Tyrosine-sulfated Proteins Research Articles

Site-Specific Incorporation of Sulfotyrosine into Proteins in Mammalian Cells.

Protein tyrosine O-sulfation (PTS) plays a crucial role in numerous extracellular protein-protein interactions. It is involved in diverse physiological processes and the development of human diseases, including AIDS and cancer. To facilitate the study of PTS in live mammalian cells, an approach for the site-specific synthesis of tyrosine-sulfated proteins (sulfoproteins) was developed. This approach takes advantage of an evolved Escherichia coli tyrosyl-tRNA synthetase to genetically encode sulfotyrosine (sTyr) into any proteins of interest (POI) in response to a UAG stop codon. Here, we give a step-by-step account of the incorporation of sTyr in HEK293T cells using the enhanced green fluorescent protein as an example. This method can be widely applied to incorporating sTyr into any POI to investigate the biological functions of PTS in mammalian cells.

Preparation of Tyrosylprotein Sulfotransferases for In Vitro One-Pot Enzymatic Synthesis of Sulfated Proteins/Peptides.

Protein tyrosine sulfation (PTS), catalyzed by membrane-anchored tyrosylprotein sulfotransferase (TPST), is one of the most common post-translational modifications of secretory and transmembrane proteins. PTS, a key modulator of extracellular protein–protein interactions, accounts for various important biological activities, namely, virus entry, inflammation, coagulation, and sterility. The preparation and characterization of TPST is fundamental for understanding the synthesis of tyrosine-sulfated proteins and for studying PTS in biology. A sulfated protein was prepared using a TPST-coupled protein sulfation system that involves the generation of the active sulfate 3′-phosphoadenosine-5′-phosphosulfate (PAPS) through either PAPS synthetase (PAPSS) or phenol sulfotransferase. The preparation of sulfated proteins was confirmed through radiometric or immunochemical assays. In this study, enzymatically active Drosophila melanogaster TPST (DmTPST) and human TPSTs (hTPST1 and hTPST2) were expressed in Escherichia coli BL21(DE3) host cells and purified to homogeneity in high yield. Our results revealed that recombinant DmTPST was particularly useful considering its catalytic efficiency and ease of preparation in large quantities. This study provides tools for high-efficiency, one-step synthesis of sulfated proteins and peptides that are useful for further deciphering the mechanisms, functions, and future applications of PTS.

Site-Specific Incorporation of Sulfotyrosine Using an Expanded Genetic Code.

Tyrosine sulfation is an important posttranslational modification found in bacteria and higher eukaryotes. However, the chemical synthesis or expression of homogenously sulfated proteins is particularly difficult, limiting our study and application of tyrosine-sulfated proteins. With the recent development of genomically recoded organisms and orthogonal translation components, we can often treat otherwise posttranslationally-modified amino acids as noncanonical amino acids (ncAAs) encoded by an expanded genetic code. Here, we describe methods for the co-translational incorporation of one or multiple sulfotyrosines into proteins using standard or genomically recoded Escherichia coli stains, thereby achieving the direct expression of site-specifically tyrosine sulfated proteins in vivo.

A second-generation expression system for tyrosine-sulfated proteins and its application in crop protection.

Posttranslational modification (PTM) of proteins and peptides is important for diverse biological processes in plants and animals. The paucity of heterologous expression systems for PTMs and the technical challenges associated with chemical synthesis of these modified proteins has limited detailed molecular characterization and therapeutic applications. Here we describe an optimized system for expression of tyrosine-sulfated proteins in Escherichia coli and its application in a bio-based crop protection strategy in rice.

Sulfation Part of Rice Defenses; Plant Hormones Sculpt Root Microbiomes

Rice plants defend against the agriculturally important bacterial pathogen Xanthomonas oryzae (Xoo) with XA21— the first characterized of more than 350 predicted rice immune receptors, according to Pamela Ronald at the University of California, Davis (UCD). Last year, she and her collaborators identified three Xoo rax genes required for activating XA21-mediated immunity in the plants that this pathogen infects. These genes suggest that tyrosine sulfation plays “a key functional role in the process,” they say, calling the newly identified tyrosine-sulfated protein that they encode “RaxX.” Details appeared in 24 July 2015 in Science Advances (2015:1:e 1500245).

Identification of Tyrosine O Sulfated Proteins in Cow Retina and the 661W Cell Line.

Lack of tyrosine O Sulfation compromises both rod and cone electroretinographic responses emphasizing the importance of this post-translational modification for vision. To identify tyrosine sulfated proteins in retina, cow retinal lysates were subjected to immunoaffinity purification using an anti-sulfotyrosine antibody. The tyrosine sulfated proteins were eluted from the column using a sulfotyrosine pentapeptide and identified using mass spectrometry. Similarly, tyrosine sulfated proteins secreted by the 661W cell line were identified. Proteins identified were vitronectin, fibronectin, fibulin 2, nidogen, collagen V alpha 2, complement component 3 and C4 and fibrinogen beta. All proteins were subjected to analysis by 'Sulfinator' to determine potential sulfated tyrosines.

Homogeneous Sulfopeptides and Sulfoproteins: Synthetic Approaches and Applications To Characterize the Effects of Tyrosine Sulfation on Biochemical Function

Post-translational modification of proteins plays critical roles in regulating structure, stability, localization, and function. Sulfation of the phenolic side chain of tyrosine residues to form sulfotyrosine (sTyr) is a widespread modification of extracellular and integral membrane proteins, influencing the activities of these proteins in cellular adhesion, blood clotting, inflammatory responses, and pathogen infection. Tyrosine sulfation commonly occurs in sequences containing clusters of tyrosine residues and is incomplete at each site, resulting in heterogeneous mixtures of sulfoforms. Purification of individual sulfoforms is typically impractical. Therefore, the most promising approach to elucidate the influence of sulfation at each site is to prepare homogeneously sulfated proteins (or peptides) synthetically. This Account describes our recent progress in both development of such synthetic approaches and application of the resulting sulfopeptides and sulfoproteins to characterize the functional consequences of tyrosine sulfation. Initial synthetic studies used a cassette-based solid-phase peptide synthesis (SPPS) approach in which the side chain sulfate ester was protected to enable it to withstand Fmoc-based SPPS conditions. Subsequently, to address the need for efficient access to multiple sulfoforms of the same peptide, we developed a divergent solid-phase synthetic approach utilizing orthogonally side chain protected tyrosine residues. Using this methodology, we have carried out orthogonal deprotection and sulfation of up to three tyrosine residues within a given sequence, allowing access to all eight sulfoforms of a given target from a single solid-phase synthesis. With homogeneously sulfated peptides in hand, we have been able to probe the influence of tyrosine sulfation on biochemical function. Several of these studies focused on sulfated fragments of chemokine receptors, key mediators of leukocyte trafficking and inflammation. For the receptor CCR3, we showed that tyrosine sulfation enhances affinity and selectivity for binding to chemokine ligands, and we determined the structural basis of these affinity enhancements by NMR spectroscopy. Using a library of CCR5 sulfopeptides, we demonstrated the critical importance of sulfation at one specific site for supporting HIV-1 infection. Demonstrating the feasibility of producing homogeneously tyrosine-sulfated proteins, in addition to smaller peptides, we have used SPPS and native chemical ligation methods to synthesize the leech-derived antithrombotic protein hirudin P6, containing both tyrosine sulfation and glycosylation. Sulfation greatly enhanced inhibitory activity against thrombin, whereas addition of glycans to the sulfated protein decreased inhibition, indicating functional interplay between different post-translational modifications. In addition, the success of the ligation approach suggests that larger sulfoproteins could potentially be obtained by ligation of synthetic sulfopeptides to expressed proteins, using intein-based technology.

The rice immune receptor XA21 recognizes a tyrosine-sulfated protein from a Gram-negative bacterium.

Surveillance of the extracellular environment by immune receptors is of central importance to eukaryotic survival. The rice receptor kinase XA21, which confers robust resistance to most strains of the Gram-negative bacterium Xanthomonas oryzae pv. oryzae (Xoo), is representative of a large class of cell surface immune receptors in plants and animals. We report the identification of a previously undescribed Xoo protein, called RaxX, which is required for activation of XA21-mediated immunity. Xoo strains that lack RaxX, or carry mutations in the single RaxX tyrosine residue (Y41), are able to evade XA21-mediated immunity. Y41 of RaxX is sulfated by the prokaryotic tyrosine sulfotransferase RaxST. Sulfated, but not nonsulfated, RaxX triggers hallmarks of the plant immune response in an XA21-dependent manner. A sulfated, 21-amino acid synthetic RaxX peptide (RaxX21-sY) is sufficient for this activity. Xoo field isolates that overcome XA21-mediated immunity encode an alternate raxX allele, suggesting that coevolutionary interactions between host and pathogen contribute to RaxX diversification. RaxX is highly conserved in many plant pathogenic Xanthomonas species. The new insights gained from the discovery and characterization of the sulfated protein, RaxX, can be applied to the development of resistant crop varieties and therapeutic reagents that have the potential to block microbial infection of both plants and animals.

Detection of tyrosine sulfation on proteins.

Tyrosine sulfation is a post-translational modification (PTM) where a sulfate group is added to a tyrosine moiety. This PTM is responsible for strengthening interaction between proteins. One of the drawbacks of studying this PTM is the lack of an antibody that can detect all tyrosine-sulfated proteins. In addition, due to the labile nature of the tyrosine sulfate, other techniques such as mass spectrometry cannot be used to study this PTM unless special modification procedures are used. This requires considerable skill and knowledge of mass spectrometry. This unit describes an in vitro technique that can be used to study tyrosine-sulfated proteins by radiolabeling the recombinant protein. The protein is then subject to barium hydroxide hydrolysis and thin-layer electrophoresis (TLE). Co-localization of radioactive tyrosine-sulfate with nonradioactive tyrosine sulfate standard spiked in before TLE analysis identifies a protein as tyrosine-sulfated protein. The advantage of this technique is that, it identifies all tyrosine-sulfated proteins without any bias and is the only technique that identifies the tyrosine sulfate residues in the protein.

Complement factor H, vitronectin, and opticin are tyrosine-sulfated proteins of the retinal pigment epithelium.

Lack of tyrosine sulfation of ocular proteins results in disorganized photoreceptor structure and drastically reduced visual function, demonstrating the importance of this post-translational modification to vision. To understand the role that tyrosine sulfation plays in the function of ocular proteins, we identified some tyrosine-sulfated proteins in the retinal pigment epithelium using two independent methods, immuno-affinity column purification with an anti-sulfotyrosine specific antibody and computer-based sequence analysis of retinal pigment epithelium secretome by means of the prediction program Sulfinator. Radioactive labeling followed by thin layer electrophoresis revealed that three proteins, vitronectin, opticin, and complement factor H (CFH), were post-translationally modified by tyrosine sulfation. The identification of vitronectin and CFH as tyrosine-sulfated proteins is significant, since both are deposited in drusen in the eyes of patients with age-related macular degeneration (AMD). Furthermore, mutations in CFH have been determined to be a major risk factor in the development of AMD. Future studies that seek to understand the role of CFH in the development of AMD should take into account the role that tyrosine sulfation plays in the interaction of this protein with its partners, and examine whether modulating sulfation provides a potential therapeutic target.

RGC-5: Are they really 661W? The saga continues

In a recent publication, Krishnamoorthy et al. demonstrated that the purported rat ganglion cell line RGC-5 is in fact of mouse origin and not derived from rat retina (Krishnamoorthy et al., 2013, Invest. Ophthalmol. Vis. Sci. 54:5712-5719). Furthermore, the authors demonstrated that the cells expressed photoreceptor markers that are also expressed in the 661W cell line. Finally, the manuscript suggested that the RGC-5 cultures may have been contaminated with the 661W cells during the initial stages of establishing the RGC-5 cell line; since the 661W cells were, at the time, in use in the laboratory that generated the RGC-5 cells (Krishnamoorthy et al., 2013, Invest. Ophthalmol. Vis. Sci. 54:5712-5719). As the originator of the 661W cells, I feel it is imperative that I caution the research community against assuming that RGC-5 stocks obtained from ATCC (catalog PTA-6600, now removed from distribution) or from other laboratories are not completely representative of the 661W cell line. My laboratory has used RGC-5 cells and found that although they share certain features with 661W cells, they also have features that are not part of the characteristics of the 661W cells; including (degree of sensitivity to light (Kanan et al. 2007, Invest. Ophthalmol. Vis. Sci. 48:40–51; Kanan et al. 2009) and pattern of expression of tyrosine sulfated proteins (Exp. Eye Res. 89:559–567)). This is not to imply that RGC-5 were derived from retinal ganglion cells, but rather to caution that they have features that are not consistent with those found in 661W cells. Because of the RGC-5 cell issue, several vision journals have adopted new policies when reviewing manuscripts using cell lines (Clark et al., 2013, Exp. Eye Res. Sep 7. [Epub ahead of print]; Beebe, D. 2013, Invest. Ophthalmol. Vis. Sci. 54:5720, Boatright et al. 2013, Mol. Vis. 19:1848–1851). It is now unlikely that any journal will accept a manuscript for publication that uses RGC-5 cells as an in vitro model for retinal ganglion cells. Despite the demonstrations that the RGC-5 cells express similar markers as 661W, those labs that continue to use these cells should not assume that they truly represent 661W cells. This unfortunate issue highlights the need to be vigilant about contamination of cultures, precise in descriptions of lines in use, and above all, quite clear in all cases about the origin and source of cell lines used in research.

Lack of Tyrosylprotein Sulfotransferase-2 Activity Results in Altered Sperm-Egg Interactions and Loss of ADAM3 and ADAM6 in Epididymal Sperm

Tyrosine O-sulfation is a post-translational modification catalyzed by two tyrosylprotein sulfotransferases (TPST-1 and TPST-2) in the trans-Golgi network. Tpst2-deficient mice have male infertility, sperm motility defects, and possible abnormalities in sperm-egg membrane interactions. Studies here show that compared with wild-type sperm, fewer Tpst2-null sperm bind to the egg membrane, but more of these bound sperm progress to membrane fusion. Similar outcomes were observed with wild-type sperm treated with the anti-sulfotyrosine antibody PSG2. The increased extent of sperm-egg fusion is not due to a failure of Tpst2-null sperm to trigger establishment of the egg membrane block to polyspermy. Anti-sulfotyrosine staining of sperm showed localization similar to that of IZUMO1, a sperm protein that is essential for gamete fusion, but we detected little to no tyrosine sulfation of IZUMO1 and found that IZUMO1 expression and localization were normal in Tpst2-null sperm. Turning to a discovery-driven approach, we used mass spectrometry to characterize sperm proteins that associated with PSG2. This identified ADAM6, a member of the A disintegrin and A metalloprotease (ADAM) family; members of this protein family are associated with multiple sperm functions. Subsequent studies revealed that Tpst2-null sperm lack ADAM6 and ADAM3. Loss of ADAM3 is strongly associated with male infertility and is observed in knockouts of male germ line-specific endoplasmic reticulum-resident chaperones, raising the possibility that TPST-2 may function in quality control in the secretory pathway. These data suggest that TPST-2-mediated tyrosine O-sulfation participates in regulating the sperm surface proteome or membrane order, ultimately affecting male fertility.

Implantation of Post‐translational Tyrosylprotein Sulfation into a Prokaryotic Expression System

Sulfation made easy: Syntheses of tyrosine-sulfated proteins were developed by coupling an in situ 3′-phosphoadenosine-5′-phosphosulfate (PAPS)-generating system and tyrosylprotein sulfotransferase (TPST) catalysis. High catalytic efficiency of TPST in vitro was demonstrated, and by using a similar strategy, sulfated proteins were produced in vivo through bacterial cultivation.

Efficient expression of tyrosine-sulfated proteins in E. coli using an expanded genetic code

Tyrosine sulfation is an important post-translational modification that occurs in higher eukaryotes and is involved in cell-cell communication, viral entry and adhesion. We describe a protocol for the heterologous expression of selectively tyrosine-sulfated proteins in Escherichia coli through the use of an expanded genetic code that co-translationally inserts sulfotyrosine in response to the amber nonsense codon, TAG. The components required for this process, an orthogonal aminoacyl-tRNA synthetase specific for sulfotyrosine and its cognate orthogonal tRNA that recognizes the amber codon, are encoded on the plasmid pSUPAR6-L3-3SY, and their use, along with a simple chemical synthesis of sulfotyrosine, are outlined in this protocol. Specifically, the gene for a protein of interest is mutated such that the codon corresponding to the desired location of tyrosine sulfate is TAG. Co-transformation of an expression vector containing this gene and pSUPAR6-L3-3SY into an appropriate E. coli strain allows the overexpression of the site-specifically sulfated protein with high efficiency and fidelity. The resulting protein contains tyrosine sulfate at any location specified by a TAG codon, making this method significantly simpler and more versatile than competing methods such as in vitro enzymatic sulfation, chemical sulfation and peptide synthesis. Once the proper expression vectors are cloned, our protocol should allow the production of the desired sulfated proteins in <1 week.

Protein tyrosine sulfation: A critical posttranslation modification in plants and animals

Protein tyrosine sulfation is a posttranslational modification restricted to proteins that transit the secretory pathway that was first described by Bettelheim in bovine fibrinopeptide B in 1954 (1). Subsequent pioneering work by Wieland Huttner's group (2, 3) and others characterized the enzyme activity responsible for the reaction, called tyrosylprotein sulfotransferase (TPST), that catalyzes the transfer of sulfate from 3′-phosphoadenosine 5′-phosphosulfate to the hydroxyl group of peptidyl tyrosine residues to form a tyrosine O4-sulfate ester. The enzymes' subcellular localization in the trans-Golgi network and its widespread tissue and cellular distribution have been well documented in animals, and several dozen tyrosine-sulfated proteins, mostly of animal origin, have been described, many of which play important roles in inflammation, hemostasis, immunity, and other processes (2–4). Finally, the general importance of protein tyrosine sulfation in protein–protein interactions has become widely accepted.

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 [ 35S]sulfate labeling of cultured HepG2 human hepatoma cells, immunoprecipitation of targeted [ 35S]sulfate-labeled hemostatic proteins, and tyrosine O-[ 35S]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.

Tyrosylprotein Sulfotransferase-2 Expression Is Required for Sulfation of RNase 9 and Mfge8 in Vivo

Protein-tyrosine sulfation is mediated by two Golgi tyrosyl-protein sulfotransferases (TPST-1 and TPST-2) that are widely expressed in vivo. However, the full substrate repertoire of this enzyme system is unknown and thus, our understanding of the biological role(s) of tyrosine sulfation is limited. We reported that whereas Tpst1(-/-) male mice have normal fertility, Tpst2(-/-) males are infertile despite normal spermatogenesis. However, Tpst2(-/-) sperm are severely defective in their motility in viscous media and in their ability to fertilize eggs. These findings suggest that sulfation of unidentified substrate(s) is crucial for normal sperm function. We therefore sought to identify tyrosine-sulfated proteins in the male genital tract using affinity chromatography on PSG2, an anti-sulfotyrosine monoclonal antibody, followed by mass spectrometry. Among the several candidate tyrosine-sulfated proteins identified, RNase 9 and Mfge8 were examined in detail. RNase 9, a catalytically inactive RNase A family member of unknown function, is expressed only in the epididymis after onset of sexual maturity. Mfge8 is expressed on mouse sperm and Mfge8(-/-) male mice are subfertile. Metabolic labeling coupled with sulfoamino acid analysis confirmed that both proteins are tyrosine-sulfated and both proteins are expressed at comparable levels in wild type, Tpst1(-/-), and Tpst2(-/-) epididymides. However, we demonstrate that RNase 9 and Mfge8 are tyrosine-sulfated in wild type and Tpst1(-/-), but not in Tpst2(-/-) mice. These findings suggest that lack of sulfation of one or both of these proteins may contribute mechanistically to the infertility of Tpst2(-/-) males.

Artifactual Sulfation of Silver-stained Proteins

Sulfation and phosphorylation are post-translational modifications imparting an isobaric 80-Da addition on the side chain of serine, threonine, or tyrosine residues. These two post-translational modifications are often difficult to distinguish because of their similar MS fragmentation patterns. Targeted MS identification of these modifications in specific proteins commonly relies on their prior separation using gel electrophoresis and silver staining. In the present investigation, we report a potential pitfall in the interpretation of these modifications from silver-stained gels due to artifactual sulfation of serine, threonine, and tyrosine residues by sodium thiosulfate, a commonly used reagent that catalyzes the formation of metallic silver deposits onto proteins. Detailed MS analyses of gel-separated protein standards and Escherichia coli cell extracts indicated that several serine, threonine, and tyrosine residues were sulfated using silver staining protocols but not following Coomassie Blue staining. Sodium thiosulfate was identified as the reagent leading to this unexpected side reaction, and the degree of sulfation was correlated with increasing concentrations of thiosulfate up to 0.02%, which is typically used for silver staining. The significance of this artifact is discussed in the broader context of sulfation and phosphorylation site identification from in vivo and in vitro experiments.

Detection and Purification of Tyrosine-sulfated Proteins Using a Novel Anti-sulfotyrosine Monoclonal Antibody

Protein tyrosine O-sulfation is a post-translational modification mediated by one of two Golgi tyrosylprotein sulfotransferases (TPST1 and TPST2) that catalyze the transfer of sulfate to tyrosine residues in secreted and transmembrane proteins. Tyrosine sulfation plays a role in protein-protein interactions in several well defined systems. Although dozens of tyrosine-sulfated proteins are known, many more are likely to exist and await description. Advancing our understanding of the importance of tyrosine sulfation in biological systems requires the development of new tools for the detection and study of tyrosine-sulfated proteins. We have developed a novel anti-sulfotyrosine monoclonal antibody (called PSG2) that binds with high affinity and exquisite specificity to sulfotyrosine residues in peptides and proteins independently of sequence context. We show that it can detect tyrosine-sulfated proteins in complex biological samples and can be used as a probe to assess the role of tyrosine sulfation in protein function. We also demonstrate the utility of PSG2 in the purification of tyrosine-sulfated proteins from crude tissue samples. Finally, Western blot analysis using PSG2 showed that certain sperm/epididymal proteins are undersulfated in Tpst2(-/-) mice. This indicates that TPST1 and TPST2 have distinct macromolecular substrate specificities and provides clues as to the molecular mechanism of the infertility of Tpst2(-/-) males. PSG2 should be widely applicable for identification of tyrosine-sulfated proteins in other systems and organisms.

Tyrosine Sulfation of Arylsulfatase A and Its Peptide

Purified human liver arylsulfatase A (ASA) as well as an ASA peptide (residues 28-39) were sulfated by tyrosyl protein sulfotransferase in vitro. The media, but not the cell lysate, of normal human fibroblasts contained a tyrosine sulfated protein (pI = 4.5-5.5). This protein was not present in either media or cell lysate of human fibroblasts lacking ASA protein. These results suggest that tyrosine sulfation facilitates secretion of ASA and that this may have pathophysiological consequences.