Expression and characterization of phosphorylated recombinant human beta-casein in Escherichia coli

An assay of human tyrosine protein kinase ABL activity using an Escherichia coli protein expression system

ABL, a human tyrosine protein kinase, and its substrate are co-expressed in Escherichia coli. Tyrosine phosphorylation of the substrate in E. coli was detected using Phos-tag SDS-PAGE. The bacterial co-expression system was used as a field for the kinase reaction to evaluate the enzymatic activity of five types of ABL kinase domain mutants. Relative to wild-type ABL, kinase activity was comparable in the H396P mutant, reduced in both Y253F and E255K mutants and undetectable in T315I and M351T mutants. These comparative results demonstrated that the phosphorylation states of the mutants correlated with their activity. The bacterial co-expression system permits rapid production of tyrosine kinase variants and provides a simple approach for examining their structure-activity relationships.

Engineering Biology to Construct Microbial Chassis for the Production of Difficult-to-Express Proteins

A large proportion of the recombinant proteins manufactured today rely on microbe-based expression systems owing to their relatively simple and cost-effective production schemes. However, several issues in microbial protein expression, including formation of insoluble aggregates, low protein yield, and cell death are still highly recursive and tricky to optimize. These obstacles are usually rooted in the metabolic capacity of the expression host, limitation of cellular translational machineries, or genetic instability. To this end, several microbial strains having precisely designed genomes have been suggested as a way around the recurrent problems in recombinant protein expression. Already, a growing number of prokaryotic chassis strains have been genome-streamlined to attain superior cellular fitness, recombinant protein yield, and stability of the exogenous expression pathways. In this review, we outline challenges associated with heterologous protein expression, some examples of microbial chassis engineered for the production of recombinant proteins, and emerging tools to optimize the expression of heterologous proteins. In particular, we discuss the synthetic biology approaches to design and build and test genome-reduced microbial chassis that carry desirable characteristics for heterologous protein expression.

An E. coli over-expression system for multiply-phosphorylated proteins and its use in a study of calcium phosphate sequestration by novel recombinant phosphopeptides

Phosphoproteins and phosphopeptides were expressed by E. coli to give yields of 30-200mg of purified protein per litre with an average degree of phosphorylation at multiple sites of 61-83%. The method employed two compatible cohabiting plasmids having low and high copy number, expressing a protein kinase and, more abundantly, the substrate (poly)peptide, respectively. It was used to phosphorylate recombinant beta-casein or osteopontin at multiple casein kinase-2 sites. Two constructs were designed to produce shorter peptides containing one or more clusters of phosphorylation sites resembling the phosphate centres of caseins. In the first, a 53-residue 6-His tagged phosphopeptide was expressed at a 5-fold higher molar yield. The second had multiple tandem repeats of a tryptic phosphopeptide sequence to give a similar increase in efficiency. Each recombinant phosphopeptide was purified (30-100mg) and small-angle X-ray scattering measurements showed that they, like certain casein and osteopontin phosphopeptides, sequester amorphous calcium phosphate to form calcium phosphate nanoclusters. In principle, the method can provide novel phosphopeptides for the control of biocalcification or be adapted for use with other kinases and cognate proteins or peptides to study the effect of specific phosphorylations on protein structure. Moreover, the insertion of a phosphate centre sequence, possibly with a linker peptide, may allow thermodynamically stable, biocompatible nanoparticles to be made from virtually any sequence.

Overexpression of post-translationally modified peptides in Escherichia coli by co-expression with modifying enzymes

Post-translational modification plays crucial roles in signal transduction in eukaryotic cells. To elucidate the biological function of a protein with a specific post-translational modification, it is necessary to isolate the modified protein. However, it is difficult to incorporate a modified amino acid into a specific position of a protein, in particular, in a large-scale preparation. In order to prepare post-translationally modified proteins in Escherichia coli (E. coli), we have constructed co-expression vectors that contain protein and corresponding enzyme genes. The protein and enzyme are co-expressed in the same E. coli cells and the protein is post-translationally modified in vivo. By using this system, the transcriptional activator cyclic-AMP-response-element-binding protein (CREB) was phosphorylated at Ser-133 and the hypoxia-inducible factor-1alpha (HIF-1alpha) was hydroxylated at Asn-803 in E. coli. Although the constructs of the proteins we used are very flexible and susceptible to degradation by proteases in E. coli when they are expressed alone, the B1 domain of streptococcal protein G (GB1) fused to the N-terminus of the proteins increased the yields dramatically. Site-specific phosphorylation of CREB and hydroxylation of HIF-1alpha were confirmed by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) and NMR. Our GB1-fusion co-expression system can be used in the same way as conventional protein expression in E. coli, making it a flexible and economical method to produce a large amount of a post-translationally modified protein.

Thermal cycling aids folding of a recombinant human beta-casein with four extra N-terminal amino acid residues

Due to the limited secondary structure, it is believed that the caseins of milk, particularly the beta-caseins (beta-CN), may be in a mostly random-coil conformation or in various structures that result from random association of hydrophobic residues. However, the self-association of the human proteins with increasing temperature (T) and in the presence of Ca2+ is reproducible, implying that they normally fold into fixed tertiary structures. A nonphosphorylated recombinant human beta-CN with four extra amino acids at the N-terminus (GSHM-) was prepared and studied by laser light scattering, analytical ultracentrifugation, fluorescence spectroscopy, turbidity, and circular dichroism. In 3.3 M urea or at 4 degrees C, the protein was monomeric, as expected. Increasing T both without and with the addition of Ca2+ ions caused self-association as it does for the nonphosphorylated native beta-CN but with a somewhat different interaction pattern. However, returning the protein to its monomeric state by reequilibration at 4 degrees C followed again by increasing T caused a shift in the pattern. Such thermal cycling eventually caused the protein to equilibrate to a particular conformation where no more change could be observed. The resulting interaction pattern was similar to that of the native protein but differed particularly in that there was more extensive self-association for the recombinant mutant. The equilibration to a stable conformation was more rapid in the presence of Ca2+ ions. This suggests that the native protein normally folds into a particular conformation which may be aided by Ca2+ in the mammary gland. Further study of a recombinant form with the native amino acid sequence is needed.

Transgenic animals and nutrition research

Transgenic animals are useful tools for the study of biological functions of proteins and secondary gene products synthesized by the action of protein catalysts. Research in nutrition and allied fields is benefiting from their use as models to contrast normal and altered metabolism. Although food, nutritional products, and ingredients from transgenic animals have not yet reached consumers, the technologies for their production are maturing and yielding exciting results in experimental and farm animals. Regulatory governmental bodies are already issuing guidelines and legislation in anticipation of the advent of these products and ingredients. This review summarizes available technology for the production of transgenic animals, discusses their scientific and commercial potential, and examines ancillary issues relevant to the field of nutrition.