Protein tyrosine sulfation, 1993--an update

Tyrosine phosphorylation of recombinant hirudin increases affinity to thrombin and antithrombotic activity

Thrombin is one of the key enzymes of the blood coagulation system and a promising target for the development of anticoagulants. One of the most specific natural thrombin inhibitors is hirudin, contained in the salivary glands of medicinal leeches. The medicinal use of recombinant hirudin is limited because of the lack of sulfation on Tyr63, resulting in a 10-fold decrease in activity compared to native (sulfated) hirudin. In the present work, a set of hirudin derivatives was tested for affinity to thrombin: phospho-Tyr63, Tyr63(carboxymethyl)Phe, and Tyr63Glu mutants, which mimic Tyr63 sulfation and Gln65Glu mutant and lysine-succinylated hirudin, which enhance the overall negative charge of hirudin, as well as sulfo-hirudin and desulfo-hirudin as references. Using steered molecular dynamics simulations with subsequent umbrella sampling, phospho-hirudin was shown to exhibit the highest affinity to thrombin among all hirudin analogs, including native sulfo-hirudin; succinylated hirudin was also prospective. Phospho-hirudin exhibited the highest antithrombotic activity in in vitro assay in human plasma. Taking into account the modern methods for obtaining phospho-hirudin and succinylated hirudin, they are prospective as anticoagulants in clinical practice.

Small leucine rich proteoglycans: Biology, function and their therapeutic potential in the ocular surface

Small leucine rich proteoglycans (SLRPs) are the largest family of proteoglycans, with 18 members that are subdivided into five classes. SLRPs are small in size and can be present in tissues as glycosylated and non-glycosylated proteins, and the most studied SLRPs include decorin, biglycan, lumican, keratocan and fibromodulin. SLRPs specifically bind to collagen fibrils, regulating collagen fibrillogenesis and the biomechanical properties of tissues, and are expressed at particularly high levels in fibrous tissues, such as the cornea. However, SLRPs are also very active components of the ECM, interacting with numerous growth factors, cytokines and cell surface receptors. Therefore, SLRPs regulate major cellular processes and have a central role in major fundamental biological processes, such as maintaining corneal homeostasis and transparency and regulating corneal wound healing. Over the years, mutations and/or altered expression of SLRPs have been associated with various corneal diseases, such as congenital stromal corneal dystrophy and cornea plana. Recently, there has been great interest in harnessing the various functions of SLRPs for therapeutic purposes. In this comprehensive review, we describe the structural features and the related functions of SLRPs, and how these affect the therapeutic potential of SLRPs, with special emphasis on the use of SLRPs for treating ocular surface pathologies.

A lentiviral vector B cell gene therapy platform for the delivery of the anti-HIV-1 eCD4-Ig-knob-in-hole-reversed immunoadhesin

Barriers to effective gene therapy for many diseases include the number of modified target cells required to achieve therapeutic outcomes and host immune responses to expressed therapeutic proteins. As long-lived cells specialized for protein secretion, antibody-secreting B cells are an attractive target for foreign protein expression in blood and tissue. To neutralize HIV-1, we developed a lentiviral vector (LV) gene therapy platform for delivery of the anti-HIV-1 immunoadhesin, eCD4-Ig, to B cells. The EμB29 enhancer/promoter in the LV limited gene expression in non-B cell lineages. By engineering a knob-in-hole-reversed (KiHR) modification in the CH3-Fc eCD4-Ig domain, we reduced interactions between eCD4-Ig and endogenous B cell immunoglobulin G proteins, which improved HIV-1 neutralization potency. Unlike previous approaches in non-lymphoid cells, eCD4-Ig-KiHR produced in B cells promoted HIV-1 neutralizing protection without requiring exogenous TPST2, a tyrosine sulfation enzyme required for eCD4-Ig-KiHR function. This finding indicated that B cell machinery is well suited to produce therapeutic proteins. Lastly, to overcome the inefficient transduction efficiency associated with VSV-G LV delivery to primary B cells, an optimized measles pseudotyped LV packaging methodology achieved up to 75% transduction efficiency. Overall, our findings support the utility of B cell gene therapy platforms for therapeutic protein delivery.

Sulfation of a FLAG tag mediated by SLC35B2 and TPST2 affects antibody recognition

A FLAG tag consisting of DYKDDDDK is an epitope tag that is frequently and widely used to detect recombinant proteins of interest. In this study, we performed a CRISPR-based genetic screening to identify factors involved in the detection of a FLAG-tagged misfolded model protein at the cell surface. In the screening, SLC35B2, which encodes 3’-phosphoadenosine-5’-phosphosulfate transporter 1, was identified as the candidate gene. The detection of FLAG-tagged misfolded proteins at the cell surface was significantly increased in SLC35B2-knockout cells. Furthermore, protein tyrosine sulfation mediated by tyrosyl-protein sulfotransferase 2 (TPST2) suppressed FLAG-tagged protein detection. Localization analysis of the FLAG-tagged misfolded proteins confirmed that defects in tyrosine sulfation are only responsible for enhancing anti-FLAG staining on the plasma membrane but not inducing the localization change of misfolded proteins on the plasma membrane. These results suggest that a FLAG tag on the misfolded protein would be sulfated, causing a reduced detection by the M2 anti-FLAG antibody. Attention should be required when quantifying the FLAG-tagged proteins in the secretory pathway.

Characterization and prediction of positional 4-hydroxyproline and sulfotyrosine, two post-translational modifications that can occur at substantial levels in CHO cells-expressed biotherapeutics

Biotherapeutics may contain a multitude of different post-translational modifications (PTMs) that need to be assessed and possibly monitored and controlled to ensure reproducible product quality. During early development of biotherapeutics, unexpected PTMs might be prevented by in silico identification and characterization together with further molecular engineering. Mass determinations of a human IgG1 (mAb1) and a bispecific IgG-ligand fusion protein (BsAbA) demonstrated the presence of unusual PTMs resulting in major +80 Da, and +16/+32 Da chain variants, respectively. For mAb1, analytical cation exchange chromatography demonstrated the presence of an acidic peak accounting for 20%. A + 79.957 Da modification was localized within the light chain complementarity-determining region-2 and identified as a sulfation based on accurate mass, isotopic distribution, and a complete neutral loss reaction upon collision-induced dissociation. Top-down ultrahigh resolution MALDI-ISD FT-ICR MS of modified and unmodified Fabs allowed the allocation of the sulfation to a specific Tyr residue. An aspartate in amino-terminal position-3 relative to the affected Tyr was found to play a key role in determining the sulfation. For BsAbA, a + 15.995 Da modification was observed and localized to three specific Pro residues explaining the +16 Da chain A, and +16 Da and +32 Da chain B variants. The BsAbA modifications were verified as 4-hydroxyproline and not 3-hydroxyproline in a tryptic peptide map via co-chromatography with synthetic peptides containing the two isomeric forms. Finally, our approach for an alert system based on in-house in silico predictors is presented. This system is designed to prevent these PTMs by molecular design and engineering during early biotherapeutic development.

Genome-wide analysis of sulfotransferase genes and their responses to abiotic stresses in Chinese cabbage (Brassica rapa L.)

Sulfotransferases (SOTs; EC 2.8.2.-), which are widespread from prokaryotes to eukaryotes, constitute a multi-protein family that plays crucial roles in plant growth, development and stress adaptation. However, this family has not been systemically investigated in Brassica rapa. Here, a genome-wide systemic analysis of SOT genes in B. rapa subsp. pekinensis, a globally cultivated vegetable, were conducted. We identified 56 SOT genes from the whole B. rapa genome using Arabidopsis SOT sequences as queries and classified them into nine groups, rather than the eight groups of previous research. 56 B. rapa SOT genes (BraSOTs) were distributed on all 10 chromosomes except for chromosome 5. Of these, 27 BraSOTs were distributed in seven clusters on five chromosomes (ChrA01, ChrA02, Chr03, ChrA07, and Chr09). Among the BraSOT proteins, 48 had only one SOT_1 domain and 6 had two, while 2 had one SOT_3 domain. Additionally, 47 BraSOT proteins contained only known SOT domains. The remaining nine proteins, five in group-VIII and two in group-IX, contained additional transmembrane domains. Specific motif regions I and IV for 3′-phosphoadenosine 5′-phosphosulfate binding were found in 41 BraSOT proteins. Introns were present in only 18 BraSOT genes, and all seven BraSOT genes in groups VIII and IX had more than three introns. To identify crucial SOTs mediating the response to abiotic stress in B. rapa, expression changes in 56 BraSOT genes were determined by quantitative RT-PCR after drought, salinity, and ABA treatments, and some BraSOT genes were associated with NaCl, drought and ABA stress, e.g. Bra017370, Bra009300, Bra027880.

Protein Interaction with Charged Macromolecules: From Model Polymers to Unfolded Proteins and Post-Translational Modifications

Interaction of proteins with charged macromolecules is involved in many processes in cells. Firstly, there are many naturally occurred charged polymers such as DNA and RNA, polyphosphates, sulfated glycosaminoglycans, etc., as well as pronouncedly charged proteins such as histones or actin. Electrostatic interactions are also important for "generic" proteins, which are not generally considered as polyanions or polycations. Finally, protein behavior can be altered due to post-translational modifications such as phosphorylation, sulfation, and glycation, which change a local charge of the protein region. Herein we review molecular modeling for the investigation of such interactions, from model polyanions and polycations to unfolded proteins. We will show that electrostatic interactions are ubiquitous, and molecular dynamics simulations provide an outstanding opportunity to look inside binding and reveal the contribution of electrostatic interactions. Since a molecular dynamics simulation is only a model, we will comprehensively consider its relationship with the experimental data.

Sulfonation, an underexploited area: from skeletal development to infectious diseases and cancer

Sulfonation is one of the most abundant cellular reactions modifying a wide range of xenobiotics as well as endogenous molecules which regulate important biological processes including blood clotting, formation of connective tissues, and functionality of secreted proteins, hormones, and signaling molecules. Sulfonation is ubiquitous in all tissues and widespread in nature (plants, animals, and microorganisms). Although sulfoconjugates were discovered over a century ago when, in 1875, Baumann isolated phenyl sulfate in the urine of a patient given phenol as an antiseptic, the significance of sulfonation and its roles in human diseases have been underappreciated until recent years. Here, we provide a current overview of the significance of sulfonation reactions in a variety of biological functions and medical conditions (with emphasis on cancer). We also discuss research areas that warrant further attention if we are to fully understand how deficiencies in sulfonation could impact human health which, in turn, could help define treatments to effect improvements in health.

Updated perspectives on the cytosolic sulfotransferases (SULTs) and SULT-mediated sulfation

The cytosolic sulfotransferases (SULTs) are Phase II detoxifying enzymes that mediate the sulfate conjugation of numerous xenobiotic molecules. While the research on the SULTs has lagged behind the research on Phase I cytochrome P-450 enzymes and other Phase II conjugating enzymes, it has gained more momentum in recent years. This review aims to summarize information obtained in several fronts of the research on the SULTs, including the range of the SULTs in different life forms, concerted actions of the SULTs and other Phase II enzymes, insights into the structure-function relationships of the SULTs, regulation of SULT expression and activity, developmental expression of SULTs, as well as the use of a zebrafish model for studying the developmental pharmacology/toxicology.

Determinants of tyrosylprotein sulfation coding and substrate specificity of tyrosylprotein sulfotransferases in metazoans

This short review likes to give a historical view on the discovery of metazoan Tyrosylprotein Sulfotransferases (TPSTs) setting its focus on the determinants of substrate specificity of these enzymes and on the hitherto knowledge of the sulfation coding mechanism. Weak points of the to-date models of sulfation coding will be outlined and a more detailed and complex view on tyrosylprotein-sulfation coding will be presented with respect to recent cellular investigations on TPSTs.

The multi-protein family of sulfotransferases in plants: composition, occurrence, substrate specificity, and functions

All members of the sulfotransferase (SOT, EC 2.8.2.-) protein family transfer a sulfuryl group from the donor 3'-phosphoadenosine 5'-phosphosulfate (PAPS) to an appropriate hydroxyl group of several classes of substrates. The primary structure of these enzymes is characterized by a histidine residue in the active site, defined PAPS binding sites and a longer SOT domain. Proteins with this SOT domain occur in all organisms from all three domains, usually as a multi-protein family. Arabidopsis thaliana SOTs, the best characterized SOT multi-protein family, contains 21 members. The substrates for several plant enzymes have already been identified, such as glucosinolates, brassinosteroids, jasmonates, flavonoids, and salicylic acid. Much information has been gathered on desulfo-glucosinolate (dsGl) SOTs in A. thaliana. The three cytosolic dsGl SOTs show slightly different expression patterns. The recombinant proteins reveal differences in their affinity to indolic and aliphatic dsGls. Also the respective recombinant dsGl SOTs from different A. thaliana ecotypes differ in their kinetic properties. However, determinants of substrate specificity and the exact reaction mechanism still need to be clarified. Probably, the three-dimensional structures of more plant proteins need to be solved to analyze the mode of action and the responsible amino acids for substrate binding. In addition to A. thaliana, more plant species from several families need to be investigated to fully elucidate the diversity of sulfated molecules and the way of biosynthesis catalyzed by SOT enzymes.

Functional implications of the Golgi and microtubular network in gonadotropes

In contrast to the widely accepted images of the Golgi apparatus as a cup-like shape, the Golgi in pituitary gonadotropes is organized as a spherical shape in which the outer and inner faces are cis- and trans-Golgi elements, respectively. At the center of the spherical Golgi, a pair of centrioles is situated as a microtubule-organizing center from which radiating microtubules isotropically extend toward the cell periphery. This review focuses on the significance of the characteristic organization of the Golgi and microtubule network in gonadotropes, considering the roles of microtubule-dependent membrane transport in the formation and maintenance of the Golgi structure. Because the highly symmetrical organization of the Golgi is possibly perturbed in response to experimental treatments of gonadotropes, monitoring of the Golgi structure in gonadotropes under various experimental conditions will be a novel in vivo approach to elucidate the biogenesis of the Golgi apparatus.

Effect of poly(phosphate) anions on glyceraldehyde-3-phosphate dehydrogenase structure and thermal aggregation: comparison with influence of poly(sulfoanions)

BACKGROUND

It is well documented that poly(sulfate) and poly(sulfonate) anions suppress protein thermal aggregation much more efficiently than poly(carboxylic) anions, but as a rule, they denature protein molecules. In this work, a polymer of different nature, i.e. poly(phosphate) anion (PP) was used to elucidate the influence of phosphate groups on stability and thermal aggregation of the model enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH).

METHODS

Isothermal titration calorimetry and differential scanning calorimetry were used for studying the protein-polyanion interactions and the influence of bound polyanions on the protein structure. The enzymatic activity of GAPDH and size of the complexes were measured. The aggregation level was determined from the turbidity.

RESULTS

Highly polymerized PP chains were able to suppress the aggregation completely, but at significantly higher concentrations as compared with poly(styrenesulfonate) (PSS) or dextran sulfate chains of the same degree of polymerization. The effect of PP on the enzyme structure and activity was much gentler as opposed to the binding of dextran sulfate or, especially, PSS that denatured GAPDH molecules with the highest efficacy caused by short PSS chains. These findings agreed well with the enhanced affinity of polysulfoanions to GAPDH.

CONCLUSIONS

The revealed trends might help to illuminate the mechanism of control of proteins functionalities by insertion of charged groups of different nature through posttranslational modifications.

GENERAL SIGNIFICANCE

Practical implementation of the results could be the use of PP chains as promising tools to suppress the proteins aggregation without noticeable loss in the enzymatic activity.

Nutritional essentiality of sulfur in health and disease

Sulfur is the seventh most abundant element measurable in the human body and is supplied mainly by the intake of methionine (Met), an indispensable amino acid found in plant and animal proteins. Met controls the initiation of protein synthesis, governs major metabolic and catalytic activities, and may undergo reversible redox processes safeguarding protein integrity. Withdrawal of Met from customary diets causes the greatest downsizing of lean body mass following either unachieved replenishment (malnutrition) or excessive losses (inflammation). These physiopathologically unrelated morbidities nevertheless stimulate comparable remethylation reactions from homocysteine, indicating that Met homeostasis benefits from high metabolic priority. Inhibition of cystathionine-β-synthase activity causes the upstream sequestration of homocysteine and the downstream drop in cysteine and glutathione. Consequently, the enzymatic production of hydrogen sulfide and the nonenzymatic reduction of elemental sulfur to hydrogen sulfide are impaired. Sulfur operates as cofactor of several enzymes critically involved in the regulation of oxidative processes. A combination of malnutrition and nutritional deprivation of sulfur maximizes the risk of cardiovascular disorders and stroke, constituting a novel clinical entity that threatens plant-eating population groups.

PredSulSite: prediction of protein tyrosine sulfation sites with multiple features and analysis

Tyrosine sulfation is a ubiquitous posttranslational modification that regulates extracellular protein-protein interactions, intracellular protein transportation modulation, and protein proteolytic process. However, identifying tyrosine sulfation sites remains a challenge due to the lability of sulfation sequences. In this study, we developed a method called PredSulSite that incorporates protein secondary structure, physicochemical properties of amino acids, and residue sequence order information based on support vector machine to predict sulfotyrosine sites. Three types of encoding algorithms-secondary structure, grouped weight, and autocorrelation function-were applied to mine features from tyrosine sulfation proteins. The prediction model with multiple features achieved an accuracy of 92.89% in 10-fold cross-validation. Feature analysis showed that the coil structure, acidic amino acids, and residue interactions around the tyrosine sulfation sites all contributed to the sulfation site determination. The detailed feature analysis in this work can help us to understand the sulfation mechanism and provide guidance for the related experimental validation. PredSulSite is available as a community resource at http://www.bioinfo.ncu.edu.cn/inquiries_PredSulSite.aspx.

Posttranslational processing of progastrin

Gastrin and cholecystokinin (CCK) are homologous hormones with important functions in the brain and the gut. Gastrin is the main regulator of gastric acid secretion and gastric mucosal growth, whereas cholecystokinin regulates gall bladder emptying, pancreatic enzyme secretion and besides acts as a major neurotransmitter in the central and peripheral nervous systems. The tissue-specific expression of the hormones is regulated at the transcriptional level, but the posttranslational phase is also decisive and is highly complex in order to ensure accurate maturation of the prohormones in a cell specific manner. Despite the structural similarities of gastrin and CCK, there are decisive differences in the posttranslational processing and secretion schemes, suggesting that specific features in the processing may have evolved to serve specific purposes. For instance, CCK peptides circulate in low picomolar concentrations, whereas the cellular expression of gastrin is expressed at higher levels, and accordingly gastrin circulates in 10-20-fold higher concentrations. Both common cancers and the less frequent neuroendocrine tumors express the gastrin gene and prohormone. But the posttranslational processing progastrin is often greatly disturbed in neoplastic cells.The posttranslational phase of the biogenesis of gastrin and the various progastrin products in gastrin gene-expressing tissues is now reviewed here. In addition, the individual contributions of the processing enzymes are discussed, as are structural features of progastrin that are involved in the precursor activation process. Thus, the review describes how the processing depends on the cell-specific expression of the processing enzymes and kinetics in the secretory pathway.

A target-specific approach for the identification of tyrosine-sulfated hemostatic proteins

A simple methodology for the identification of hemostatic proteins that are subjected to posttranslational tyrosine sulfation was developed. The procedure involves sequence analysis of members of the three hemostatic pathways using the Sulfinator prediction algorithm, followed by [(35)S]sulfate labeling of cultured HepG2 human hepatoma cells, immunoprecipitation of targeted [(35)S]sulfate-labeled hemostatic proteins, and tyrosine O-[(35)S]sulfate analysis of immunoprecipitated proteins. Three new tyrosine-sulfated hemostatic proteins-protein S, prekallikrein, and plasminogen-were identified. Such a target-specific approach will allow investigation of tyrosine-sulfated proteins of other biochemical/physiological pathways/processes and contribute to a better understanding of the functional role of posttranslational tyrosine sulfation.

Tyrosine modification enhances metal-ion binding

Tyrosine sulfation is a common modification of many proteins, and the ability to phosphorylate tyrosine residues is an intrinsic property of many growth-factor receptors. In the present study, we have utilized the peptide hormone CCK(8) (cholecystokinin), which occurs naturally in both sulfated and unsulfated forms, as a model to investigate the effect of tyrosine modification on metal-ion binding. The changes in absorbance and fluorescence emission on Fe(3+) binding indicated that tyrosine sulfation or phosphorylation increased the stoichiometry from 1 to 2, without greatly affecting the affinity (0.6-2.8 microM at pH 6.5). Measurement of Ca(2+) binding with a Ca(2+)-selective electrode revealed that phosphorylated CCK(8) bound two Ca(2+) ions. CCK(8) and sulfated CCK(8) each bound only one Ca(2+) ion with lower affinity. Binding of Ca(2+), Zn(2+) or Bi(3+) to phosphorylated CCK(8) did not cause any change in absorbance, but substantially increased the change in absorbance on subsequent addition of Fe(3+). The results of the present study demonstrate that tyrosine modification may increase the affinity of metal-ion binding to peptides, and imply that metal ions may directly regulate many signalling pathways.

Tyrosine O-sulfation

The O-sulfation of tyrosine residues of plasma membrane and secretory proteins that transit through the secretory pathway of eukaryotic cells is a widespread post-translational modification. This enzymatic reaction is catalyzed by trans-Golgi-associated tyrosylprotein sulfotransferases, which recognize tyrosine residues located in a specific acidic amino acid sequence. Tyrosine sulfation promotes extracellular protein-protein interactions involved in diverse biological processes, ranging from the receptor binding of regulatory peptides to the interaction of viral envelope proteins with the cell surface. This unit outlines procedures to determine whether a protein of interest contains sulfated tyrosine residues, using methods based on labeling proteins with inorganic [35S]-sulfate, alkaline hydrolysis, and one-dimensional thin-layer electrophoresis.

Choice of cellular protein expression system

Recombinant protein expression has become a standard laboratory tool, and a wide variety of systems and techniques are now in use. Because there are so many systems to choose from, the investigator has to be careful to use the combination that will give the best results for the protein being studied. This overview unit discusses expression and production choices, including post-translational modifications (e.g., glycosylation, acylation, sulfation, and removal of N-terminal methionine), in vivo and in vitro folding, and influence of downstream elements on expression.

The essentiality of sulfur is closely related to nitrogen metabolism: a clue to hyperhomocysteinaemia

N and S metabolisms are closely interwoven throughout both the plant and animal kingdoms. The essentiality of S relates to its participation in the structure of S-containing amino acids (SAA), to its inclusion in many sulfonated molecules, and to a myriad of metabolic and catalytic reactions of vital importance. Methionine (Met) is the indispensable SAA supplied by food proteins and its plasma homeostasis is achieved via a number of highly efficient regulatory mechanisms. In all conditions characterised by a negative body protein balance such as in dietary restriction or cytokine-induced hypercatabolic losses, N and S endogenous pools manifest parallel tissue depletion rates. Adaptive conservation of N and S body stores is reached by a functional restraint of the trans-sulfuration cascade, through the depression of cystathionine β-synthase activity. As a result, upstream accumulation of homocysteine favours its re-methylation conversion to Met which helps maintain metabolic pathways of survival value. In addition to the measurement of vitamin indices, that of plasma transthyretin, a sensitive marker of protein nutritional status, is proposed to identify the fluctuations of the total body N component accountable for the alterations of homocysteine concentrations in body fluids.

Sulfation, the up-and-coming post-translational modification: its role and mechanism in protein-protein interaction

Tyrosine sulfation is a post-translational modification entailing covalent attachment of sulfate to tyrosine residues. It takes place in the trans-Golgi, is necessary for the bioactivity of some proteins, and improves their ability to interact with other proteins. In the present work, we show that a protein containing a sulfated tyrosine with a delocalized negative charge forms a salt bridge with another protein if it has two or more adjacent arginine residues containing positive delocalized charges. These noncovalent complexes are so stable that, when submitted to collision induced dissociation, the peptides forming the complex dissociate. Just one covalent bond fragments, the covalent bond between the tyrosine oxygen and the SO3 sulfur, and is represented by the appearance of a new peak (basic peptide + SO3), suggesting that in some instances covalent bonds will break down before the noncovalent bonds between the arginine guanidinium and SO3 dissociate. The data implies that the dissociation pathway is preferred; however, fragmentation between tyrosine and the sulfate residue is a major pathway.

Structural Differences in the Hinge Region of the Glycoprotein Hormone Receptors: Evidence from the Sulfated Tyrosine Residues

Tyrosine sulfation is a late posttranslational modification of proteins that takes place in the Golgi network. In the past few years, this process has been identified as an important modulator of protein-protein interactions. Sulfated tyrosine residues have recently been identified in the C-terminal, so-called hinge region of the ectodomain of glycoprotein hormone receptors [TSH, LH/chorionic gonadotropin (CG), and FSH receptors] and were shown to play an important role in the interaction with their natural ligands. The position of two sulfated tyrosine residues in a Y-D/E-Y motif appears perfectly conserved in the alignment of TSH and LH receptors from different species, and site-directed mutagenesis experiments demonstrated that sulfation of the first residue of this motif was responsible for the functional effect on hormone binding. In contrast, the corresponding motif is not conserved in the FSH receptor, in which the first tyrosine residue is missing: the Y-D/E-Y motif is replaced by F(333)DY(335). We extend here our previous observation that, in this case, it is sulfation of the second sole tyrosine residue in the motif that is functionally important. An LH/CG receptor harboring an F(331)DY(333) motif (i.e. displaying decreased sensitivity to human CG) was used as a backbone in which short portions of the FSH receptor were substituted. Segments from the FSH receptor capable of restoring sensitivity to human CG were identified by transfection of the chimeras in COS-7 cells. These experiments identified key amino acid residues in the hinge region of the FSH receptor associated with the functional role of the second sulfated tyrosine residue in a Y-D/E-Y motif, allowing for efficient hormone binding. The experiments represent strong evidence that structural differences in the hinge regions of FSH and LH/CG receptors play a significant role in hormone-receptor-specific recognition.

The plasminogen-like molecule apically secreted by epithelial thyroid cells is sulfated

Plasminogen (Pl), a circulating protease synthesized in the liver, is also present in several tissues. In the thyroid gland a Pl-like protease was found in the apical lumen where it is involved, through its proteolytic activity, in luminal degradation of thyroglobulin (Tg). Here, we showed for the first time that the Pl-like protease apically secreted by epithelial thyroid cells is sulfated, both on tyrosine residue(s) and on oligosaccharide side chains. The Pl molecule is composed of a large N-terminal moiety made of five distinct Kringle domains (K1-K5) separated by small peptidic fragments, and of a C-terminal domain with serine protease activity. Using a software tool able to predict tyrosine sulfation sites in protein sequences we localized the potential tyrosine sulfation sites of Pl. Then, we became aware that, whatever the species considered, at least three of the four potential tyrosine sulfation sites of Pl were located on Kringle sites, and more precisely, for K1, on the highly conserved binding domain of K1. We determined with the same software tool which potential sulfation sites were the most likely to be really sulfated. We hypothesize that the sulfation of these sites modulates the binding properties of Pl.

Analysis of acidic peptides with a matrix-assisted laser desorption/ionization mass spectrometry using positive and negative ion modes with additive monoammonium phosphate

Acidic PTMs such as phosphorylation and sulfonation of proteins are known to play important roles in many cellular processes including signal transductions and protein-protein interactions. In MS, the acidic modified peptides, that have negative charge, are observable in negative ion mode rather than in positive ion mode. Moreover, addition of ammonium salt into MALDI matrix solution improves the relative intensity of ionization of the phosphorylated peptide to unmodified one. We demonstrate that a combination of the negative ion mode and addition of ammonium salt is more effective in the ionization of the acidic modified peptides. We applied this method to 2-DE separated proteins of Caenorhabditis elegans. As a result, 42 spots were identified as modified proteins, of which 34 proteins were nonoverlapping unique proteins. Furthermore, our study revealed that pI shifts of the DIM-1 and MLC-1 proteins in the 2-DE gel were attributed to the presence of the acidic modifications. The negative ion mode together with the addition of ammonium salt provides us a useful method to detect the phosphorylation and/or sulfonation of protein in a simple manner.

Tissue Distribution and Ontogeny of Sulfotransferase Enzymes in Mice

Sulfotransferases (Sults) are phase-II conjugation enzymes that catalyze the transfer of a sulfonate group from 3'-phosphoadenosine 5'-phosphosulfate (PAPS) to target endo and xenobiotics. PAPS is formed from inorganic sulfate by the action of the enzyme PAPS synthase (PAPSs). In the present study, the tissue distribution and developmental changes in the mRNA expression of 11 Sult isozymes and 2 PAPSs isoforms in mice were quantified. Sult1a1, 1b1, 1c1, 1c2, 1d1, 1e1, 2a1/2, 2b1, 3a1, 4a1, 5a1, PAPSs1, and PAPSs2 mRNA expression was quantified in 14 tissues from male and female mice using the branched DNA signal amplification assay. Sult2a1/2 and 3a1 expression were highest in liver; Sult1b1, 2b1, and PAPSs2 in small intestine; Sult1a1 in large intestine; Sult1c2 in stomach; Sult1d1 in kidney; Sult1e1 in placenta; and Sult4a1 in brain. Sult1c1, 5a1, and PAPSs1 were ubiquitously expressed in most tissues. These enzymes demonstrated three different ontogenic expression patterns in liver. Sult1a1, 1c2, 1d1, 2a1/2, and PAPSs2 hepatic expression gradually increased from birth until about 3 weeks of age and then declined somewhat thereafter, Sult1c1 expression was highest before birth and declined after that, and Sult3a1 mRNA expression was very low in fetal livers and remained low until 30 days of age, when expression in females dramatically increased, whereas it never increased in males. The organ-specific distribution of Sults as well as the different expression of the Sults in young animals may affect the pharmacokinetic behavior and organ-specific toxicity of xenobiotics.

Protein tyrosine-O-sulfation analysis by exhaustive product ion scanning with minimum collision offset in a NanoESI Q-TOF tandem mass spectrometer

Tyrosine-O-sulfated peptides were studied by nanoESI Q-TOF mass spectrometry and were found to exhibit an abundant loss of SO3 in positive ion mode under the usually nonfragmenting conditions of survey spectrum acquisition. A new strategy for the detection of tyrosine-O-sulfated peptides in total protein digests was designed based on exhaustive product ion scanning at the collision offset conditions typical for the recording of survey spectra (minimum collision offset). From these data, Q-TOF neutral loss scans for loss of 80/z and Q-TOF precursor ions scans were extracted. The specificity of this approach for analysis of tyrosine-O-sulfation was tested using a tryptic digest of bovine serum albumin spiked with sulfated hirudin (1:1 and 1000:1 molar ratio of BSA to sulfated hirudin, respectively) and using an in-solution digest of the recombinant extracellular domain of thyroid stimulating hormone receptor (ECD-TSHr). For both examples, the combination of in silico neutral loss scans for 80/z and subsequent in silico precursor ion scans resulted in a specific identification of sulfated peptides. In the analysis of recombinant ECD-TSHr, a doubly sulfated peptide could be identified in this way. Surprisingly, approximately 1/4 of the product ion spectra acquired from the tryptic digest of ECD-TSHr at minimum collision offset exhibited sequence-specific ions suitable for peptide identification. Complementary ion pairs were frequently observed, which either were b2/y(max-2) pairs or were induced by cleavage N-terminal to proline. MS/MS analysis at minimum collision offset followed by extraction of neutral loss and precursor ion scans is ideally suited for highly sensitive detection of analyte ions which exhibit facile gas-phase decomposition reactions.

Cell type-specific transgene expression of the prion protein in Xenopus intermediate pituitary cells

The cellular form of prion protein (PrPC) is anchored to the plasma membrane of the cell and expressed in most tissues, but predominantly in the brain, including in the pituitary gland. Thus far, the biosynthesis of PrPC has been studied only in cultured (transfected) tumour cell lines and not in primary cells. Here, we investigated the intracellular fate of PrPCin vivo by using the neuroendocrine intermediate pituitary melanotrope cells of the South-African claw-toed frog Xenopus laevis as a model system. These cells are involved in background adaptation of the animal and produce high levels of its major secretory cargo proopiomelanocortin (POMC) when the animal is black-adapted. The technique of stable Xenopus transgenesis in combination with the POMC gene promoter was used as a tool to express Xenopus PrPC amino-terminally tagged with the green fluorescent protein (GFP-PrPC) specifically in the melanotrope cells. The GFP-PrPC fusion protein was expressed from stage-25 tadpoles onwards to juvenile frogs, the expression was induced on a black background and the fusion protein was subcellularly located mainly in the Golgi apparatus and at the plasma membrane. Pulse-chase metabolic cell labelling studies revealed that GFP-PrPC was initially synthesized as a 45-kDa protein that was subsequently stepwise glycosylated to 48-, 51-, and eventually 55-kDa forms. Furthermore, we revealed that the mature 55-kDa GFP-PrPC protein was sulfated, anchored to the plasma membrane and cleaved to a 33-kDa product. Despite the high levels of transgene expression, the subcellular structures as well as POMC synthesis and processing, and the secretion of POMC-derived products remained unaffected in the transgenic melanotrope cells. Hence, we studied PrPC in a neuroendocrine cell and in a well-defined physiological context.

Ion-selective enrichment of tyrosine-sulfated peptides from complex protein digests

We have developed a novel procedure for concentrating sulfated peptides, as a front end to mass spectrometric analysis, based on ion-selective interaction of sulfate ions with anion exchangers. Ions with a higher charge and smaller solvated ion radius, such as sulfate ions, have higher retention in an ion exchanger due to their greater degree of coulombic interactions. We tested the effectiveness of this approach for enrichment and identification of sulfated peptides using a tryptic digest of bovine serum albumin spiked with model sulfated peptide (molar ratio 20:1) and using a tryptic digest of bovine fibrinogen. Sulfated peptides are identified by mass spectrometry in which both the molecular ion and its specific fragment ion produced by facile loss of SO(3) are detected. In both experiments, sulfated peptides were strongly retained on the anion exchanger and were eluted by higher concentrations of competing ion with minimal contamination of nonsulfated peptides. Using this procedure, we determined that the 13-amino acid C-terminal peptide of the minor gamma'-chain of bovine fibrinogen contains sulfated tyrosine.

Role of sulfated tyrosines of thyroglobulin in thyroid hormonosynthesis

Our previous studies showed that sulfated tyrosines (Tyr-S) are involved in thyroid hormone synthesis and that Tyr(5), the main hormonogenic site of thyroglobulin (Tg), is sulfated. In the present paper, we studied the role of Tyr-S in the formation and activity of the peroxidase-Tg complex. Results show that noniodinated (35)SO(3)-Tg specifically binds (Kd=1.758 microM) to immobilized lactoperoxidase (LPO) via Tyr-S linkage by using saturation binding and competition experiments. We found that NIFEY-S, a 15-amino acid peptide corresponding to the NH2-end sequence of Tg and containing the hormonogenic acceptor Tyr5-S, was a better competitor than cholecystokinin and Tyr-S. 35SO3-Tg, iodinated without peroxidase, bound to LPO with a Kd (1.668 microM) similar to that of noniodinated Tg, suggesting that 1) its binding occurs via Tyr-S linkage and 2) Tyr-S requires peroxidase to be iodinated, whereas nonsulfated Tyr does not. Iodination of NIFEY-S with [125I]iodide showed that Tyr5-S iodination increased with LPO concentration, whereas iodination of a nonsulfated peptide containing the donor Tyr130 was barely dependent on LPO concentration. Enzymatic hydrolysis of iodinated Tg or NIFEY-S showed that the amounts of sulfated iodotyrosines also depended on LPO amount. Sulfated iodotyrosines were detectable in the enzyme-substrate complex, suggesting they have a short life before the coupling reaction occurs. Our data suggest that after Tyr-S binding to peroxidase where it is iodinated, the sulfate group is removed, releasing an iodophenoxy anion available for coupling with an iodotyrosine donor.

Requirement of tyrosylprotein sulfotransferase-A for proper cuticle formation in the nematode C. elegans

Tyrosine O-sulfation is one of the post-translational modification processes that occur to membrane proteins and secreted proteins in eukaryotes. Tyrosylprotein sulfotransferase (TPST) is responsible for this modification, and in this report, we describe the expression pattern and the biological role of TPST-A in the nematode Caenorhabditis elegans. We found that TPST-A was mainly expressed in the hypodermis, especially in the seam cells. Reduction of TPST-A activity by RNAi caused severe defects in cuticle formation, indicating that TPST-A is involved in the cuticle formation in the nematode. We found that RNAi of TPST-A suppressed the roller phenotype caused by mutations in the rol-6 collagen gene, suggesting that sulfation of collagen proteins may be important for proper organization of the extracellular cuticle matrix. The TPST-A RNAi significantly decreased the dityrosine level in the worms, raising the possibility that the sulfation process may be a pre-requisite for the collagen tyrosine cross-linking.

Suppression of DHEA sulfotransferase (Sult2A1) during the acute-phase response

The acute-phase response (APR) induces alterations in lipid metabolism, and our data suggest that this is associated with suppression of type II nuclear hormone receptors that are key regulators of fatty acid, cholesterol, and bile acid metabolism. Recently, the farnesoid X receptor (FXR), constitutive androstane receptor (CAR), and pregnane X receptor (PXR) were found to regulate DHEA sulfotransferase (Sult2A1), which plays an important role in DHEA sulfation and detoxification of bile acids. Because FXR, PXR, and CAR are suppressed during the APR, we hypothesized that Sult2A1 is downregulated during the APR. To induce the APR, mice were treated with LPS, which will then trigger the release of various cytokines, and the mRNA levels of Sult2A1 and the sulfate donor 3'-phosphoadenosine 5'-phosphosulfate synthase 2 (PAPSS2), as well as the enzyme activity of Sult2A1, were determined in the liver. We found that mRNA levels of Sult2A1 decrease in a time- and dose-dependent manner during the LPS-induced APR. Similar changes were observed in the mRNA levels of PAPSS2, the major synthase of PAPS in the liver. Moreover, hepatic Sult2A1 activity and serum levels of DHEA-sulfate (DHEA-S) were significantly decreased in LPS-treated animals. These results suggest that decreased levels or activities of FXR, PXR, and CAR during the APR could contribute to decreases in Sult2A1, resulting in decreased sulfation of DHEA and lower circulating level of DHEA-S. Finally, we found that both TNF and IL-1 caused a significant decrease in the mRNA level of Sult2A1 in Hep3B human hepatoma cells, suggesting that the proinflammatory cytokines TNF and IL-1 mediate the inhibitory effect of LPS on Sult2A1 mRNA level. Our study provides a possible mechanism by which infection and inflammation are associated with altered steroid metabolism and cholestasis.

Biological significance of nitric oxide-mediated protein modifications

Nitric oxide (NO), despite an apparently simple diatomic structure, has a wide variety of functions in both physiology and pathology and within every major organ system. It has become an increasingly important scientific challenge to decipher how this wide range of activity is achieved. To this end a number of investigators have begun to explore how NO-mediated posttranslational modifications of proteins may represent mechanisms of cellular signaling. These modifications include: 1). binding to metal centers; 2). nitrosylation of thiol and amine groups; 3). nitration of tyrosine, tryptophan, amine, carboxylic acid, and phenylalanine groups; and 4). oxidation of thiols (both cysteine and methionine residues) and tyrosine. However, two particular modifications have recently received much attention, nitrosylation of thiols to produce S-nitrosothiol and nitration of tyrosine residues to produce nitrotyrosine. It is the purpose of this review to examine the possibility that these modifications may play a role in NO-mediated signaling.

Identification of tyrosine sulfation in extracellular leucine-rich repeat proteins using mass spectrometry

Multiple and variable tyrosine sulfation in extracellular class II leucine-rich repeat proteins/proteoglycans were characterized by mass spectrometry. The sulfogroup on tyrosine is labile and is released from peptides under normal mass spectrometric conditions. Thus, special approaches must be considered in order to identify this modification. By using a combination of mass spectrometry studies operating in negative and positive ion mode, tyrosine sulfation could be identified. In positive mode, the peptides normally appeared non-sulfated, whereas in negative mode a mixture of sulfated and non-sulfated species was observed. A combination of peptides released by different proteinases was used to obtain details on the locations of sulfate groups. Multiple tyrosine sulfates were observed in the N-terminal region of fibromodulin (up to 9 sites), osteoadherin (up to 6 sites), and lumican (2 sites). Osteoadherin contains two additional sulfated tyrosine residues close to its C terminus. We also identified an error in the published sequence of bovine fibromodulin, resulting in the replacement of Thr37 by Tyr37-Gly38, thus increasing its homology with its human counterpart.

Identification of differentially expressed genes associated with colorectal cancer liver metastasis

The molecular mechanisms underlying successful metastasis of primary colorectal tumour to the liver remain unknown. We have used suppression subtractive hybridisation (SSH) and reverse Northern dot blot analysis to profile the mRNA expression patterns of a primary colorectal cancer and its liver metastasis. After SSH and reverse Northern dot blot analysis, differential expression was confirmed in 17 clones from the forward, and 13 clones from the reverse subtracted cDNA library. Four clones showed no significant sequence identities with any known sequences in the GenBank data base and likely to represent novel genes whose up- or down-regulation is associated with colorectal liver metastasis. Interestingly, one of the 13 down-regulated clones displayed 99% sequence identity with the BRCA1 tumour suppressor gene. Since promoter methylation is a direct cause of transcription silencing of the BRCA1 gene in approximately 10-20% of human breast cancer we further investigated its promoter methylation status in ten primary colorectal tumour samples, but revealed no evidence of promoter methylation.

Urinary excretion of the uraemic toxin p-cresol in the rat: contribution of glucuronidation to its metabolization

BACKGROUND

Increasing evidence indicates that lipophilic and/or protein-bound substances such as p-cresol are responsible for adverse physiological alterations in uraemic patients. To better understand the evolution of p-cresol disposition in renal failure and dialysis patients, it is necessary to determine its kinetic characteristics and biotransformation pathways.

METHODS

We studied the biotransformation of p-cresol after intravenous injection of the compound in eight rats with normal renal function. Urine was collected in four 1 h intervals. To evaluate the presence of p-cresol metabolites, beta-glucuronidase was added to urine samples and the isolated unidentified chromatographic peak observed in previous experiments was submitted to tandem mass spectrometry (MS/MS) analysis.

RESULTS

Administration of p-cresol produced a p-cresol peak and an unknown peak, suggesting biotransformation of the compound. Addition of beta-glucuronidase to urine samples and incubation at 37 degrees C resulted in a marked decrease in the unidentified peak height (P<0.001) together with an increase in p-cresol peak height (P<0.001), suggesting that the unidentified peak was composed, at least in part, of p-cresylglucuronide. Mass spectrometry (MS) and MS/MS analysis of the isolated unidentified peak confirmed the presence of p-cresylglucuronide. Linear regression between the peak height of p-cresylglucuronide before enzyme treatment and the increase in p-cresol peak height after enzyme treatment in samples incubated with beta-glucuronidase allowed us to calculate the amount of p-cresylglucuronide as its p-cresol equivalents. This revealed that 64% of the injected p-cresol was excreted as glucuronide. There was no change in peak heights when sulphatase was added to the urine. When p-cresol and p-cresylglucuronide levels were combined, approximately 85% of all administered p-cresol was recovered in the urine. In addition, the combined urinary excretion of p-cresol and p-cresylglucuronide was more than four times greater than excretion of p-cresol by itself (P<0.01).

CONCLUSIONS

In rats with normal renal function, intravenous administration of p-cresol results in immediate and extensive metabolization of the compound into p-cresylglucuronide. The elimination of p-cresol from the body depends largely on the urinary excretion of this metabolite.

Changes in ultrastructural characteristics of endolymphatic sac ribosome-rich cells of the rat during development

It has recently been demonstrated that endolymphatic sac (ES) ribosome-rich (dark) cells respond to induced endolymph changes and are thus likely to be involved in endolymph homeostasis. Therefore, we studied the ultrastructural characteristics of rat ES ribosome-rich cells during development in order to determine the cellular distribution of organelles involved in protein metabolism, secretion and absorption, indicative for their contribution to endolymph homeostasis. During embryonal stages ribosome-rich cells contain a limited number and variety of organelles and are predominantly involved in the production of components for cell growth and differentiation. In the young adult stage (P60) three different states of ribosome-rich cells may be distinguished. State A resembles a cell with only limited metabolic activities whereas state B is characterized by numerous different intracellular organelles and is considered to be involved in production and secretion as well as absorption and degradation of complex proteins. A third cellular state, state C, is filled with phagolysosomes and contains very few other organelles. This is considered to be a final (pre)apoptotic state. Autoradiography data suggest that ES ribosome-rich cells are capable of synthesis and secretion of tyrosine-containing proteins and may thus be involved in regulation of the osmolarity of endolymph based on the capacity to bind cations as well as water molecules. In addition, ES ribosome-rich cells appear to synthesize and secrete fucosylated glycoproteins into the endolymph. In conclusion, the present data suggest that ES ribosome-rich cells are actively involved in endolymph homeostasis through secretion and absorption of complex proteins and it is hypothesized that they are able to adapt their function or activities in response to changes in endolymph composition.

Isolation, identification, and synthesis of a disulfated sulfakinin from the central nervous system of an arthropods the white shrimp Litopenaeus vannamei

Two myotropic peptides displaying tyrosyl sulfation have been isolated from an extract of central nervous systems (brain, suboesophageal ganglion, thoracic ganglia, and ventral nerve cord) of the white shrimp Litopenaeus vannamei. Both peptides were identified by mass spectrometry and belong to the sulfakinin family of neuropeptides, which are characterized by the C-terminal hexapeptide Y(SO(3)H)GHMRF-NH(2) preceded by two acidic amino acid residues. Pev-SK 1 (AGGSGGVGGEY(SO(3)H)DDY(SO(3)H)GH(L/I) RF-NH(2)) has two sulfated tyrosyl residues and a unique (L/I) for M substitution in the C-terminal sequence. Pev-SK 2 (pQFDEY(SO(3)H)GHMRF-NH(2)) fully complies with the typical sulfakinin core sequence and is blocked by a pyroglutamyl residue. Synthetic analogs (sulfated and unsulfated) were synthesized and the tyrosyl sulfations were confirmed by myotropic activity studies and co-elution with the native fractions. Pev-SK 1 is the first disulfated neuropeptide elucidated in the phylum of the arthropoda, with the only other reported disulfated neuropeptide, called cionin, found in a protochordate. The similarities in amino acid sequence and posttranslational modifications of the crustacean sulfakinins and protochordate cionin provide further evidence for the hypothesis stating that gastrin/CCK, cionin, and sulfakinins originate from a common ancestral gastrin/CCK-like peptide.

The hormonogenic tyrosine 5 of porcine thyroglobulin is sulfated

Our previous results showed that sulfated tyrosines of thyroglobulin (Tg), the molecular support of thyroid hormonosynthesis, are involved in the hormonogenic process. Moreover, the consensus sequence required for tyrosine sulfation is present in most of the hormonogenic sites. These observations suggest that tyrosine sulfation might play a critical role in the hormonogenic process. In this paper we studied the putative sulfation of tyrosine 5 contained in the preferential hormonogenic site. Porcine thyrocytes were cultured with thyrotropin but without iodide to preserve the sulfation state of tyrosine 5 and then incubated or not with [35S]sulfate. Secreted Tg was purified and submitted to peptide sequence analysis which confirmed the known peptide sequence of the NH(2) extremity of Tg:NIFEYQV. The treatment of [35S]sulfate-labeled Tg by leucine aminopeptidase, which sequentially digested its amino-terminal extremity, released the same amino acids and further analysis by thin layer chromatography showed that the tyrosine was sulfated. We concluded that tyrosine 5 is sulfated but the role of sulfate group in the hormonogenic process remains to be elucidated.

Sulfonation and molecular action

The sulfonation of endogenous molecules is a pervasive biological phenomenon that is not always easily understood, and although it is increasingly recognized as a function of fundamental importance, there remain areas in which significant cognizance is still lacking or at most minimal. This is particularly true in the field of endocrinology, in which the sulfoconjugation of hormones is a widespread occurrence that is only partially, if at all, appreciated. In the realm of steroid/sterol sulfoconjugation, the discovery of a novel gene that utilizes an alternative exon 1 to encode for two sulfotransferase isoforms, one of which sulfonates cholesterol and the other pregnenolone, has been an important advance. This is significant because cholesterol sulfate plays a crucial role in physiological systems such as keratinocyte differentiation and development of the skin barrier, and pregnenolone sulfate is now acknowledged as an important neurosteroid. The sulfonation of thyroglobulin and thyroid hormones has been extensively investigated and, although this transformation is better understood, there remain areas of incomplete comprehension. The sulfonation of catecholamines is a prevalent modification that has been extensively studied but, unfortunately, remains poorly understood. The sulfonation of pituitary glycoprotein hormones, especially LH and TSH, does not affect binding to their cognate receptors; however, sulfonation does play an important role in their plasma clearance, which indirectly has a significant effect on biological activity. On the other hand, the sulfonation of distinct neuroendocrine peptides does have a profound influence on receptor binding and, thus, a direct effect on biological activity. The sulfonation of specific extracellular structures plays an essential role in the binding and signaling of a large family of extracellular growth factors. In summary, sulfonation is a ubiquitous posttranslational modification of hormones and extracellular components that can lead to dramatic structural changes in affected molecules, the biological significance of which is now beginning to be appreciated.

Site-directed mutagenesis of platelet glycoprotein Ib alpha demonstrating residues involved in the sulfation of tyrosines 276, 278, and 279

The interaction between platelet glycoprotein (GP) Ib alpha and von Willebrand factor (VWF) is essential for initiation of hemostasis. The sulfation of the 3 tyrosine residues 276, 278, and 279 in GPIb alpha is an important posttranslational modification that seems to promote the interaction with VWF. The environment where sulfation of tyrosines occurs has been proposed to contain highly acidic residues. This investigation has examined the highly acidic region from Asp249 to Asp287 in the mature GPIb alpha protein. Changes to most of the carboxylic acids in this region resulted in decreased reactivity to VWF. Only 3 mutants (Glu270Gln, Asp283Asn, Asp283Asn/Glu285Gln/Asp287Asn) resulted in the abolition of sulfation. Two novel mutations were also created. First, a deletion of the 7 amino acids from Tyr276 to Glu282 led to a loss of sulfation and totally abolished VWF binding in the presence of botrocetin. This confirms that it is these 3 tyrosines that undergo sulfation and that this region is crucial for botrocetin-mediated VWF binding. The second mutation involves changing the lysine residues at 253, 258, and 262 to alanine. This also led to distinct changes in VWF binding and abolition of sulfation.

Baseline resolution of isobaric phosphorylated and sulfated peptides and nucleotides by electrospray ionization FTICR ms: another step toward mass spectrometry-based proteomics

Electrospray ionization broadband FTICR mass spectrometry at a mass resolving power, m/delta m50% > or = 400,000 has achieved the first direct mass spectral resolution of phosphorylated and sulfated peptides (or nucleotides) of the same nominal mass. The elemental composition difference in each case is PH versus S (9.5 mDa), requiring a minimum mass resolving power ((m2 - m1)/ml) of 118,000 (C terminal amidated cholecystekinin fragment 26-33 (CCK-8), DY(PO3H2)MGWMDF-NH2 versus DY(SO3H)MGWMDF-NH2) or 65,400 (adenosine triphosphate vs 3-phosphoadenosine 5'-phosphosulfate). The isobaric mass doublets were detected in broadband mode (400 < m/z <1400) in the presence of dozens of other species. It is therefore now possible to distinguish phosphorylated from sulfated peptides, even when both species are present at the same time in a protein digest.

Stabilization of a tyrosine O-sulfate residue by a cationic functional group: formation of a conjugate acid-base pair

Sulfated tyrosine [Tyr(SO3H)]-containing peptides showed characteristic peak patterns in their liquid secondary-ion mass spectrometry (LSIMS) spectra. Protonated molecules were desulfated more easily than their deprotonated counterparts. Therefore, the stabilities of the Tyr(SO3H) residues were well-reflected by peak patterns in their positive-ion spectra. These intrinsic peak patterns were investigated by comparing the behavior of each Tyr(SO3H) residue in acidic solution. As the peptide chain was lengthened and the number of cationic functional groups increased, the peak representing the [MH]+ of a Tyr(SO3H)-containing peptide became more prominent than that representing the desulfated [MH-SO3]+. These alterations in peptide structure also increased the stability of the Tyr(SO3H) residue in acidic solution. Based on the desulfation mechanism of an aryl monosulfate, we predicted that intramolecular cationic functional groups would stabilize Tyr(SO3H) residues by forming conjugate acid-base pairs (or salt bridges) both in the gaseous phase and in acidic solution. In accordance with this theory, Arg residues would take primary responsibility for this self-stabilization within Tyr(SO3H)-containing peptides. Moreover, a long peptide backbone was expected to have a weak protective effect against desulfation of the [MH]+ in the gaseous phase. Tyr(SO3H) residues were also stabilized by adding an external basic peptide containing multiple Arg residues. Formation of such intermolecular acid-base pairs was demonstrated by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) which detected conjugated peptide ions. The energetically favorable formation of conjugate acid-base pairs prompted by Tyr(SO3H) residues might be a driving force for protein folding and protein-protein interaction.

Inhibition of the vacuolar H+-ATPase perturbs the transport, sorting, processing and release of regulated secretory proteins

Vacuolar H+-ATPases (V-ATPases) are multisubunit enzymes that acidify various intracellular organelles, including secretory pathway compartments. We have examined the effects of the specific V-ATPase inhibitor bafilomycin A1 (Baf) on the intracellular transport, sorting, processing and release of a number of neuroendocrine secretory proteins in primary Xenopus intermediate pituitary cells. Ultrastructural examination of Baf-treated intermediate pituitary cells revealed a reduction in the amount of small dense-core secretory granules and the appearance of vacuolar structures in the trans-Golgi area. Pulse-chase incubations in combination with immunoprecipitation analysis showed that in treated cells, the proteolytic processing of the newly synthesized prohormone proopiomelanocortin, prohormone convertase PC2 and secretogranin III (SgIII) was inhibited, and an intracellular accumulation of intact precursor forms and intermediate cleavage products became apparent. Moreover, we found that treated cells secreted considerable amounts of a PC2 processing intermediate and unprocessed SgIII in a constitutive fashion. Collectively, these data indicate that in the secretory pathway, V-ATPases play an important role in creating the microenvironment that is essential for proper transport, sorting, processing and release of regulated secretory proteins.