Functional expression of the CMP-sialic acid transporter in Escherichia coli and its identification as a simple mobile carrier

Delivery of Nucleotide Sugars to the Mammalian Golgi: A Very Well (un)Explained Story

Nucleotide sugars (NSs) serve as substrates for glycosylation reactions. The majority of these compounds are synthesized in the cytoplasm, whereas glycosylation occurs in the endoplasmic reticulum (ER) and Golgi lumens, where catalytic domains of glycosyltransferases (GTs) are located. Therefore, translocation of NS across the organelle membranes is a prerequisite. This process is thought to be mediated by a group of multi-transmembrane proteins from the SLC35 family, i.e., nucleotide sugar transporters (NSTs). Despite many years of research, some uncertainties/inconsistencies related with the mechanisms of NS transport and the substrate specificities of NSTs remain. Here we present a comprehensive review of the NS import into the mammalian Golgi, which consists of three major parts. In the first part, we provide a historical view of the experimental approaches used to study NS transport and evaluate the most important achievements. The second part summarizes various aspects of knowledge concerning NSTs, ranging from subcellular localization up to the pathologies related with their defective function. In the third part, we present the outcomes of our research performed using mammalian cell-based models and discuss its relevance in relation to the general context.

In vitro Measurement of CMP-Sialic Acid Transporter Activity in Reconstituted Proteoliposomes

Nucleotide-sugar transporters (NSTs) facilitate eukaryotic cellular glycosylation by transporting nucleotide-sugar conjugates into the Golgi lumen and endoplasmic reticulum for use by glycosyltransferases, while also transferring nucleotide monophosphate byproducts to the cytoplasm. Mutations in this family of proteins can cause a number of significant cellular pathologies, and wild type members can act as virulence factors for many parasites and fungi. Here, we describe an in vitro assay to measure the transport activity of the CMP-sialic acid transporter (CST), one of seven NSTs found in mammals. While in vitro transport assays have been previously described for CST, these studies failed to account for the fact that 1) commercially available stocks of CMP-sialic acid (CMP-Sia) are composed of ~10% of the higher-affinity CMP and 2) CMP-Sia is hydrolyzed into CMP and sialic acid in aqueous solutions. Herein we describe a method for treating CMP-Sia with a nonselective phosphatase, Antarctic phosphatase, to convert all free CMP to cytidine. This allows us to accurately measure substrate affinities and transport kinetics for purified CST reconstituted into proteoliposomes.

Stable Protein Sialylation in Physcomitrella

Recombinantly produced proteins are indispensable tools for medical applications. Since the majority of them are glycoproteins, their N-glycosylation profiles are major determinants for their activity, structural properties and safety. For therapeutical applications, a glycosylation pattern adapted to product and treatment requirements is advantageous. Physcomitrium patens (Physcomitrella, moss) is able to perform highly homogeneous complex-type N-glycosylation. Additionally, it has been glyco-engineered to eliminate plant-specific sugar residues by knock-out of the β1,2-xylosyltransferase and α1,3-fucosyltransferase genes (Δxt/ft). Furthermore, Physcomitrella meets wide-ranging biopharmaceutical requirements such as GMP compliance, product safety, scalability and outstanding possibilities for precise genome engineering. However, all plants, in contrast to mammals, lack the capability to perform N-glycan sialylation. Since sialic acids are a common terminal modification on human N-glycans, the property to perform N-glycan sialylation is highly desired within the plant-based biopharmaceutical sector. In this study, we present the successful achievement of protein N-glycan sialylation in stably transformed Physcomitrella. The sialylation ability was achieved in a Δxt/ft moss line by stable expression of seven mammalian coding sequences combined with targeted organelle-specific localization of the encoded enzymes responsible for the generation of β1,4-galactosylated acceptor N-glycans as well as the synthesis, activation, transport and transfer of sialic acid. Production of free (Neu5Ac) and activated (CMP-Neu5Ac) sialic acid was proven. The glycosidic anchor for the attachment of terminal sialic acid was generated by the introduction of a chimeric human β1,4-galactosyltransferase gene under the simultaneous knock-out of the gene encoding the endogenous β1,3-galactosyltransferase. Functional complex-type N-glycan sialylation was confirmed via mass spectrometric analysis of a stably co-expressed recombinant human protein.

Nucleotide Sugar Transporter SLC35 Family Structure and Function

The covalent attachment of sugars to growing glycan chains is heavily reliant on a specific family of solute transporters (SLC35), the nucleotide sugar transporters (NSTs) that connect the synthesis of activated sugars in the nucleus or cytosol, to glycosyltransferases that reside in the lumen of the endoplasmic reticulum (ER) and/or Golgi apparatus. This review provides a timely update on recent progress in the NST field, specifically we explore several NSTs of the SLC35 family whose substrate specificity and function have been poorly understood, but where recent significant progress has been made. This includes SLC35 A4, A5 and D3, as well as progress made towards understanding the association of SLC35A2 with SLC35A3 and how this relates to their potential regulation, and how the disruption to the dilysine motif in SLC35B4 causes mislocalisation, calling into question multisubstrate NSTs and their subcellular localisation and function. We also report on the recently described first crystal structure of an NST, the SLC35D2 homolog Vrg-4 from yeast. Using this crystal structure, we have generated a new model of SLC35A1, (CMP-sialic acid transporter, CST), with structural and mechanistic predictions based on all known CST-related data, and includes an overview of reported mutations that alter transport and/or substrate recognition (both de novo and site-directed). We also present a model of the CST-del177 isoform that potentially explains why the human CST isoform remains active while the hamster CST isoform is inactive, and we provide a possible alternate access mechanism that accounts for the CST being functional as either a monomer or a homodimer. Finally we provide an update on two NST crystal structures that were published subsequent to the submission and during review of this report.

Coexpression of CMP-sialic acid transporter reduces N-glycolylneuraminic acid levels of recombinant glycoproteins in Chinese hamster ovary cells

Recombinant glycoproteins expressed in Chinese hamster ovary (CHO) cells contain two forms of sialic acids; N-acetylneuraminic acid (Neu5Ac) as a major type and N-glycolylneuraminic acid (Neu5Gc) as a minor type. The Neu5Gc glycan moieties in therapeutic glycoproteins can elicit immune responses because they do not exist in human. In the present work, to reduce Neu5Gc levels of recombinant glycoproteins from CHO cell cultures, we coexpressed cytidine-5'-monophosphate-sialic acid transporter (CMP-SAT) that is an antiporter and transports cytosolic CMP-sialic acids (both forms) into Golgi lumen. When human erythropoietin was used as a target human glycoprotein, coexpression of CMP-SAT resulted in a significant decrease of Neu5Gc level by 41.4% and a notable increase of Neu5Ac level by 21.2%. This result could be reasonably explained by our hypothesis that the turnover rate of Neu5Ac to Neu5Gc catalyzed by CMP-Neu5Ac hydroxylase would be reduced through facilitated transportation of Neu5Ac into Golgi apparatus by coexpression of CMP-SAT. We confirmed the effects of CMP-SAT coexpression on the decrease of Neu5Gc level and the increase of Neu5Ac level using another glycoprotein human DNase I. Therefore, CMP-SAT coexpression might be an effective strategy to reduce the levels of undesired Neu5Gc in recombinant therapeutic glycoproteins from CHO cell cultures.

Structural basis for mammalian nucleotide sugar transport

Nucleotide-sugar transporters (NSTs) are critical components of the cellular glycosylation machinery. They transport nucleotide-sugar conjugates into the Golgi lumen, where they are used for the glycosylation of proteins and lipids, and they then subsequently transport the nucleotide monophosphate byproduct back to the cytoplasm. Dysregulation of human NSTs causes several debilitating diseases, and NSTs are virulence factors for many pathogens. Here we present the first crystal structures of a mammalian NST, the mouse CMP-sialic acid transporter (mCST), in complex with its physiological substrates CMP and CMP-sialic acid. Detailed visualization of extensive protein-substrate interactions explains the mechanisms governing substrate selectivity. Further structural analysis of mCST’s unique lumen-facing partially-occluded conformation, coupled with the characterization of substrate-induced quenching of mCST’s intrinsic tryptophan fluorescence, reveals the concerted conformational transitions that occur during substrate transport. These results provide a framework for understanding the effects of disease-causing mutations and the mechanisms of this diverse family of transporters.

The cells in our body are tiny machines which, amongst other things, produce proteins. One of the production steps involves a compartment in the cell called the Golgi, where proteins are tagged and packaged before being sent to their final destination. In particular, sugars can be added onto an immature protein to help to fold it, stabilize it, and to affect how it works.

Before sugars can be attached to a protein, they need to be ‘activated’ outside of the Golgi by attaching to a small molecule known as a nucleotide. Then, these ‘nucleotide-sugars’ are ferried across the Golgi membrane and inside the compartment by nucleotide-sugar transporters, or NSTs. Humans have seven different kinds of NSTs, each responsible for helping specific types of nucleotide-sugars cross the Golgi membrane. Changes in NSTs are linked to several human diseases, including certain types of epilepsy; these proteins are also important for dangerous microbes to be able to infect cells. Yet, scientists know very little about how the transporters recognize their cargo, and how they transport it.

To shed light on these questions, Ahuja and Whorton set to uncover for the first time the 3D structure of a mammalian NST using a method known as X-ray crystallography. This revealed how nearly every component of this transporter is arranged when the protein is bound to two different molecules: a specific nucleotide, or a type of nucleotide-sugar. The results help to understand how changes in certain components of the NST can lead to a problem in the way the protein works. Ultimately, this knowledge may be useful to prevent diseases linked to faulty NSTs, or to stop microbes from using the transporters to their own advantage.

Functional expression of a human GDP-L-fucose transporter in Escherichia coli

OBJECTIVES

To investigate the translocation of nucleotide-activated sugars from the cytosol across a membrane into the endoplasmatic reticulum or the Golgi apparatus which is an important step in the synthesis of glycoproteins and glycolipids in eukaryotes.

RESULTS

The heterologous expression of the recombinant and codon-adapted human GDP-L-fucose antiporter gene SLC35C1 (encoding an N-terminal OmpA-signal sequence) led to a functional transporter protein located in the cytoplasmic membrane of Escherichia coli. The in vitro transport was investigated using inverted membrane vesicles. SLC35C1 is an antiporter specific for GDP-L-fucose and depending on the concomitant reverse transport of GMP. The recombinant transporter FucT1 exhibited an activity for the transport of ³H-GDP-L-fucose with a V_max of 8 pmol/min mg with a K_m of 4 µM. The functional expression of SLC35C1 in GDP-L-fucose overproducing E. coli led to the export of GDP-L-fucose to the culture supernatant.

CONCLUSIONS

The export of GDP-L-fucose by E. coli provides the opportunity for the engineering of a periplasmatic fucosylation reaction in recombinant bacterial cells.

Disease mutations in CMP-sialic acid transporter SLC35A1 result in abnormal α-dystroglycan O-mannosylation, independent from sialic acid

Binding of cellular α-dystroglycan (α-DG) to its extracellular matrix ligands is fully dependent on a unique O-mannose-linked glycan. Disrupted O-mannosylation is the hallmark of the muscular dystrophy-dystroglycanopathy (MDDG) syndromes. SLC35A1, encoding the transporter of cytidine 5'-monophosphate-sialic acid, was recently identified as MDDG candidate gene. This is surprising, since sialic acid itself is dispensable for α-DG-ligand binding. In a novel SLC35A1-deficient cell model, we demonstrated a lack of α-DG O-mannosylation, ligand binding and incorporation of sialic acids. Removal of sialic acids from HAP1 wild-type cells after incorporation or preventing sialylation during synthesis did not affect α-DG O-mannosylation or ligand binding but did affect sialylation. Lentiviral-mediated complementation with the only known disease mutation p.Q101H failed to restore deficient O-mannosylation in SLC35A1 knockout cells and partly restored sialylation. These data indicate a role for SLC35A1 in α-DG O-mannosylation that is distinct from sialic acid metabolism. In addition, human SLC35A1 deficiency can be considered as a combined disorder of α-DG O-mannosylation and sialylation, a novel variant of the MDDG syndromes.

Short N-terminal region of UDP-galactose transporter (SLC35A2) is crucial for galactosylation of N-glycans

UDP-galactose transporter (UGT) and UDP-N-acetylglucosamine transporter (NGT) form heterologous complexes in the Golgi apparatus (GA) membrane. We aimed to identify UGT region responsible for galactosylation of N-glycans. Chimeric proteins composed of human UGT and either NGT or CMP-sialic acid transporter (CST) localized to the GA, and all but UGT/CST chimera corrected galactosylation defect in UGT-deficient cell lines, although at different efficiency. Importantly, short N-terminal region composed of 35 N-terminal amino-acid residues of UGT was crucial for galactosylation of N-glycans. The remaining molecule must be derived from NGT not CST, confirming that the role played by UGT and NGT is coupled.

Expression, solubilisation, and purification of a functional CMP-sialic acid transporter in Pichia pastoris

Membrane proteins, including solute transporters play crucial roles in cellular function and have been implicated in a variety of important diseases, and as such are considered important targets for drug development. Currently the drug discovery process is heavily reliant on the structural and functional information discerned from high-resolution crystal structures. However, membrane protein structure determination is notoriously difficult, due in part to challenges faced in their expression, solubilisation and purification. The CMP-sialic acid transporter (CST) is considered to be an attractive target for drug discovery. CST inhibition reduces cancer cell sialylation and decreases the metastatic potential of cancer cells and to date, no crystal structure of the CST, or any other nucleotide sugar transporter exists. Here we describe the optimised conditions for expression in Pichia pastoris, solubilisation using n-nonyl β-d-maltopyranoside (NM) and single step purification of a functional CST. Importantly we show that despite being able to solubilise and purify the CST using a number of different detergents, only NM was able to maintain CST functionality.

Structure and function of nucleotide sugar transporters: Current progress

The proteomes of eukaryotes, bacteria and archaea are highly diverse due, in part, to the complex post-translational modification of protein glycosylation. The diversity of glycosylation in eukaryotes is reliant on nucleotide sugar transporters to translocate specific nucleotide sugars that are synthesised in the cytosol and nucleus, into the endoplasmic reticulum and Golgi apparatus where glycosylation reactions occur. Thirty years of research utilising multidisciplinary approaches has contributed to our current understanding of NST function and structure. In this review, the structure and function, with reference to various disease states, of several NSTs including the UDP-galactose, UDP-N-acetylglucosamine, UDP-N-acetylgalactosamine, GDP-fucose, UDP-N-acetylglucosamine/UDP-glucose/GDP-mannose and CMP-sialic acid transporters will be described. Little is known regarding the exact structure of NSTs due to difficulties associated with crystallising membrane proteins. To date, no three-dimensional structure of any NST has been elucidated. What is known is based on computer predictions, mutagenesis experiments, epitope-tagging studies, in-vitro assays and phylogenetic analysis. In this regard the best-characterised NST to date is the CMP-sialic acid transporter (CST). Therefore in this review we will provide the current state-of-play with respect to the structure–function relationship of the (CST). In particular we have summarised work performed by a number groups detailing the affect of various mutations on CST transport activity, efficiency, and substrate specificity.

Characterisation of CMP-sialic acid transporter substrate recognition

CMP-sialic acid transporter: We report an in-depth, multidisciplinary, structural study that has identified the amino acid residues intimately involved in CMP-sialic acid transporter (CST) substrate specificity. Our data provide a significant contribution towards a better understanding the structure-function relationship of this important family of transporters and the rational design of CST inhibitors.

The targeted expression of nucleotide sugar transporters to the E. coli inner membrane

The heterologous expression of functional mammalian integral membrane proteins still represents a significant hurdle towards evaluating the relationship between their structure and function. We have therefore utilised the OmpA signal sequence to deliberately target the expression of a mammalian nucleotide sugar transporter, the murine CMP-sialic acid transporter, to the E. coli inner membrane. The functionality of the recombinant CMP-sialic acid transporter could then be evaluated either following the spheroplasting of E. coli cells or through the isolation of the E. coli inner membrane and the formation of mixed phosphatidylcholine-inner membrane proteoliposomes.

A single UDP-galactofuranose transporter is required for galactofuranosylation in Aspergillus fumigatus

Galactofuranose (Galf) containing molecules have been described at the cell surface of several eukaryotes and shown to contribute to the virulence of the parasite Leishmania major and the fungus Aspergillus fumigatus. It is anticipated that a number of the surface glycoconjugates such as N-glycans or glycolipids are galactofuranosylated in the Golgi apparatus. This raises the question of how the substrate for galactofuranosylation reactions, UDP-Galf, which is synthesized in the cytosol, translocates into the organelles of the secretory pathway. Here we report the first identification of a Golgi-localized nucleotide sugar transporter, named GlfB, with specificity for a UDP-Galf. In vitro transport assays established binding of UDP-Galf to GlfB and excluded transport of several other nucleotide sugars. Furthermore, the implication of glfB in the galactofuranosylation of A. fumigatus glycoconjugates and galactomannan was demonstrated by a targeted gene deletion approach. Our data reveal a direct connection between galactomannan and the organelles of the secretory pathway that strongly suggests that the cell wall-bound polysaccharide originates from its glycosylphosphatidylinositol-anchored form.

Nucleotide sugar transporters of the Golgi apparatus

The Golgi apparatus is the major site of protein, lipid and proteoglycan glycosylation. The glycosylation enzymes, as well as kinases and sulfatases that catalyze phosphorylation and sulfation, are localized within the Golgi cisternae in characteristic distributions that frequently reflect their order in a particular pathway (Kornfeld and Kornfeld 1985; Colley 1997). The glycosyl-transferases, sulfotransferases and kinases are “transferases” that require activated donor molecules for the reactions they catalyze. For eukaryotic, fungal and protozoan glycosyltransferases these are the nucleotide sugars UDP-N-acetylglucosamine (UDP-GlcNAc), UDP-galactose (UDP-Gal), GDP-fucose (GDP-Fuc), CMP-sialicacid (CMP-Sia), UDP-glucuronicacid (UDP-GlcA), GDP-mannose (GDP-Man), and UDP-xylose (UDP-Xyl) (Hirschberg et al. 1998). For the kinases, ATP functions as the donor, while for the sulfotransferases, adenosine 3′-phosphate 5′-phosphate (PAPS) acts as the donor (Hirschberg et al. 1998). The active sites of all these enzymes are oriented towards the lumen of the Golgi cisternae. This necessitates the translocation of their donors from the cytosol into the lumenal Golgi compartments. In this chapter we will focus on the structure, function and localization of the Golgi nucleotide sugar transporters (NSTs), and highlight the diseases and developmental defects associated with defective transporters. We direct the reader to several excellent reviews on Golgi transporters for additional details and references (Hirschberg et al. 1998; Berninsone and Hirschberg 2000; Gerardy-Schahn et al. 2001; Handford et al. 2006; Caffaro and Hirschberg 2006).

Targeting the expression of functional murine CMP-sialic acid transporter to the E. coli inner membrane

The heterologous expression of functional mammalian integral membrane proteins still represents a significant hurdle towards their crystallization and structure elucidation. We have therefore explored the use of the OmpA signal sequence to deliberately target the expression of the murine CMP-sialic acid transporter, a Golgi-resident protein with 10 putative transmembrane domains, to the Escherichia coli inner membrane. Here, we show that the expression of an OmpA signal sequence-FLAG-CMP-sialic acid transporter fusion protein in E. coli results in the targeting and insertion of recombinant protein within the inner membrane. Significantly, functionality was confirmed by the ability of spheroplasted E. coli and mixed phosphatidylcholine-E. coli inner membrane proteoliposomes incorporating recombinant CMP-sialic acid transporter to accumulate CMP-sialic acid in vitro.

High-throughput fluorescent-based optimization of eukaryotic membrane protein overexpression and purification in Saccharomyces cerevisiae

Eukaryotic membrane proteins are often difficult to produce in large quantities, which is a significant obstacle for further structural and biochemical investigation. Based on the analysis of 43 eukaryotic membrane proteins, we present a cost-effective high-throughput approach for rapidly screening membrane proteins that can be overproduced to levels of >1 mg per liter in Saccharomyces cerevisiae. We find that 70% of the well expressed membrane proteins tested in this system are stable, targeted to the correct organelle, and monodisperse in either Fos-choline 12 (FC-12) or n-dodecyl-beta-D-maltoside. We illustrate the advantage of such an approach, with the purification of monodisperse human and yeast nucleotide-sugar transporters to unprecedented levels. We estimate that our approach should be able to provide milligram quantities for at least one-quarter of all membrane proteins from both yeast and higher eukaryotic organisms.

The CMP-sialic acid transporter is localized in the medial-trans Golgi and possesses two specific endoplasmic reticulum export motifs in its carboxyl-terminal cytoplasmic tail

The addition of sialic acid to glycoproteins and glycolipids requires Golgi sialyltransferases to have access to their glycoconjugate substrates and nucleotide sugar donor, CMP-sialic acid. CMP-sialic acid is transported into the lumen of the Golgi complex through the CMP-sialic acid transporter, an antiporter that also functions to transport CMP into the cytosol. We localized the transporter using immunofluorescence and deconvolution microscopy to test the prediction that it is broadly distributed across the Golgi stack to serve the many sialyltransferases involved in glycoconjugate sialylation. The transporter co-localized with ST6GalI in the medial and trans Golgi, showed partial overlap with a medial Golgi marker and little overlap with early Golgi or trans Golgi network markers. Endoplasmic reticulum-retained forms of sialyltransferases did not redistribute the transporter from the Golgi to the endoplasmic reticulum, suggesting that transporter-sialyltransferase complexes are not involved in transporter localization. Next we evaluated the role of the transporter's N- and C-terminal cytoplasmic tails in its trafficking and localization. The N-tail was not required for either endoplasmic reticulum export or Golgi localization. The C-tail was required for endoplasmic reticulum export and contained di-Ile and terminal Val motifs at its very C terminus that function as independent endoplasmic reticulum export signals. Deletion of the last four amino acids of the C-tail (IIGV) eliminated these export signals and prevented endoplasmic reticulum export of the transporter. This form of the transporter supplied limited amounts of CMP-sialic acid to Golgi sialyltransferases but was unable to completely rescue the transporter defect of Lec2 Chinese hamster ovary cells.

Identification and characterization of a novel Drosophila 3'-phosphoadenosine 5'-phosphosulfate transporter

Sulfation of macromolecules requires the translocation of a high energy form of nucleotide sulfate, i.e. 3'-phosphoadenosine 5'-phosphosulfate (PAPS), from the cytosol into the Golgi apparatus. In this study, we identified a novel Drosophila PAPS transporter gene dPAPST2 by conducting data base searches and screening the PAPS transport activity among the putative nucleotide sugar transporter genes in Drosophila. The amino acid sequence of dPAPST2 showed 50.5 and 21.5% homology to the human PAPST2 and SLALOM, respectively. The heterologous expression of dPAPST2 in yeast revealed that the dPAPST2 protein is a PAPS transporter with an apparent K(m) value of 2.3 microm. The RNA interference of dPAPST2 in cell line and flies showed that the dPAPST2 gene is essential for the sulfation of cellular proteins and the viability of the fly. In RNA interference flies, an analysis of the genetic interaction between dPAPST2 and genes that contribute to glycosaminoglycan synthesis suggested that dPAPST2 is involved in the glycosaminoglycan synthesis and the subsequent signaling. The dPAPST2 and sll genes showed a similar ubiquitous distribution. These results indicate that dPAPST2 may be involved in Hedgehog and Decapentaplegic signaling by controlling the sulfation of heparan sulfate.