Labeling of the glycoprotein subunit of (Na,K)ATPase with fluorescent probes

Structure and Function of Na,K-ATPase-The Sodium-Potassium Pump

Na,K-ATPase is an ubiquitous enzyme actively transporting Na-ions out of the cell in exchange for K-ions, thereby maintaining their concentration gradients across the cell membrane. Since its discovery more than six decades ago the Na-pump has been studied extensively and its vital physiological role in essentially every cell has been established. This article aims at providing an overview of well-established biochemical properties with a focus on Na,K-ATPase isoforms, its transport mechanism and principle conformations, inhibitors, and insights gained from crystal structures. © 2021 American Physiological Society. Compr Physiol 11:1-21, 2021.

Site-specific modification of recombinant proteins: a novel platform for modifying glycoproteins expressed in E. coli

The site-specific modification of proteins is expected to be an important capability for the synthesis of bioconjugates in the future. However, the traditional repertoire of reactions available for the direct modification of proteins suffers from lack of specificity, necessitating costly downstream processing to isolate the specific species of interest. (1) Here, we use a well-established, glycan-specific chemistry to PEGylate model glycoproteins, each containing a unique reactive GalNAc attached to a specifically engineered threonine residue. By engineering E. coli to execute the initial steps of human, mucin-type O-glycosylation, we were able to obtain homogeneous site-specifically modified glycoproteins with fully human glycan linkages. Two mucin-based reporters as well as several fusion proteins containing eight-amino-acid GalNAc-T recognition sequences were glycosylated in this engineered glycocompetent strain of E. coli. The use of one sequence in particular, PPPTSGPT, resulted in site-specific glycan occupancy of approximately 69% at the engineered threonine. The GalNAc present on the purified glycoprotein was oxidized by galactose oxidase and then coupled to hydroxylamine functionalized 20 kDa PEG in the presence of aniline. The glycoprotein could be converted to the PEGylated product at approximately 85% yield and >98% purity as determined by comparison to the products of control reactions.

Hyaluronic Acid-Based Microgels and Microgel Networks for Vocal Fold Regeneration

Vocal fold scarring disrupts the viscoelastic properties of the lamina propria that are critical for normal phonation. There is a clinical need for the development of advanced biomaterials that approximate the mechanical properties of the lamina propria for in vivo vocal fold regeneration. We have developed hyaluronic acid (HA)-based microgels and cross-linked microgel networks with tunable degradation and mechanical properties. HA microgels were prepared by cross-linking HA derivatives carrying hydrazide (HAADH) and aldehyde (HAALD) functionalities within the inverse emulsion droplets. Alternatively, poly(ethylene glycol) dialdehyde (PEGDiALD) was employed in place of HAALD. Microgels based on HAADH/HAALD are more resistant to enzymatic degradation than those generated from HAADH/PEGDiALD. In vitro cytotoxicity studies using vocal fold fibroblasts indicate that microgels synthesized from HAADH/HAALD are essentially nontoxic, whereas microgels derived from HAADH/PEGDiALD exhibit certain adverse effects on the cultured cells at high concentration (> or =2 mg/mL). These microgels exhibit residual functional groups that can be used as reactive handles for covalent conjugation of therapeutic molecules. The presence of residual functional groups also allows for subsequent cross-linking of the microgels with other reactive polymers, giving rise to doubly cross-linked networks (DXNs) with tunable viscoelasticity. Mechanical measurements using a torsional wave apparatus indicate that HA-based DXNs exhibit elastic moduli that are similar to those of vocal fold lamina propria at frequencies close to the range of human phonation. These HA-based microgel systems are promising candidates for the treatment of vocal fold scarring, not just as biocompatible filler materials, but as smart entities that can repair focal defects, smooth the vocal fold margin, and potentially soften and dissolve scar tissue.

Excited-state dynamics of the fluorescent probe Lucifer Yellow in liquid solutions and in heterogeneous media

The photophysics of the dye Lucifer Yellow ethylenediamine (LYen) has been investigated in various polar solvents. The main deactivation pathways of its first singlet excited state are the fluorescence and the intersystem crossing. In water, non-radiative decay by intermolecular proton transfer becomes a significant deactivation channel. The early fluorescence dynamics, which was investigated in liquids and in reverse micelles, was found to depend substantially on the environment. An important static quenching of LYen by tryptophan and indole occurring in the subpicosecond timescale was observed. The use of the fluorescence dynamics of LYen as a local probe is illustrated by preliminary results obtained with a biotinylated Lucifer Yellow derivative complexed with avidin.

Surface engineering of living myoblasts via selective periodate oxidation

Cell surface molecules are vital for normal cell activity. To study the functions of these molecules or manipulate cell behavior, the ability to decorate cell surfaces with bioactive molecules of our choosing is a potentially powerful technique. Here, we describe the molecular engineering of living L6 myoblast monolayers via selective periodate oxidation of sialic acid residues and the application of this surface modification in the artificial aggregation of cells. The aldehyde groups generated by this reaction were used to selectively ligate a model molecule, biotin hydrazide, to the cell surfaces. Flow cytometry analysis after staining with fluorescently conjugated avidin revealed a concentration-dependent increase in fluorescence compared to untreated cells with a maximal shift of 345.1 +/- 27.4-fold and an EC(50) of 17.4 +/- 1.1 microM. This mild oxidation reaction did not affect cell number, viability, or morphology. We then compared this chemical technique with the metabolic incorporation of reactive cell surface ketone groups using N-levulinoylmannosamine (ManLev). In this cell line, only a 22.3-fold fluorescence shift was observed compared to untreated cells when myoblasts were incubated with a high concentration of ManLev for 48 hours. Periodate oxidation was then used to modify myoblast surfaces to induce cell aggregation. Crosslinking biotinylated myoblasts, which do not spontaneously aggregate in culture, with avidin resulted in the rapid formation of millimeter-sized, multicellular structures. These data indicate that sodium periodate treatment is an effective, noncytotoxic method for the in vitro molecular engineering of living cell surfaces with the potential for cell biology and tissue engineering applications.

Chemoenzymatic synthesis of biotinylated nucleotide sugars as substrates for glycosyltransferases

The enzymatic oxidation of uridine 5'-diphospho-alpha-D-galactose (UDP-Gal) and uridine 5'-diphospho-N-acetyl-alpha-D-galactosamine (UDP-GalNAc) with galactose oxidase was combined with a chemical biotinylation step involving biotin-epsilon-amidocaproylhydrazide in a one-pot synthesis. The novel nucleotide sugar derivatives uridine 5'-diphospho-6-biotin-epsilon-amidocaproylhydrazino-alpha-D-galactose (UDP-6-biotinyl-Gal) and uridine 5'-diphospho-6-biotin-epsilon-amidocaproylhydrazino-N-acetyl-alpha-D-galactosamine (UDP-6-biotinyl-GalNAc) were synthesized on a 100-mg scale and characterized by mass spectrometry (fast atom bombardment and matrix-assisted laser desorption/ionization time of flight) and one/two dimensional NMR spectroscopy. It could be demonstrated for the first time, by use of UDP-6-biotinyl-Gal as a donor substrate, that the human recombinant galactosyltransferases beta3Gal-T5, beta4Gal-T1, and beta4Gal-T4 mediate biotinylation of the neoglycoconjugate bovine serum albumin-p-aminophenyl N-acetyl-beta-D-glucosaminide (BSA-(GlcNAc)17) and ovalbumin. The detection of the biotin tag transferred by beta3Gal-T5 onto BSA-(GlcNAc)17 with streptavidin-enzyme conjugates gave detection limits of 150 pmol of tagged GlcNAc in a Western blot analysis and 1 pmol of tagged GlcNAc in a microtiter plate assay. The degree of Gal-biotin tag transfer onto agalactosylated hybrid N-glycans present at the single glycosylation site of ovalbumin was dependent on the Gal-T used (either beta3Gal-T5, beta4Gal-T4, or beta4Gal-T1), which indicates that the acceptor specificity may direct the transfer of the Gal-biotin tag. The potential of this biotinylated UDP-Gal as a novel donor substrate for human galactosyltransferases lies in the targeting of distinct acceptor structures, for example, under-galactosylated glycoconjugates, which are related to diseases, or in the quality control of glycosylation of recombinant and native glycoproteins.

Interactions of the human class II major histocompatibility complex protein HLA-DR4 with a peptide ligand demonstrated by affinity capillary electrophoresis

The interactions of empty recombinant major histocompatibility complex (MHC) class II molecules (DRA1*0101/DRB1*0401) with a known peptide ligand [the HA(307-319) fragment of influenza virus hemagglutinin] were studied by capillary electrophoresis. Using an alkaline buffer system with the addition of non-ionic or zwitterionic detergent and high sensitivity laser-induced fluorescence detection, both slowly and rapidly equilibrating binding could be demonstrated. This was accomplished using a pre-equilibration approach as well as migration shift experiments where receptor molecules were added to the electrophoresis buffer. This system may be useful for the study of both peptide binding to MHC molecules and screening for inhibition or amplification of binding by other ligands as well as for the study of the interactions of T-cell receptors with MHC-peptide complexes.

The carbohydrate moieties of the beta-subunit of Na+, K(+)-ATPase: their lateral motions and proximity to the cardiac glycoside site

The beta-subunit associated with the catalytic (alpha) subunit of the mammalian Na+, K(+) -ATPase is a transmembrane glycoprotein with three extracellularly located N-glycosylation sites. Although beta appears to be essential for a functional enzyme, the role of beta and its sugars remains unknown. In these studies, steady-state and dynamic fluorescence measurements of the fluorophore lucifer yellow (LY) covalently linked to the carbohydrate chains of beta have demonstrated that the bound probes are highly solvent exposed but restricted in their diffusional motions. Furthermore, the probes' environments on beta were not altered by Na+ or K+ or ouabain-induced enzyme conformational changes, but both divalent cation and oligomycin addition evoked modest changes in LY fluorescence. Frequency domain measurements reflecting the Förster fluorescence energy transfer (FET) occurring between anthroylouabain (AO) bound to the cardiac glycoside receptor site on alpha and the carbohydrate-linked LY demonstrated their close proximity (18 A). Additional FET determinations made between LY as donor and erythrosin-5-isothiocyanate, covalently bound at the enzyme's putative ATP binding site domain, indicated that a distance of about 85 A separates these two regions and that this distance is reduced upon divalent cation binding and increased upon the Na+E1-->K+E2 conformational transition. These data suggest a model for the localization of the terminal moieties of the oligosaccharides that places them, on average, about 18 A from the AO binding site and this distance or less from the extracellular membrane surface.

Inhibition of Na(+)-pump expression by impairment of protein glycosylation is independent of the reduced sodium entry into the cell

Previous studies indicate that inhibition of protein N-glycosylation reduces Na(+)-pump activity. Since this effect is preceded by an inhibition of the entry of sodium into the cell, it is unclear whether the reduced Na(+)-pump is produced by the inactivation of protein glycosylation per se or by the lower intracellular sodium concentration. We compared the effects of tunicamycin, which inhibits protein glycosylation, and amiloride, which inhibits the entry of sodium into the cell, on the expression of the Na(+)-pump activity in A6 cells. The short-circuit current across A6 epithelia, which corresponds to sodium ions transported through the Na+ channel and the Na(+)-pump, was almost totally inhibited after 24-hr treatment with 1 microgram/ml tunicamycin. The maximal Na(+)-pump activity, measured after permeabilizing the apical cell membrane with amphotericin B, was only 30% inhibited. This inhibition increased to 80% after 72-hr treatment with tunicamycin. Thus, tunicamycin inhibits the activities of both the apical Na+ channel and the basolateral Na(+)-pump. However, the reduced number of Na(+)-pump molecules, as well as the inhibition of the Na(+)-pump activity, were not observed when the Na+ channel was inhibited for 72-hr with amiloride. Thus, the reduced Na(+)-pump expression produced by inactivation of protein glycosylation is not secondary to reduced entry of sodium into the cell.

The galactose-binding sites of the cytotoxic lectin ricin can be chemically blocked in high yield with reactive ligands prepared by chemical modification of glycopeptides containing triantennary N-linked oligosaccharides

A glycopeptide containing a triantennary N-linked oligosaccharide from fetuin was modified by a series of chemical and enzymic reactions to afford a reagent that contained a terminal residue of 6-(N-methylamino)-6-deoxy-D-galactose on one branch of the triantennary structure and terminal galactose residues on the other two branches. Binding assays and gel filtration experiments showed that this modified glycopeptide could bind to the sugar-binding sites of ricin. The ligand was activated at the 6-(N-methylamino)-6-deoxy-D-galactose residue by reaction with cyanuric chloride. The resulting dichlorotriazine derivative of the ligand reacts with ricin, forming a stable covalent linkage. The reaction was confined to the B-chain and was inhibited by lactose. Bovine serum albumin and ovalbumin were not modified by the activated ligand under similar conditions, and we conclude, therefore, that the reaction of the ligand with ricin B-chain was dependent upon specific binding to sugar-binding sites. Ricin that had its galactose-binding sites blocked by the covalent reaction with the activated ligand was purified by affinity chromatography. The major species in this fraction was found to contain 2 covalently linked ligands per ricin B-chain, while a minor species contained 3 ligands per B-chain. The cytotoxicity of blocked ricin was at least 1000-fold less than that of native ricin for cultured cells in vitro, even though the activity of the A-chain in a cell-free system was equal to that from native ricin. Modified ricin that contained only 1 covalently linked ligand was also purified. This fraction retained an ability to bind to galactose affinity columns, although with a lower affinity than ricin, and was only 5- to 20-fold less cytotoxic than native ricin.

Chemical modification as an approach to elucidation of sodium pump structure-function relations

Chemical modification of specific residues in enzymes, with the characterization of the type of inhibition and properties of the modified activity, is an established approach in structure-function studies of proteins. This strategy has become more productive in recent years with the advances made in obtaining primary sequence information from gene-cloning technologies. This article discusses the application of chemical modification procedures to the study of the Na(+)-K(+)-ATPase protein. A wide array of information has become available about the kinetics, enzyme structure, and various conformational states as a result of the combined use of inhibitors, ligands, modifiers, and proteolytic enzymes. We will review a variety of reagents and approaches that have been employed to arrive at structure-function correlates and discuss critically the limits and ambiguities in the type of information obtained from these methodologies. Chemical modification of the Na(+)-pump protein has already provided a body of data and will, we anticipate, guide the efforts of mutagenesis studies in the future when suitable expression systems become available.

Detailed structural analysis of exposed domains of membrane-bound Na+,K+-ATPase. A model of transmembrane arrangement

Exposed regions of the alpha- and beta-subunits of membrane-bound Na+,K+-ATPase were in turn hydrolyzed with trypsin. Resistance of the beta-subunit to proteolysis was shown to be due mainly to the presence of disulfide bridge(s) in the molecule. A model for the spatial organisation of the enzyme in the membrane was proposed on the basis of detailed structural analysis of extramembrane regions of both subunits.

A fluorometric method for the determination of functional (Na,K)-ATPase and cardiac glycoside receptors

A method to measure high-affinity binding sites for fluorescent ligands is described. The method is applied to the determination of (Na,K)-ATPase using the fluorescent ouabain derivative anthroylouabain (P. A. G. Fortes (1977) Biochemistry 16, 531-540). The method consists of measurements of the fluorescence intensities of a saturating concentration of anthroylouabain in the presence and absence of (Na,K)-ATPase and ouabain. These data and the fluorescence enhancement factor upon anthroylouabain binding are used to calculate the concentration of binding sites. The measurements can be done in a few minutes and 10 to 100 pmoles of ouabain sites is sufficient for accurate determinations. Because phosphorylation of (Na,K)-ATPase is necessary to bind anthroylouabain, only functional enzyme is detected by this method.