Engineering a soluble extracellular erythropoietin receptor (EPObp) in Pichia pastoris to eliminate microheterogeneity, and its complex with erythropoietin

Chemical and Enzymatic Synthesis of Sialylated Glycoforms of Human Erythropoietin

Recombinant human erythropoietin (EPO) is the main therapeutic glycoprotein for the treatment of anemia in cancer and kidney patients. The in-vivo activity of EPO is carbohydrate-dependent with the number of sialic acid residues regulating its circulatory half-life. EPO carries three N-glycans and thus obtaining pure glycoforms provides a major challenge. We have developed a robust and reproducible chemoenzymatic approach to glycoforms of EPO with and without sialic acids. EPO was assembled by sequential native chemical ligation of two peptide and three glycopeptide segments. The glycopeptides were obtained by pseudoproline-assisted Lansbury aspartylation. Enzymatic introduction of the sialic acids was readily accomplished at the level of the glycopeptide segments but even more efficiently on the refolded glycoprotein. Biological recognition of the synthetic EPOs was shown by formation of 1:1 complexes with recombinant EPO receptor.

A synthetic approach to 'click' neoglycoprotein analogues of EPO employing one-pot native chemical ligation and CuAAC chemistry

The clinical significance of batch-wise variability on the pharmacokinetics and potency of commercial erythropoietin (EPO), prepared recombinantly as a heterogeneous mixture of glycoforms, necessitates the development of synthetic strategies to afford homogenous EPO formulations. Herein we present a previously unexplored and divergent route towards 'click' neoglycoprotein analogues of EPO, employing one-pot native chemical ligation (NCL) of alkynylated peptides and copper-catalysed azide-alkyne cycloaddition (CuAAC) with azido monosaccharides. By design, our synthetic platform permits glycosylation at virtually any stage, providing flexibility for the synthesis of various glycoforms for biological analysis. Insights obtained from attempted folding of our 'click' neoglycoprotein EPO analogue, bearing four different neutral sugar moieties, highlight the important role played by the charged oligosaccharides present in native EPO glycoproteins.

Directed evolution of the epidermal growth factor receptor extracellular domain for expression in yeast

The extracellular domain of epidermal growth factor receptor (EGFR-ECD) has been engineered through directed evolution and yeast surface display using conformationally-specific monoclonal antibodies (mAbs) as screening probes for proper folding and functional expression in Saccharomyces cerevisiae. An EGFR mutant with four amino acid changes exhibited binding to the conformationally-specific mAbs and human epidermal growth factor, and showed increased soluble secretion efficiency compared with wild-type EGFR. Full-length EGFR containing the mutant EGFR-ECD was functional, as assayed by EGF-dependent autophosphorylation and intracellular MAPK signaling in mammalian cells, and was expressed and localized at the plasma membrane in yeast. This approach should enable engineering of other complex mammalian receptor glycoproteins in yeast for genetic, structural, and biophysical studies.

Erythropoietin: the story of hypoxia and a finely regulated hematopoietic hormone

The dramatic increase in knowledge during the last half century about the hormone erythropoietin is reviewed. The description of these events has been separated into two parts. The first part describes how the rapid changes in response to tissue oxygenation in the kidneys changes the rate of erythropoietin production. The second part describes how changes in erythropoietin concentrations act on erythroid progenitor cells, resulting in prompt changes in rates of erythrocyte production. Together these two aspects of erythropoietin biology provide an explanation for the tight physiological regulation of the numbers of circulating erythrocytes and, in a more general manner, provide a model for the control of the numbers of other specific blood cells by their respective hematopoietic growth factors.

Synthetic Erythropoietic Proteins: Tuning Biological Performance by Site-Specific Polymer Attachment

Chemical synthesis in combination with precision polymer modification allows the systematic exploration of the effect of protein properties, such as charge and hydrodynamic radius, on potency using defined, homogeneous conjugates. A series of polymer-modified synthetic erythropoiesis proteins were constructed that had a polypeptide chain similar to the amino acid sequence of human erythropoietin but differed significantly in the number and type of attached polymers. The analogs differed in charge from +5 to -26 at neutral pH and varied in molecular weight from 30 to 54 kDa. All were active in an in vitro cell proliferation assay. However, in vivo potency was found to be strongly dependent on overall charge and size. The trends observed in this study may serve as starting points for the construction of more potent synthetic EPO analogs in the future.

Soluble mimics of a chemokine receptor: chemokine binding by receptor elements juxtaposed on a soluble scaffold

Despite the broad biological importance of G protein-coupled receptors (GPCRs), ligand recognition by GPCRs remains poorly understood. To explore the roles of GPCR extracellular elements in ligand binding and to provide a tractable system for structural analyses of GPCR/ligand interactions, we have developed a soluble protein that mimics ligand recognition by a GPCR. This receptor analog, dubbed CROSS5, consists of the N-terminal and third extracellular loop regions of CC chemokine receptor 3 (CCR3) displayed on the surface of a small soluble protein, the B1 domain of Streptococcal protein G. CROSS5 binds to the CCR3 ligand eotaxin with a dissociation equilibrium constant of 2.9 +/- 0.8 microM and competes with CCR3 for eotaxin binding. Control proteins indicate that juxtaposition of both CCR3 elements is required for optimal binding to eotaxin. Moreover, the affinities of CROSS5 for a series of eotaxin mutants are highly correlated with the apparent affinities of CCR3 for the same mutants, demonstrating that CROSS5 uses many of the same interactions as does the native receptor. The strategy used to develop CROSS5 could be applied to many other GPCRs, with a variety of potential applications.

Domain-level antibody epitope mapping through yeast surface display of epidermal growth factor receptor fragments

Individual domains from extracellular proteins are potential reagents for biochemical characterization of ligand/receptor interactions and antibody binding sites. Here, we describe an approach for the identification and characterization of stable protein domains with cell surface display in Saccharomyces cerevesiae, using the epidermal growth factor receptor (EGFR) as a model system. Fragments of the EGFR were successfully expressed on the yeast cell surface. The yeast-displayed EGFR fragments were properly folded, as assayed with conformationally specific EGFR antibodies. Heat denaturation of yeast-displayed EGFR proteins distinguished between linear and conformational antibody epitopes. In addition, EGFR-specific antibodies were categorized based on their ability to compete ligand binding, which has been shown to have therapeutic implications. Overlapping EGFR antibody epitopes were determined based on a fluorescent competitive binding assay. Yeast surface display is a useful method for identifying stable folded protein domains from multidomain extracellular receptors, as well as characterizing antibody binding epitopes, without the need for soluble protein expression and purification.

Mechanistic diversity of cytokine receptor signaling across cell membranes

Circulating cytokines bind to specific receptors on the cell outer surface to evoke responses inside the cell. Binding of cytokines alters the association between receptor molecules that often cross the membrane only once in a single alpha-helical segment. As a consequence, association of protein domains on the inside of the membrane are also altered. Increasing evidence suggests that an initial "off-state" of associated receptors is perturbed, and brought to an activated state that leads to intracellular signaling and eventually effects a change in DNA transcription. The initial detection event that transduces the change in receptor association is sensitive to both proximity and orientation of the receptors, and probably also to the time that the activated state or receptor association is maintained. Ultimately, a cascade of phosphorylation events is triggered. The initial kinases are sometimes part of the intracellular domains of the receptors. The kinases can also be separate proteins that may be pre-associated with intracellular domains of the receptors, or can be recruited after the intracellular association of the activated receptors. We focus here on each of the cases for which structures of the activated cytokine-receptor complexes are known, in a search for underlying mechanisms. The variations in modes of association, stoichiometries of receptors and cytokines, and orientations before and after activation of these receptors are almost as great as the number of complexes themselves. The principles uncovered nevertheless illustrate the basis for high specificity and fidelity in cytokine-mediated signaling.

Designing proteins to crystallize through beta-strand pairing

Inherent difficulties in growing protein crystals are major concerns within structural biology and particularly in structural proteomics. Here, we describe a novel approach of engineering target proteins by surface mutagenesis to increase the odds of crystallizing the molecules. To this end, we have exploited our recent triad-hypothesis using proteins with crystallographically defined beta-structures as the principal models. Crystal packing analyses of 182 protein structures belonging to 21 different superfamilies implied that the propensities to crystallize could be engineered into target proteins by replacing short segments, 5-6 residues, of their beta-strands with 'cassettes' of suitable packing motifs. These packing motifs will generate specific crystal packing interactions that promote crystallization. Key features of the primary and tertiary structures of such packing motifs have been identified for immunoglobulins. Further, packing motifs have been engineered successfully into six model antibodies without disturbing their capabilities to be produced, their immunoreactivity and their overall structure. Preliminary crystallization analyses have also been performed. Taken together, the procedures outline a rational protocol for crystallizing proteins by surface mutagenesis. The importance of these findings is discussed in relation to the crystallization of proteins in general.

Production of humanized Fab fragment against human high affinity IgE receptor in Pichia pastoris

Binding of allergen-IgE complexes to the high affinity IgE receptor (Fc epsilonRI) on mast cells and basophils leads to the release of various mediaters such as histamine. Fab fragments prepared by the papain digestion of humanized antibody against human Fc epsilonRI inhibited the release of histamine from human basophils. Here we established an expression system to directly produce Fab fragments of the humanized anti-human Fc epsilonRI antibody in methylotropic yeast, P. pastoris. Fab fragments were efficiently secreted into the medium at a concentration of 10-40 mg/L using a signal sequence from the P. pastoris phosphatase gene. They were consisted of disulfide-linked light and heavy chains correctly starting from the first amino acid residues by proper cleavage of the signal peptides. The obtained Fab fragments inhibited the binding between IgE and Fc epsilonRI as efficiently as the counterpart prepared by papain digestion of the whole antibody.

Recombinant protein expression in Pichia pastoris

The methylotrophic yeast Pichia pastoris is now one of the standard tools used in molecular biology for the generation of recombinant protein. P. pastoris has demonstrated its most powerful success as a large-scale (fermentation) recombinant protein production tool. What began more than 20 years ago as a program to convert abundant methanol to a protein source for animal feed has been developed into what is today two important biological tools: a model eukaryote used in cell biology research and a recombinant protein production system. To date well over 200 heterologous proteins have been expressed in P. pastoris. Significant advances in the development of new strains and vectors, improved techniques, and the commercial availability of these tools coupled with a better understanding of the biology of Pichia species have led to this microbe's value and power in commercial and research labs alike.

The structure, organization, activation and plasticity of the erythropoietin receptor

Dimerization of the erythropoietin receptor has long been accepted as the singular step in its mechanism of activation. Recent studies have revealed a regulator process for activation that is dependent on the actual configuration of the receptor-ligand dimer assembly. This aspect of the receptor subunit assembly appears to extend to the unliganded receptor, which can dimerize on the cell surface and diminish any spontaneous background signaling in the absence of ligand. This self-recognition, as well as the multiple ligand binding capabilities of the receptor binding site, is consistent with an emerging theme of plasticity in protein-protein and ligand-receptor interactions.