Recombinant bovine α-lactalbumin obtained by limited proteolysis of a fusion protein expressed at high levels in Escherichia coli

Cell factory-based milk protein biomanufacturing: Advances and perspectives

The increasing global population and protein demand cause global challenges for food supply. Fueled by significant developments in synthetic biology, microbial cell factories are constructed for the bioproduction of milk proteins, providing a promising approach for scalable and cost-effective production of alternative proteins. This review focused on the synthetic biology-based microbial cell factory construction for milk protein bioproduction. The composition, content, and functions of major milk proteins were first summarized, especially for caseins, α-lactalbumin, and β-lactoglobulin. An economic analysis was performed to determine whether cell factory-based milk protein production is economically viable for industrial production. Cell factory-based milk protein production is proved to be economically viable for industrial production. However, there still exist some challenges for cell factory-based milk protein biomanufacturing and application, including the inefficient production of milk proteins, insufficient investigation of protein functional property, and insufficient food safety evaluation. Constructing new high-efficiency genetic regulatory elements and genome editing tools, coexpression/overexpression of chaperone genes, and engineering protein secretion pathways and establishing a cost-effective protein purification method are possible ways to improve the production efficiency. Milk protein biomanufacturing is one of the promising approaches to acquiring alternative proteins in the future, which is of great importance for supporting cellular agriculture.

Transcription factor expression uniquely identifies most postembryonic neuronal lineages in the Drosophila thoracic central nervous system

Most neurons of the adult Drosophila ventral nerve cord arise from a burst of neurogenesis during the third larval instar stage. Most of this growth occurs in thoracic neuromeres, which contain 25 individually identifiable postembryonic neuronal lineages. Initially, each lineage consists of two hemilineages--'A' (Notch(On)) and 'B' (Notch(Off))--that exhibit distinct axonal trajectories or fates. No reliable method presently exists to identify these lineages or hemilineages unambiguously other than labor-intensive lineage-tracing methods. By combining mosaic analysis with a repressible cell marker (MARCM) analysis with gene expression studies, we constructed a gene expression map that enables the rapid, unambiguous identification of 23 of the 25 postembryonic lineages based on the expression of 15 transcription factors. Pilot genetic studies reveal that these transcription factors regulate the specification and differentiation of postembryonic neurons: for example, Nkx6 is necessary and sufficient to direct axonal pathway selection in lineage 3. The gene expression map thus provides a descriptive foundation for the genetic and molecular dissection of adult-specific neurogenesis and identifies many transcription factors that are likely to regulate the development and differentiation of discrete subsets of postembryonic neurons.

In vitro folding of β-1,4galactosyltransferase and polypeptide-α-N-acetylgalactosaminyltransferase from the inclusion bodies

The aim of this article is to present a unique in vitro folding technique for glycosyltransferases to generate active proteins that can be used for X-ray crystallographic and bioconjugation protocols. Although a number of in vitro refolding methods are available, β1,4galactosyltransferases in large quantities can be made using the current protocol only. This technique is not only limited to glycosyltransferases alone but has been successfully used to refold single-chain antibodies and other molecules. Although this in vitro folding method is quite similar to other methods, it differs from them by the use of S-sulfonation of the inclusion bodies before setting up the in vitro refolding of the protein.

Increased expression, folding and enzyme reaction rate of recombinant human insulin by selecting appropriate leader peptide

Five new expression vectors for recombinant human insulin production (pPT-B5Kpi, pPT-T10Rpi, pPT-T13Rpi, pPT-H27Rpi, pPT-B5Rpi), which have different sizes and leader peptide structure, were constructed and compared based on their expression level, yields of S-sulfonated preproinsulin (SSPPI) and folded proinsulin and enzymatic conversion rate. The ranking of expression level of the five fused proinsulins was H27R≫T10R > B5K >T13R≈B5R. In particular, the expression level of H27R was more than double (60-70%) the level of the other fused proinsulins, and this high expression level led to large amounts of SSPPI, folded proinsulin and insulin. Changes to the leader peptide structure affected not only protein expression level, but also refolding yield because the leader peptide affects protein conformation and hydrophobicity. The refolding yield of H27R was 85% at 500L pilot scale. This high refolding yield was caused by the hydrophilic character of H27R. However, the β-mercaptoethanol concentration needed for refolding and the pH required to precipitate impurities after refolding had to be changed for high refolding yield. To avoid using CNBr, which is used to cleave fusion proteins, we used lysine and arginine linkers to connect the fusion protein and proinsulin. This fusion protein could be simultaneously cleaved by trypsin during enzymatic conversion to eliminate the C-peptide. The length and kind of leader peptide did not affect the enzyme reaction rate. Only the leader peptide linker connecting the B-chain influenced enzyme reaction rate. By testing several leader peptides, we constructed a new strain with 30% increased productivity based on expression level, refolding yield and enzyme reaction.

dbx mediates neuronal specification and differentiation through cross-repressive, lineage-specific interactions with eve and hb9

Individual neurons adopt and maintain defined morphological and physiological phenotypes as a result of the expression of specific combinations of transcription factors. In particular, homeodomain-containing transcription factors play key roles in determining neuronal subtype identity in flies and vertebrates. dbx belongs to the highly divergent H2.0 family of homeobox genes. In vertebrates, Dbx1 and Dbx2 promote the development of a subset of interneurons, some of which help mediate left-right coordination of locomotor activity. Here, we identify and show that the single Drosophila ortholog of Dbx1/2 contributes to the development of specific subsets of interneurons via cross-repressive, lineage-specific interactions with the motoneuron-promoting factors eve and hb9 (exex). dbx is expressed primarily in interneurons of the embryonic, larval and adult central nervous system, and these interneurons tend to extend short axons and be GABAergic. Interestingly, many Dbx(+) interneurons share a sibling relationship with Eve(+) or Hb9(+) motoneurons. The non-overlapping expression of dbx and eve, or dbx and hb9, within pairs of sibling neurons is initially established as a result of Notch/Numb-mediated asymmetric divisions. Cross-repressive interactions between dbx and eve, and dbx and hb9, then help maintain the distinct expression profiles of these genes in their respective pairs of sibling neurons. Strict maintenance of the mutually exclusive expression of dbx relative to that of eve and hb9 in sibling neurons is crucial for proper neuronal specification, as misexpression of dbx in motoneurons dramatically hinders motor axon outgrowth.

Generation of novel chimeric LacdiNAcS by gene fusion of alpha-lactalbumin and beta1,4-galactosyltransferase 1

Novel chimeric lacdiNAc (GalNAc(beta1-4)GlcNAc) synthase (c-LacdiNAcS) was generated by gene fusion of alpha-lactalbumin (alpha-LA) and beta1,4-galactosyltransferase 1 (beta1,4-GalT1). c-LacdiNAcS was expressed in Lec8 Chinese hamster ovary (Lec8 CHO) cells and exhibited N-acetylgalactosaminyltransferase (GalNAcT) activity in the absence of exogenous alpha-LA as well as other glycosyltransferase activities including lactose synthase (LacS), and beta1,4-GalT. These glycosyltransferase activities of c-LacdiNAcS were compared to those activities induced in LacS system under the co-presence of bovine beta1,4-GalT1 and alpha-LA, indicating that each domain of alpha-LA and beta1,4-GalT1 on c-LacdiNAcS is not only folding correctly, but also interacting together. Furthermore, c-LacdiNAcS was found to be auto-lacdiNAcylated and can synthesize lacdiNAc structures on cellular glycoproteins, demonstrating that GalNAcT activity of c-LacdiNAcS is functional in Lec8 CHO cells.

Controlling orientations of immobilized oligopeptides using N-terminal cysteine labels

This letter reports a strategy of using N-terminal cysteine labels for controlling the immobilization of oligopeptides on aldehyde-terminated surfaces through the formation of stable thiazolidine rings. We also study the effect of cysteine position (either N-terminal or C-terminal) and lysine residue on the immobilization of oligopeptides. On the basis of our ellipsometry and quartz crystal microbalance (QCM) results, we conclude that the proposed immobilization strategy is highly site-specific. It works only when cysteine is in the N-terminal position, and the formation of thiazolidine is much faster than the formation of imines between lysine residues and aldehydes, even in the presence of a reducing agent such as NaBH(3)CN. By labeling an oligopeptide CSNKTRIDEANNKATKML with an N-terminal cysteine, we immobilize this oligopeptide on an aldehyde-terminated surface and investigate the enzymatic activity of trypsin acting on the oligopeptide. It is found that trypsin is able to cleave the immobilized oligopeptide having a single anchoring point at the N-terminal cysteine. No cleavage is observed when the oligopeptide is immobilized through multiple anchoring points at lysine residues.

alpha-Lactalbumin (LA) stimulates milk beta-1,4-galactosyltransferase I (beta 4Gal-T1) to transfer glucose from UDP-glucose to N-acetylglucosamine. Crystal structure of beta 4Gal-T1 x LA complex with UDP-Glc

beta-1,4-Galactosyltransferase 1 (Gal-T1) transfers galactose (Gal) from UDP-Gal to N-acetylglucosamine (GlcNAc), which constitutes its normal galactosyltransferase (Gal-T) activity. In the presence of alpha-lactalbumin (LA), it transfers Gal to Glc, which is its lactose synthase (LS) activity. It also transfers glucose (Glc) from UDP-Glc to GlcNAc, constituting the glucosyltransferase (Glc-T) activity, albeit at an efficiency of only 0.3-0.4% of Gal-T activity. In the present study, we show that LA increases this activity almost 30-fold. It also enhances the Glc-T activity toward various N-acyl substituted glucosamine acceptors. Steady state kinetic studies of Glc-T reaction show that the K(m) for the donor and acceptor substrates are high in the absence of LA. In the presence of LA, the K(m) for the acceptor substrate is reduced 30-fold, whereas for UDP-Glc it is reduced only 5-fold. In order to understand this property, we have determined the crystal structures of the Gal-T1.LA complex with UDP-Glc x Mn(2+) and with N-butanoyl-glucosamine (N-butanoyl-GlcN), a preferred sugar acceptor in the Glc-T activity. The crystal structures reveal that although the binding of UDP-Glc is quite similar to UDP-Gal, there are few significant differences observed in the hydrogen bonding interactions between UDP-Glc and Gal-T1. Based on the present kinetic and crystal structural studies, a possible explanation for the role of LA in the Glc-T activity has been proposed.

Crystal structure of lactose synthase reveals a large conformational change in its catalytic component, the beta1,4-galactosyltransferase-I

The lactose synthase (LS) enzyme is a 1:1 complex of a catalytic component, beta1,4-galactosyltransferse (beta4Gal-T1) and a regulatory component, alpha-lactalbumin (LA), a mammary gland-specific protein. LA promotes the binding of glucose (Glc) to beta4Gal-T1, thereby altering its sugar acceptor specificity from N-acetylglucosamine (GlcNAc) to glucose, which enables LS to synthesize lactose, the major carbohydrate component of milk. The crystal structures of LS bound with various substrates were solved at 2 A resolution. These structures reveal that upon substrate binding to beta4Gal-T1, a large conformational change occurs in the region comprising residues 345 to 365. This repositions His347 in such a way that it can participate in the coordination of a metal ion, and creates a sugar and LA-binding site. At the sugar-acceptor binding site, a hydrophobic N-acetyl group-binding pocket is found, formed by residues Arg359, Phe360 and Ile363. In the Glc-bound structure, this hydrophobic pocket is absent. For the binding of Glc to LS, a reorientation of the Arg359 side-chain occurs, which blocks the hydrophobic pocket and maximizes the interactions with the Glc molecule. Thus, the role of LA is to hold Glc by hydrogen bonding with the O-1 hydroxyl group in the acceptor-binding site on beta4Gal-T1, while the N-acetyl group-binding pocket in beta4Gal-T1 adjusts to maximize the interactions with the Glc molecule. This study provides details of a structural basis for the partially ordered kinetic mechanism proposed for lactose synthase.

Crystal structures of apo- and holo-bovine alpha-lactalbumin at 2. 2-A resolution reveal an effect of calcium on inter-lobe interactions

High affinity binding of Ca(2+) to alpha-lactalbumin (LA) stabilizes the native structure and is required for the efficient generation of native protein with correct disulfide bonds from the reduced denatured state. A progressive increase in affinity of LA conformers for Ca(2+) as they develop increasingly native structures can account for the tendency of the apo form to assume a molten globule state and the large acceleration of folding by Ca(2+). To investigate the effect of calcium on structure of bovine LA, x-ray structures have been determined for crystals of the apo and holo forms at 2.2-A resolution. In both crystal forms, which were grown at high ionic strength, the protein is in a similar global native conformation consisting of alpha-helical and beta-subdomains separated by a cleft. Even though alternative cations and Ca(2+) liganding solvent molecules are absent, removal of Ca(2+) has only minor effects on the structure of the metal-binding site and a structural change was observed in the cleft on the opposite face of the molecule adjoining Tyr(103) of the helical lobe and Gln(54) of the beta-lobe. Changes include increased separation of the lobes, loss of a buried solvent molecule near the Ca(2+)-binding site, and the replacement of inter- and intra-lobe H-bonds of Tyr(103) by interactions with new immobilized water molecules. The more open cleft structure in the apo protein appears to be an effect of calcium binding transmitted via a change in orientation of helix H3 relative to the beta-lobe to the inter-lobe interface. Calcium is well known to promote the folding of LA. The results from the comparison of apo and holo structures of LA provide high resolution structural evidence that the acceleration of folding by Ca(2+) is mediated by an effect on interactions between the two subdomains.

Gene fragment polymerization gives increased yields of recombinant human proinsulin C-peptide

A multimerization strategy to improve yields upon recombinant production of the 31-aa human proinsulin C-peptide is presented. Gene fragments encoding the C-peptide were assembled using specific head-to-tail multimerization. DNA constructs encoding one, three or seven copies of the C-peptide gene, fused to a serum albumin binding affinity tag, were expressed intracellularly in Escherichia coli. The three fusion proteins were produced at similar levels (approximately 50 mg/l) and were proteolytically stable during production. Enzymatic digestion by trypsin-carboxypeptidase B treatment of the fusion proteins was shown to efficiently release native C-peptide, as determined by mass spectrometry, reverse-phase chromatography and a radioimmunoassay. The quantitative yields of C-peptide obtained from the three different fusion proteins suggest that this multimerization strategy could provide a cost-efficient production scheme for the C-peptide, and that this strategy could be useful also for production of other recombinant peptides.

Molecular divergence of lysozymes and alpha-lactalbumin

The vast number of proteins that sustain the currently living organisms have been generated from a relatively small number of ancestral genes that has involved a variety of processes. Lysozyme is an ancient protein whose origin goes back an estimated 400 to 600 million years. This protein was originally a bacteriolytic defensive agent and has been adapted to serve a digestive function on at least two occasions, separated by nearly 40 million years. The origins of the related goose type and T4 phage lysozyme that are distinct from the more common C type are obscure. They share no discernable amino acid sequence identity and yet they possess common secondary and tertiary structures. Lysozyme C gene also gave rise, after gene duplication 300 to 400 million years ago, to a gene that currently codes for alpha-lactalbumin, a protein expressed only in the lactating mammary gland of all but a few species of mammals. It is required for the synthesis of lactose, the sugar secreted in milk. alpha-Lactalbumin shares only 40% identity in amino acid sequence with lysozyme C, but it has a closer spatial structure and gene organization. Although structurally similar, functionally they are quite distinct. Specific amino acid substitutions in alpha-lactalbumin account for the loss of the enzyme activity of lysozyme and the acquisition of the features necessary for its role in lactose synthesis. Evolutionary implications are as yet unclear but are being unraveled in many laboratories.

Structural and functional analysis of negatively charged milk proteins with anti-HIV activity

Several polyanionic reagents such as dextran sulfates, heparin sulfates, and negatively charged proteins have been reported to exhibit anti-HIV activity in vitro. Particularly potent inhibition has been reported for the milk protein beta-lactoglobulin (betaLG) on modification by 3-hydroxyphthalic anhydride (3HP). The introduction of multiple negatively charged carboxyl groups along the polypeptide backbone obviously leads to repulsion within the protein molecule and this is likely to affect the specific tertiary, and perhaps also secondary, structure of the protein. We used several biophysical techniques to probe the structural changes that occur on 3HP modification of betaLG. The results suggest that the protein becomes largely unstructured on chemical modification. Although a profound anti-HIV activity was measured for 3HP-betaLG, similar antiviral effects were observed with two other 3HP-modified milk proteins, alpha-lactalbumin and alpha(S2)-casein, but not with the unmodified proteins. Most potent inhibition of HIV-1 replication was obtained with 3HP-modified alpha-lactalbumin, which also demonstrated the least cytotoxicity. These combined results indicate that HIV inhibition is a general property of negatively charged polypeptides and do not support a model in which the negatively charged 3HP-betaLG protein interacts in a structure-specific manner with the CD4 cell surface receptor for HIV-1 entry.

Integrated production of human insulin and its C-peptide

The potential for the development of an integrated process for production of human insulin and its C-peptide in Escherichia coli has been investigated. Human proinsulin was produced intracellularly in E. coli fused to two synthetic IgG-binding domains (ZZ) derived from staphylococcal protein A. High expression levels (3 g/l culture) of the gene product, which accumulated as inclusion bodies, was obtained. Solubilization of inclusion bodies by oxidative sulfitolysis and subsequent renaturation was performed directly after cell lysis and pellet wash. IgG affinity chromatography was used for efficient recovery of pure proinsulin fusion protein in a single step. Monomers of the proinsulin fusion protein constituted approximately 70%. A single step conversion of the fusion protein into insulin and C-peptide by trypsin and carboxypeptidase B treatment was achieved by engineering the junction between proinsulin and its affinity handle, ZZ. Characterization of the cleavage products by reversed phase chromatography (RPC) verified that human insulin and C-peptide were generated and that the ZZ affinity handle was resistant to cleavage. Human insulin and C-peptide were recovered with high yields by preparative reversed-phase high performance liquid chromatography (RP-HPLC). The potential use of the presented scheme for large-scale production of recombinant insulin and/or its C-peptide is discussed.

Single-step trypsin cleavage of a fusion protein to obtain human insulin and its C peptide

The kinetics for trypsin cleavage of different fusion proteins, consisting of human proinsulin and two IgG-binding domains (ZZ), were investigated. To achieve simultaneous removal of the fusion tag and processing of proinsulin to insulin and free C peptide, three versions of the ZZ-proinsulin fusion protein were generated, having different trypsin-sensitive cleavage sites, Arg, Lys-Arg or Lys. The ZZ-proinsulin fusion proteins which accumulated as inclusion bodies in Escherichia coli cells were solubilized, refolded and purified by IgG affinity chromatography. The yield of ZZ-proinsulin monomers exceeded 90%. The kinetics for the trypsin cleavage revealed unexpected differences when comparing the three linkers and it was found that the single arginine linker was most efficiently processed. Characterization of the cleavage products by reverse-phase chromatography, mass spectrometry and N-terminal sequencing verified that human insulin and C peptide were generated. The results demonstrate that high yields of native insulin, C peptide and affinity tag can be achieved by simultaneous cleavage of a fusion protein at three different trypsin-sensitive sites in a single step. The implications for production and recovery of various recombinant proteins are discussed.

Conformation of GroEL-bound alpha-lactalbumin probed by mass spectrometry

The conformation of a three-disulphide derivative of bovine alpha-lactalbumin bound to the molecular chaperone GroEL has been investigated by monitoring directly its hydrogen exchange kinetics using electrospray ionization mass spectrometry. The bound protein is weakly protected from exchange to an extent closely similar to that of an uncomplexed molten globule state of the three-disulphide protein. Binding to GroEL in this system appears to involve relatively disordered partly folded states resembling intermediates formed in the very early stages of kinetic folding of many proteins in vitro.

Structure and function of milk protein genes

Interspecies comparisons of cDNA and mosaic milk protein genes have confirmed their high rate of evolution, but the overall gene organization has been conserved. The three Ca-sensitive casein genes, which share common motifs in the promoter region and contain similar sequences that encode signal peptide and multiple phosphorylation sites, probably derived from a common ancestor. alpha s1- and alpha s2-casein genes, divided into many small exons, undergo complex splicing, and the deleted caseins arise from exon skipping. The four bovine casein genes are clustered on 200 kb of chromosome 6. alpha-Lactalbumin and beta-lactoglobulin pseudogenes occur in ruminants. Study of the expression of native and modified milk protein genes in mammary cell lines and transgenic animals and DNA footprinting have shown the occurrence of important regulatory motifs in the proximal 5' flanking region, including one recognized by a specific mammary nuclear factor. Good stage- and tissue-specific expression has been obtained in transgenic animals with milk protein genes having less than a 3-kb 5' flanking region. Better knowledge of both the structure and function of milk protein genes, which has already allowed the use of powerful techniques for the rapid identification of alleles, offers the potential for the genetic modification of milk composition.

Efficient expression of bovine alpha-lactalbumin in Saccharomyces cerevisiae

A synthetic gene encoding the mature bovine alpha-lactalbumin fused to the preproregion of the yeast alpha-mating factor has been expressed and secreted at high level in Saccharomyces cerevisiae under the control of the alpha-mating promoter. Growth conditions were found to be critical for the expression: recombinant alpha-lactalbumin could only be detected in the medium provided the culture was grown at neutral pH. The secreted bovine alpha-lactalbumin is enzymatically active and identical to the whey protein, as confirmed by SDS/PAGE, IEF, ultraviolet and CD spectral analysis, and amino-terminal sequence determination.

Expression of honeybee prepromelittin as a fusion protein in Escherichia coli

Strategies for the expression of precursors of eukaryotic secreted proteins as part of fused proteins in Escherichia coli have been explored. A fusion protein with beta-galactosidase at the N-terminal end and honeybee prepromelittin at the C-terminal end (beta-gal-pM) was expressed in low amounts as a cleaved polypeptide, from which the promelittin portion had been removed. Inclusion in the induction culture of 10 mM MgCl2 or 8.3% (v/v) ethanol, inhibitors of signal peptidase, gave rise to the full-length beta-gal-pM fusion protein. The results suggest that a soluble recombinant fusion protein with a signal peptide in an internal location 660 residues from the N-terminus is recognized by the E. coli translocation apparatus in the inner membrane and by leader peptidase. High-level production (about 45% of total cellular proteins) of prepromelittin was achieved when it was part of a fusion protein at the C-terminus of a truncated insoluble polypeptide from bacteriophage gene 10. This fusion protein separated into inclusion bodies in an aggregated form. In contrast, attempts to express prepromelittin by itself or at the N-terminal end of a fusion with mouse dihydrofolate reductase (pM-DHFR) proved unsuccessful.

Expression and secretion of goat alpha-lactalbumin as an active protein in Saccharomyces cerevisiae

An expression plasmid for goat alpha-lactalbumin in Saccharomyces cerevisiae, pSKA100, was constructed into a shuttle vector, pYG100, by inserting cDNA which encodes goat pre-alpha-lactalbumin and two-thirds of the 3'-non-coding region. The goat alpha-lactalbumin was expressed under the yeast glyceraldehyde 3-phosphate dehydrogenase (GPD) promoter and terminator of pYG100 and secreted in the growth medium for yeast as a precise mature protein, possessing specific activity essentially the same as that of authentic goat alpha-lactalbumin in lactose synthesis.