What Happens If a Human Galectin Enters the Endoplasmic Reticulum?

Mammalian galectins have no signal peptide, and it is not known what would happen if a galectin is directed to take the classical export route. The corresponding engineering of galectin-specific cDNA will answer questions on the fate of a signal peptide-bearing protein variant after its entry into the endoplasmic reticulum (ER). Affinity chromatography and mass-spectrometric analysis of occupancy of potential N-glycosylation sites for the galectin, binding and functional assays with cells as well as subcellular fractionation by density gradient ultracentrifugation and immunocytochemical colocalization with ER/Golgi markers report on aspects of the consequences of letting a galectin enter new territory. Applying these methods will help to clarify why galectins are leaderless and thus produced by free ribosomes.

Characterizing ligand-induced conformational changes in clinically relevant galectin-1 by HN/H2O (D2O) exchange.

Glycans of cellular glycoconjugates serve as biochemical signals for a multitude of (patho)physiological processes via binding to their receptors (e.g. lectins). In the case of human adhesion/growth-regulatory galectin-1 (Gal-1), small angle neutron scattering and fluorescence correlation spectroscopy have revealed a significant decrease of its gyration radius and increase of its diffusion coefficient upon binding lactose, posing the pertinent question on the nature and region(s) involved in the underlying structural alterations. Requiring neither a neutron source nor labeling, diffusion measurements by 1H NMR spectroscopy are shown here to be sufficiently sensitive to detect this ligand-induced change. In order to figure out which region(s) of Gal-1 is (are) affected at the level of peptides, we first explored the use of H/D exchange mass spectrometry (HDX MS). Hereby, we found a reduction in proton exchange kinetics beyond the lactose-binding site. The measurement of fast HN/H2O exchange by phase-modulated NMR clean chemical exchange (CLEANEX) NMR on 15N-labeled Gal-1 then increased the spatial resolution to the level of individual amino acids. The mapped regions with increased protection from HN/H2O (D2O) exchange that include the reduction of solvent exposure around the interface can underlie the protein's compaction. These structural changes have potential to modulate this galectin's role in lattice formation on the cell surface and its interaction(s) with protein(s) at the F-face.

How presence of a signal peptide affects human galectins-1 and -4: Clues to explain common absence of a leader sequence among adhesion/growth-regulatory galectins.

BACKGROUND: Galectins are multifunctional effectors, which all share absence of a signal sequence. It is not clear why galectins belong to the small set of proteins, which avoid the classical export route. METHODS: Products of recombinant galectin expression in P. pastoris were analyzed by haemagglutination, gel filtration and electrophoresis and lectin blotting as well as mass spectrometry on the level of tryptic peptides and purified glycopeptides(s). Density gradient centrifugation and confocal laser scanning microscopy facilitated localization in transfected human and rat cells, proliferation assays determined activity as growth mediator. RESULTS: Directing galectin-1 to the classical secretory pathway in yeast produces N-glycosylated protein that is active. It cofractionates and -localizes with calnexin in human cells, only Gal-4 is secreted. Presence of N-glycan(s) reduces affinity of cell binding and growth regulation by Gal-1. CONCLUSIONS: Folding and activity of a galectin are maintained in signal-peptide-directed routing, N-glycosylation occurs. This pathway would deplete cytoplasm and nucleus of galectin, presence of N-glycans appears to interfere with lattice formation. GENERAL SIGNIFICANCE: Availability of glycosylated galectins facilitates functional assays to contribute to explain why galectins invariably avoid classical routing for export.

Design-functionality relationships for adhesion/growth-regulatory galectins.

Glycan-lectin recognition is assumed to elicit its broad range of (patho)physiological functions via a combination of specific contact formation with generation of complexes of distinct signal-triggering topology on biomembranes. Faced with the challenge to understand why evolution has led to three particular modes of modular architecture for adhesion/growth-regulatory galectins in vertebrates, here we introduce protein engineering to enable design switches. The impact of changes is measured in assays on cell growth and on bridging fully synthetic nanovesicles (glycodendrimersomes) with a chemically programmable surface. Using the example of homodimeric galectin-1 and monomeric galectin-3, the mutual design conversion caused qualitative differences, i.e., from bridging effector to antagonist/from antagonist to growth inhibitor and vice versa. In addition to attaining proof-of-principle evidence for the hypothesis that chimera-type galectin-3 design makes functional antagonism possible, we underscore the value of versatile surface programming with a derivative of the pan-galectin ligand lactose. Aggregation assays with N,N'-diacetyllactosamine establishing a parasite-like surface signature revealed marked selectivity among the family of galectins and bridging potency of homodimers. These findings provide fundamental insights into design-functionality relationships of galectins. Moreover, our strategy generates the tools to identify biofunctional lattice formation on biomembranes and galectin-reagents with therapeutic potential.

Chicken GRIFIN: Structural characterization in crystals and in solution.

Despite its natural abundance in lenses of vertebrates the physiological function(s) of the galectin-related inter-fiber protein (GRIFIN) is (are) still unclear. The same holds true for the significance of the unique interspecies (fish/birds vs mammals) variability in the capacity to bind lactose. In solution, ultracentrifugation and small angle X-ray scattering (at concentrations up to 9 mg/mL) characterize the protein as compact and stable homodimer without evidence for aggregation. The crystal structure of chicken (C-)GRIFIN at seven pH values from 4.2 to 8.5 is reported, revealing compelling stability. Binding of lactose despite the Arg71Val deviation from the sequence signature of galectins matched the otherwise canonical contact pattern with thermodynamics of an enthalpically driven process. Upon lactose accommodation, the side chain of Arg50 is shifted for hydrogen bonding to the 3-hydroxyl of glucose. No evidence for a further ligand-dependent structural alteration was obtained in solution by measuring hydrogen/deuterium exchange mass spectrometrically in peptic fingerprints. The introduction of the Asn48Lys mutation, characteristic for mammalian GRIFINs that have lost lectin activity, lets labeled C-GRIFIN maintain capacity to stain tissue sections. Binding is no longer inhibitable by lactose, as seen for the wild-type protein. These results establish the basis for detailed structure-activity considerations and are a step to complete the structural description of all seven members of the galectin network in chicken.

Application of Hydrogen/Deuterium Exchange-Mass Spectrometry to Biopharmaceutical Development Requirements: Improved Sensitivity to Detection of Conformational Changes.

The usefulness of the higher-order structure information provided by hydrogen/deuterium exchange mass spectrometry (HDX-MS) in the protein therapeutic field is undisputed; however, its applicability as a method for critical quality and comparability assessment has until now not been demonstrated. Here we present results demonstrating for the first time the applicability of the HDX-MS technique to monitor structural changes due to methionine oxidation at sensitivity levels realistic to the requirements of biopharmaceutical research and development. For the analyzed heavy chain marker peptides deuterium uptake differences due to oxidation at the conserved methionine in position 254 were significantly verifiable at the lowest increase (1%) through spiked oxidized IgG1.

Mapping the Interactions of Selective Biochemical Probes of Antibody Conformation by Hydrogen-Deuterium Exchange Mass Spectrometry.

Protein-based pharmaceuticals represent the fastest growing group of drugs in development in the pharmaceutical industry. One of the major challenges in the discovery, development, and distribution of biopharmaceuticals is the assessment of changes in their higher-order structure due to chemical modification. Here, we investigated the interactions of three different biochemical probes (Fab s) generated to detect conformational changes in a therapeutic IgG1 antibody (mAbX) by local hydrogen-deuterium exchange mass spectrometry (HDX-MS). We show that two of the probes target the Fc part of the antibody, whereas the third probe binds to the hinge region. Through HDX-ETD, we could distinguish specific binding patterns of the Fc -binding probes on mAbX at the amino-acid level. Preliminary surface plasmon resonance (SPR) experiments showed that these domain-selective Fab probes are sensitive to conformational changes in distinct regions of a full-length therapeutic antibody upon oxidation.

Development, preclinical safety, formulation, and stability of clinical grade bevacizumab-800CW, a new near infrared fluorescent imaging agent for first in human use.

There is a dire need for better visualization of cancer and analysis of specific targets in vivo. Molecular imaging with fluorescence is gaining more and more attention, as it allows detection of these targets and has advantages over radioactivity, such as no radiation dose, and lower costs. A key challenge in optical imaging however, is translation of the newly developed tracers from pre-clinical phase to clinical application. We describe the development and safety testing of clinical grade bevacizumab-800CW, an antibody-based targeted agent for non-invasive imaging of vascular endothelial growth factor A (VEGF-A). Development included implementing the manufacturing process and analytical methods according to current Good Manufacturing Practice (cGMP), formulation studies, extended characterization and stability testing. For safety pharmacology an extended single dose toxicity study in mice was performed. Bevacizumab-800CW was formulated in isotonic phosphate buffered sodium chloride solution at pH 7. The production was robust and showed a reproducible labeling efficiency, and no impurities. The binding affinity to VEGF-A remained intact. The optimized product meets all release specifications, is stable up to at least 3months and its characteristics did not significantly differ from the unlabeled bevacizumab. Toxicity testing in mice showed no remarkable findings. In conclusion, sterile bevacizumab-800CW (6mg=6ml) can be produced in stock according to current Good Manufacturing Practice. It is ready for first-in-human use.

Combining Crystallography and Hydrogen-Deuterium Exchange to Study Galectin-Ligand Complexes.

The physiological significance arising from translating information stored in glycans into cellular effects explains the interest in structurally defining lectin-carbohydrate recognition. The relatively small set of adhesion/growth-regulatory galectins in chicken makes this system attractive to study the origins of specificity and divergence. Cell binding by using glycosylation mutants reveals binding of the N-terminal domain of chicken galectin-8 (CG-8N) to α-2,3-sialylated and galactose-terminated glycan chains. Cocrystals with lactose and its 3'-sialylated derivative disclose Arg58 as a key contact for the carboxylic acid and differences in loop lengths to the three homodimeric chicken galectins. Monitoring hydrogen-deuterium exchange by mass spectrometry revealed an effective reduction of deuteration after ligand binding within the contact area. In addition, evidence for changes in solvent accessibility of amide protons beyond this site was obtained. Their detection, which highlights the sensor capacity of this technique, encourages systematic studies on galectins and beyond.

New open-chain tetrapyrroles as chromophores in the plant photoreceptor phytochrome.

A series of six open-chain tetrapyrroles has been synthesized and used as chromophores for the plant photoreceptor protein phytochrome. The novel chromophores vary in the size of substituents 17 and 18 at ring D. This ring undergoes maximal conformational change upon light excitation ( Z --> E photoisomerization of the 15,16-double bond). Instead of methyl and vinyl substituents (positions 17, 18) as present in the native chromophore phytochromobilin, dimethyl, methyl and isopropyl, methyl and tert-butyl, ethyl and methyl, vinyl and methyl, and isopropyl and methyl substituents have been generated. All novel chromophores assemble with the apoprotein. The obtained chromoproteins show hypsochromic shifts of the absorbance maxima by 10 nm maximally, compared to the native pigment, except for the 17-isopropyl-18-methyl-substituted compound which showed a 100 nm hypsochromic shift of selectively the P r form. The assembly kinetics were slowed down in correlation to the increasing size of the substituents, with stronger effects for modified substituents at position 17. The thermal stability of the photoinduced P fr form for the 18-isopropyl and the 18- tert butyl substituents was even greater than that of the native pigments. Those chromophores carrying substituents at position 17 larger than the methyl group (ethyl and isopropyl) showed a very low stability of the respective P fr forms. Time-resolved detection of the P r to P fr conversion (laser-induced flash photolysis) revealed a slower formation of the P fr form for those chromophores carrying larger substituents at position 18, whereas the rise and decay kinetics of the early intermediates are only moderately changed. Introduction of larger substituents at position 17 (ethyl, vinyl, and isopropyl) causes drastic changes in the kinetics; in particular the formation of the first thermally stable intermediate, I 700, is significantly slowed, making a detection of its rise possible.

Femtosecond kinetics of photoconversion of the higher plant photoreceptor phytochrome carrying native and modified chromophores.

The photoprocesses of native (phyA of oat), and of C-terminally truncated recombinant phytochromes, assembled instead of the native phytochromobilin with phycocyanobilin (PCB-65 kDa-phy) and iso-phycocyanobilin (iso-PCB-65 kDa-phy) chromophores, have been studied by femtosecond transient absorption spectroscopy in both their red absorbing phytochrome (P(r)) and far-red absorbing phytochrome (P(fr)) forms. Native P(r) phytochrome shows an excitation wavelength dependence of the kinetics with three main picosecond components. The formation kinetics of the first ground-state intermediate I(700), absorbing at approximately 690 nm, is mainly described by 28 ps or 40 ps components in native and PCB phytochrome, respectively, whereas additional approximately 15 and 50 ps components describe conformational dynamics and equilibria among different local minima on the excited-state hypersurface. No significant amount of I(700) formation can be observed on our timescale for iso-PCB phytochrome. We suggest that iso-PCB-65 kDa-phy either interacts with the protein differently leading to a more twisted and/or less protonated configuration, or undergoes P(r) to P(fr) isomerization primarily via a different configurational pathway, largely circumventing I(700) as an intermediate. The isomerization process is accompanied by strong coherent oscillations due to wavepacket motion on the excited-state surface for both phytochrome forms. The femto- to (sub-)nanosecond kinetics of the P(fr) forms is again quite similar for the native and the PCB phytochromes. After an ultrafast excited-state relaxation within approximately 150 fs, the chromophores return to the first ground-state intermediate in 400-800 fs followed by two additional ground-state intermediates which are formed with 2-3 ps and approximately 400 ps lifetimes. We call the first ground-state intermediate in native phytochrome I(fr 750), due to its pronounced absorption at that wavelength. The other intermediates are termed I(fr 675) and pseudo-P(r). The absorption spectrum of the latter already closely resembles the absorption of the P(r) chromophore. PCB-65 kDa-phy shows a very similar kinetics, although many of the detailed spectral features in the transients seen in native phy are blurred, presumably due to wider inhomogeneous distribution of the chromophore conformation. Iso-PCB-65 kDa-phy shows similar features to the PCB-65 kDa-phy, with some additional blue-shift of the transient spectra of approximately 10 nm. The sub-200 fs component is, however, absent, and the picosecond lifetimes are somewhat longer than in 124 kDa phytochrome or in PCB-65 kDa-phy. We interpret the data within the framework of two- and three-dimensional potential energy surface diagrams for the photoisomerization processes and the ground-state intermediates involved in the two photoconversions.

Phytochromes with noncovalently bound chromophores: the ability of apophytochromes to direct tetrapyrrole photoisomerization.

Chromophore-apoprotein interactions were studied with recombinant apoproteins, oat phytochrome (phyA) and CphB of the cyanobacterium Calothrix PCC7601, which were both incubated with the bilin compounds biliverdin (BV) IXalpha, phycocyanobilin (PCB) and the 3'-methoxy derivative of PCB. Previously it was shown that CphB and its homolog in Calothrix, CphA, show strong sequence similarities with each other and with the phytochromes of higher and lower plants, despite the fact that CphB carries a leucine instead of a cysteine at the chromophore attachment position and thus holds the chromophore only noncovalently. CphA binds tetrapyrrole chromophores in a covalent, phytochrome-like manner. For both eyanobacterial phytochromes, red and far-red light-induced photochemistry has been reported. Thus, the role of the binding site of CphB in directing the photochemistry of noncovalently bound tetrapyrroles was analyzed in comparison with the apoprotein from phyA phytochrome. Both the aforementioned compounds, which were used as chromophores, are not able to form covalent bonds with a phytochrome-type apoprotein because of their chemical structure (vinyl group at position 3 or methoxy group at position 3'). The BV adducts of both apoproteins showed phytochrome-like photochemistry (formation of red and far-red-absorbing forms of phytochrome [P(r) and P(fr) forms]). However, incubation of the oat apophytochrome with BV primarily yields a 700 nm form from which the P(r)-P(fr) photochemistry can be initiated and to which the system relaxes in the dark after illumination. The results for CphB were compared with a CphB mutant where the chromophore-binding cysteine had been introduced, which, upon incubation with PCB, shows spectral properties nearly identical with its (covalently binding) CphA homolog. A comparison of the spectral properties (P(r) and P(fr) forms) of all the PCB- and BV-containing chromoproteins reveals that the binding site of the cyanobacterial apoprotein is better suited than the plant (oat) phytochrome to noncovalently incorporate the chromophore and to regulate its photochemistry. Our findings support the proposal that the recently identified phytochrome-like prokaryotic photoreceptors, which do not contain a covalently bound chromophore, may trigger a light-induced physiological response.