membranes. The binding of these peptide ligands to the ECD of GC-C results in the synthesis of cyclic GMP in cells, which, in turn, regulates a variety of intracellular physiologic processes. As the cloning of GC-C, its physiologic functions of each domain have been vigorously investigated. The structural characterization of the ligand-binding domain of the receptor promises to provide important clues for better understanding of the mechanisms of receptor recognition and activation. Recently, structural data for each domain of membrane-bound GCs and related proteins has become available. Coupling information obtained from such work and validation of structure-function relationships of GC-C and its ligands should allow for three-dimensional mapping of their interaction site in detail. Our approach to this issue involved designing photoaffinity-labeling ST(a) analogs, capable of binding covalently to the ligand-binding region of the ECD of GC-C. The photoaffinity-labeling ligand was used to covalently label a soluble form of the recombinant ECD protein. Mass spectrometric analyses of an endoproteinase digest of the ECD revealed that the ligand specifically bound to a narrow region contained in the membrane-proximal subdomain of the ECD of GC-C. These results will enable us to identify the possible binding motifs within the ligand-binding domain by computer modeling. In this review, we summarize the available data on the recognition mechanism between ST(a) and GC-C at the molecular level.

Engineering disulfide bonds of the novel human beta-defensins hBD-27 and hBD-28: differences in disulfide formation and biological activity among human beta-defensins

Human beta-defensins comprise a large number of peptides that play a functional role in the innate and adaptive immune system. Recently, clusters of new beta-defensin genes with predominant expression in testicular tissue have been discovered on different chromosomes by bioinformatics. beta-Defensins share a common pattern of three disulfides that are essential for their biological effects. Here we report for the first time the chemical synthesis of the new fully disulfide-bonded beta-defensins hBD-27 and hBD-28, and compare the results with synthetic procedures to obtain the known hBD-2 and hBD-3. While hBD-27 was readily converted into a product with the desired disulfide pattern by oxidative folding, hBD-28 required a selective protective group strategy to introduce the three disulfide bonds. The established synthetic processes were applied to the synthesis of hBD-2, which, like hBD-27, was accessible by oxidative folding, whereas hBD-3 required a selective strategy comparable to hBD-28. Experimental work demonstrated that trityl, acetamidomethyl, and t-butyl are superior to other protection strategies. However, the suitable pairwise arrangement of the protective groups can be different, as shown here for hBD-3 and hBD-28. Determination of the minimum inhibitory concentration against different bacteria revealed that hBD-27, in contrast to other beta-defensins tested, has virtually no antimicrobial activity. Compared to the other peptides tested, hBD-27 showed almost no cytotoxic activity, measured by hemoglobin release of erythrocytes. This might be due to the low positive net charge, which is significantly higher for hBD-2, hBD-3, and hBD-28.

Total chemical synthesis and oxidative folding of delta-conotoxin PVIA containing an N-terminal propeptide

Small disulfide-rich peptides are translated as larger precursors typically containing an N-terminal prepro sequence. In this study, we investigated the role of a propeptide in the oxidative folding of an extremely hydrophobic delta-conotoxin, PVIA. delta-Conotoxin PVIA (delta-PVIA) is a 29-amino acid neurotoxin stabilized by three disulfide bridges. Previous folding studies on delta-conotoxins revealed that their poor folding properties resulted from their hydrophobicity. However, low folding yields of delta-PVIA could be improved by the presence of a nonionic detergent, which acted as a chemical chaperone. delta-PVIA provided an attractive model to investigate whether the hydrophilic propeptide region could function as an intramolecular chaperone. A 58-amino acid precursor for delta-PVIA (pro-PVIA), containing the N-terminal propeptide covalently attached to the mature conotoxin, was synthesized using native chemical ligation. Oxidative folding of pro-PVIA resulted in a very low accumulation of the correctly folded form, comparable to that for the mature conotoxin delta-PVIA. Our results are in accord with the relevant data previously observed for alpha- and omega-conotoxins, indicating that conotoxin prepro sequences are so-called class II propeptides, which are not directly involved in the oxidative folding. We hypothesize that these propeptide regions may be important for interactions with protein folding catalysts and sorting receptors during the secretory process.

Functional analysis of chemically synthesized derivatives of the human CC chemokine CCL15/HCC-2, a high affinity CCR1 ligand

The CCL15 is a human CC chemokine that activates the receptors, CCR1 and CCR3. Unlike other chemokines, it contains an unusually long N-terminal domain of 31 amino acids preceding the first cysteine residue and a third disulfide bond. To elucidate the functional role of distinct structural determinants, a series of sequential amino-terminal truncated and point-mutated CCL15 derivatives as well as mutants lacking the third disulfide bond and the carboxy-terminal alpha-helix were synthesized using 9-fluorenylmethoxycarbonyl (Fmoc) chemistry. We demonstrate that a truncation of 24 amino acid residues (delta24-CCL15) converts the slightly active 92-residue delta0-CCL15 into a potent agonist of CC chemokine receptor 1 (CCR1) and a weak agonist of CCR3 in cell-based assays. The biological activity decreases from delta24-CCL15 to delta29-CCL15, and re-increases from delta29-CCL15 to delta30-CCL15. Thus, an exocyclic N-terminal region of only one amino acid residue is sufficient for efficient CCR1 activation. As none of the peptides investigated except for delta24-CCL15 activates CCR3, we suggest that CCR1 is the major receptor for CCL15 in vivo. Further we demonstrate that the third disulfide bond of CCL15 and an exchange of tyrosine in position 70 by a leucine residue, which is conserved in CXC chemokines, do not alter the interaction with CCR1. In contrast, a CCL15 derivative lacking the carboxy-terminal alpha-helix exhibits a complete loss of tertiary structure and hence loss of CCR1 agonistic and binding activity. This study demonstrates that specific protein residues in chemokines, which contribute to receptor-ligand interaction, vary significantly between chemokines and cannot be extrapolated using data from functionally related chemokines.

Solution structure of human proguanylin: the role of a hormone prosequence

The endogenous ligand of guanylyl cyclase C, guanylin, is produced as the 94-amino-acid prohormone proguanylin, with the hormone guanylin located at the COOH terminus of the prohormone. The solution structure of proguanylin adopts a new protein fold and consists of a three-helix bundle, a small three-stranded beta-sheet of two NH2-terminal strands and one COOH-terminal strand, and an unstructured linker region. The sequence corresponding to guanylin is fixed in its bioactive topology and is involved in interactions with the NH2-terminal beta-hairpin: the hormone region (residues 80-94) partly wraps around the first 4 NH2-terminal residues that thereby shield parts of the hormone surface. These interactions provide an explanation for the negligible bioactivity of the prohormone as well as the important role of the NH2-terminal residues in the disulfide-coupled folding of proguanylin. Since the ligand binding region of guanylyl cyclase C is predicted to be located around an exposed beta-strand, the intramolecular interactions observed between guanylin and its prosequence may be comparable with the guanylin/receptor interaction.

A novel guanylin family (guanylin, uroguanylin, and renoguanylin) in eels: possible osmoregulatory hormones in intestine and kidney

As the intestine is an essential organ for fish osmoregulation, the intestinal hormone guanylins may perform major functions, especially in euryhaline fish such as eels and salmonids. From the intestine of an eel, we identified cDNAs encoding three distinct guanylin-like peptides. Based on the sequence of mature peptide and sites of production, we named them guanylin, uroguanylin, and renoguanylin. Renoguanylin is a novel peptide that possesses the characteristics of both guanylin and uroguanylin and was abundantly expressed in the kidney. By immunohistochemistry, guanylin was localized exclusively in goblet cells, but not enterochromaffin cells, of the intestine. After transfer of eels from fresh water to seawater, mRNA expression of guanylin and uroguanylin did not change for 3 h, but it increased after 24 h. The increase was profound (2-6-fold) after adaptation to seawater. The expression of uroguanylin was also up-regulated in the kidney of seawater-adapted eels, but that of renoguanylin was not so prominent as other guanylins in both intestine and kidney. Collectively, the novel eel guanylin family appears to have important functions for seawater adaptation, particularly long-term adaptation. Eel guanylin may be secreted from goblet cells into the lumen with mucus in response to increased luminal osmolality and act on the epithelium to regulate water and salt absorption.

Identification and characterization of two novel clostridial bacteriocins, circularin A and closticin 574

Two novel antibacterial peptides of clostridial species were purified, N-terminally sequenced, and characterized. Moreover, their structural genes were identified. Closticin 574 is an 82-amino-acid bacteriocin produced by Clostridium tyrobutyricum ADRIAT 932. The supernatant of the producing strain showed a high level of activity against the indicator strain C. tyrobutyricum. The protein is synthesized as a preproprotein that is possibly secreted via the general secretion pathway, after which it is hydrolyzed at an Asp-Pro site. Circularin A is produced by Clostridium beijerinckii ATCC 25752 as a prepeptide of 72 amino acids. Cleavage of the prepeptide between the third leucine and fourth valine residues followed by a head-to-tail ligation between the N and C termini creates a circular antimicrobial peptide of 69 amino acids. The unusually small circularin A leader peptide of three amino acids is cleaved off in this process. The supernatant of C. beijerinckii ATCC 25752 showed a broad antibacterial activity range.

The micro domain responsible for ligand-binding of guanylyl cyclase C

Guanylyl cyclase C (GC-C), a member of membrane-bound guanylyl cyclases, is a receptor protein for guanylin and uroguanylin. The binding of a ligand to the extracellular domain of GC-C (ECD(GC-C)) triggers signal transduction, resulting in the regulation of intestinal fluids and electrolytes. A previous study proposed that a ligand-binding site on GC-C is localized near the transmembrane region. To further investigate the mechanism by which GC-C is activated, the C-terminal polypeptide (Met341-Gln407) of ECD(GC-C) (the micro domain), which includes the ligand-binding site, was over-expressed in Escherichia coli and its ligand-binding ability was examined. The micro domain showed ligand-binding activity (IC(50)=1 x 10(-8) M). This result clearly indicates that a ligand-binding site is located in close proximity to the membrane-bound region, and that the micro domain is capable of independently binding the ligand, without assistance from other domains. The use of this micro binding domain in the study of interactions between GC-C and ligands could be a useful tool and could lead to a better understanding of GC-C signal transduction.

Chemical synthesis of beta-defensins and LEAP-1/hepcidin

A large and steadily growing subfamily of antimicrobially active peptides of animals and plants is formed by the defensins, which are highly disulfide-bonded, cationic peptides with a molecular mass of about 4 kDa. The synthesis of the human beta-defensins 1 and 2 (hBD-1, hBD-2) as well as of the novel murine beta-defensins 7 and 8 (mBD-7 and mBD-8) is reported. The peptides were synthesized by solid-phase peptide synthesis using fluorenylmethoxycarbonyl chemistry. The linear products were oxidized in the presence of the cysteine/cystine redox system to the biologically active molecules. The correct disulfide connectivity of the resulting cyclic products was partly verified by mass spectrometry and sequence analysis of the fragments obtained after tryptic cleavage. In addition, the recently discovered antimicrobially active human peptide LEAP-1/hepcidin, which contains four disulfide bonds, was successfully synthesized and subsequently oxidized. For Liver-expressed anti microbial peptide (LEAP)-1/hepcidin and hBD-1, the identity of native and synthetic peptides was demonstrated by high-pressure liquid chromatography and capillary electrophoretic analysis. The general synthetic procedure is suitable to rapidly perform the total chemical synthesis of novel fully bioactive defensins, which are expected to be identified soon, as well as of structurally modified analogs.

High salt intake increases uroguanylin expression in mouse kidney

The intestinal peptides, guanylin and uroguanylin, may have an important role in the endocrine control of renal function. Both peptides and their receptor, guanylyl cyclase C (GC-C), are also expressed within the kidney, suggesting that they may act locally in an autocrine/paracrine fashion. However, their physiological regulation within the kidney has not been studied. To begin to address this issue, we evaluated the distribution of uroguanylin and guanylin messenger RNA (mRNA) in the mouse nephron and the regulation of renal expression by changes in dietary salt/water intake. Expression was determined in 1) wild-type mice, 2) two strains of receptor-guanylyl cyclase-deficient mice (ANP-receptor-deficient, GC-A-/-, and GC-C-deficient mice); and 3) cultured renal epithelial (M-1) cells, by RT-PCR, Northern blotting and immunocytochemistry. Renal uroguanylin messenger RNA expression was higher than guanylin and had a different distribution pattern, with highest levels in the proximal tubules, whereas guanylin was mainly expressed in the collecting ducts. Uroguanylin expression was significantly lower in GC-C-/- mice than in GC-A-/- and wild-types, suggesting that absence of a receptor was able to down-regulate ligand expression. Salt-loading (1% NaCl in drinking water) increased uroguanylin-mRNA expression by >1.8-fold but had no effect on guanylin expression. Uroguanylin but not guanylin transcripts were detected in M-1 cells and increased in response to hypertonic media (+NaCl or mannitol). Our results indicate that high-salt intake increases uroguanylin but not guanylin expression in the mouse kidney. The synthesis of these peptides by tubular epithelium may contribute to the local control of renal function and its adaptation to dietary salt.

Dual function of the propeptide of prouroguanylin in the folding of the mature peptide: disulfide-coupled folding and dimerization

Guanylyl cyclase activating peptide II (GCAP-II), an endogenous ligand of guanylyl cyclase C, is produced via the processing of the precursor protein (prepro-GCAP-II). We have previously shown that the propeptide in pro-GCAP-II functions as an intramolecular chaperone in the proper folding of the mature peptide, GCAP-II (Hidaka, Y., Ohno, M., Hemmasi, B., Hill, O., Forssmann, W.-G., and Shimonishi, Y. (1998) Biochemistry 37, 8498-8507). Here, we report an essential region in pro-GCAP-II for the correct disulfide pairing of the mature peptide, GCAP-II. Five mutant proteins, in which amino acid residues were sequentially deleted from the N terminus, and three mutant proteins of pro-GCAP-II, in which N-terminal 6, 11, or 17 amino acid residues were deleted, were overproduced using Escherichia coli or human kidney 293T cells, respectively. Detailed analysis of in vivo or in vitro folding of these mutant proteins revealed that one or two amino acid residues at the N terminus of pro-GCAP-II are critical, not only for the chaperone function in the folding but also for the net stabilization of pro-GCAP-II. In addition, size exclusion chromatography revealed that pro-GCAP-II exists as a dimer in solution. These data indicate that the propeptide has two roles in proper folding: the disulfide-coupled folding of the mature region and the dimerization of pro-GCAP-II.

Intramolecular chaperones: polypeptide extensions that modulate protein folding

Several prokaryotic and eukaryotic proteins are synthesized as precursors in the form of pre-pro-proteins. While the pre-regions function as signal peptides that are involved in transport, the propeptides can often catalyze correct folding of their associated proteins. Such propeptides have been termed intramolecular chaperones. In cases where propeptides may not directly catalyze the folding reaction, it appears that they can facilitate processes such as structural organization and oligomerization, localization, sorting and modulation of enzymatic activity and stability of proteins. Based on the available literature it appears that propeptides may actually function as 'post-translational modulators' of protein structure and function. Propeptides can be classified into two broad categories: Class I propeptides that function as intramolecular chaperones and directly catalyze the folding reaction; and Class II propeptides that are not directly involved in folding.

Regulated, side-directed secretion of proguanylin from isolated rat colonic mucosa

Guanylin, an activator of the guanylyl cyclase C receptor in the apical membrane of intestinal epithelium, modulates intestinal fluid and electrolyte transport. The bioactive 15-amino acid peptide originally isolated from rat intestine represents the C-terminal part of a longer, 115-residue prepropeptide. The aim of the present study was to characterize the direction and molecular form in which guanylin is secreted from the colonic mucosa, as well as the mechanisms that trigger its secretion. Isolated rat colonic mucosa was mounted in Ussing chambers, allowing the separate determination of apical and basolateral release. After HPLC purification, two different molecular forms of guanylin were identified in the apical incubation media by combining a bioassay for guanylyl cyclase C activation, a specific guanylin enzyme-linked immunosorbent assay and mass spectrometry, as well as sequence analysis: a bioactive form coeluting with synthetic 15-residue guanylin and the 94-residue propeptide, guanylin-22-115. The basal concentration of proguanylin at the apical side of epithelia was about 15-fold higher, compared with that of the small, bioactive peptide. In the basolateral incubation media, no proguanylin and only very low amounts of bioactive guanylin were detected. Incubation with carbachol led to a significant increase of about 7-fold in the release of proguanylin to both sides of the isolated epithelia. On the apical side, a concomitant increase of the small, bioactive peptide was observed; whereas, on the basolateral side, its concentration remained unchanged. Vasoactive intestinal peptide or the NO-donor S-nitroso-N-acetylpenicillamine did not affect guanylin secretion. Our results suggest that, in the intestine, guanylin is secreted mainly to the luminal side of the epithelium. The peptide is released as a 94-residue propeptide, which is then processed to a smaller, bioactive form (luminocrine secretion). Carbachol stimulates the release of proguanylin to both sides of the intestinal mucosa, but a parallel increase in the bioactive C-terminal derivative only occurs on the apical side. In vivo, the basolateral release could be a source of circulating proguanylin, which might be processed proteolytically to the active peptide in distant target tissues (endocrine secretion).