Characterizing Recombinant Proteins

Overview of the characterization of recombinant proteins

This overview provides guidelines for the characterization of recombinantly expressed proteins (e.g., verifying primary structure and appropriate post-translational modifications), along with methodologies for characterizing the proteins according to size, X-ray structure, absorbance, biological activity, and subunit structure. A flow chart presents a suggested path for fully characterizing recombinant protein and involves equipment for HPLC, mass spectrometry, circular dichroism, NMR and fluorescence spectroscopy. Also covered are sources and consequences of contamination in protein solutions.

The beta-defensin gallinacin-6 is expressed in the chicken digestive tract and has antimicrobial activity against food-borne pathogens

Food-borne pathogens are responsible for most cases of food poisoning in developed countries and are often associated with poultry products, including chicken. Little is known about the role of beta-defensins in the chicken digestive tract and their efficacy. In this study, the expression of chicken beta-defensin gallinacin-6 (Gal-6) and its antimicrobial activity against food-borne pathogens were investigated. Reverse transcription-PCR analysis showed high expression of Gal-6 mRNA in the esophagus and crop, moderate expression in the glandular stomach, and low expression throughout the intestinal tract. Putative transcription factor binding sites for nuclear factor kappa beta, activator protein 1, and nuclear factor interleukin-6 were found in the Gal-6 gene upstream region, which suggests a possible inducible nature of the Gal-6 gene. In colony-counting assays, strong bactericidal and fungicidal activity was observed, including bactericidal activity against food-borne pathogens Campylobacter jejuni, Salmonella enterica serovar Typhimurium, Clostridium perfringens, and Escherichia coli. Treatment with 16 mug/ml synthetic Gal-6 resulted in a 3 log unit reduction in Clostridium perfringens survival within 60 min, indicating fast killing kinetics. Transmission electron microscopy examination of synthetic-Gal-6-treated Clostridium perfringens cells showed dose-dependent changes in morphology after 30 min, including intracellular granulation, cytoplasm retraction, irregular septum formation in dividing cells, and cell lysis. The high expression in the proximal digestive tract and broad antimicrobial activity suggest that chicken beta-defensin gallinacin-6 plays an important role in chicken innate host defense.

Characterization of monoclonal antibody glycosylation: comparison of expression systems and identification of terminal alpha-linked galactose

CAMPATH-1H is a recombinant humanized murine monoclonal immunoglobulin (IgG1) which recognizes the CDw52 antigen of human lymphocytes, and has been the subject of clinical trials for the treatment of non-Hodgkin's lymphoma and rheumatoid arthritis. Peptide mapping by liquid chromatography-mass spectrometry was used to confirm the predicted amino acid sequences and profile glycosylation for two CAMPATH isotypes expressed in a murine myeloma cell line (NS0) and a single isotype expressed in both Chinese hamster ovary (CHO) and NS0 lines. The three major glycoforms identified in CAMPATH are fucosylated biantennary structures, containing zero, one, or two galactose residues. Glycosylation of the IgG1 form of CAMPATH expressed in CHO cells is consistent with normal human IgG. However, IgG1 and IgG4 expressed in NS0 cells include two potentially immunogenic glycoforms which contain either one or two nonreducing terminal alpha-linked galactose residues. Oligosaccharide structures were characterized by a combination of tandem mass spectrometry, methylation analysis, and exoglycosidase digestion. The strategy used here is designed to be widely applicable to recombinant glycoproteins.

Specific cleavage of immunoglobulin G by copper ions

The hinge region of a recombinant-DNA-produced human IgG1 (Campath 1H) is specifically cleavable at a single copper-sensitive peptide bond, yielding a distinct fragment resolved by size-exclusion high-performance liquid chromatography. This novel metal ion-catalysed cleavage at slightly alkaline pH is inhibited by EDTA and its rate is reduced at slightly acidic conditions (pH 5-6) and accelerated by increasing concentrations of cupric ion and higher temperature. Complete cleavage was observed after incubation at pH 8 for 24 h with 1 mM CuCl2. Sequence analysis determined the cleavage site to be the Lys226-Thr227 bond in the hinge-region sequence DKTHT. Cleavage of other IgGs was observed to varying degrees, and specific cleavage of synthetic peptides containing this pentapeptide sequence was also observed.

Phosphoric acid entrapment leads to apparent protein heterogeneity

Recombinant proteins produced in prokaryotes or eukaryotes show certain types of heterogeneity due to post-translational modifications. Some preparations of a soluble interferon gamma receptor, produced in Escherichia coli, appeared as a double band with slightly different mobilities in non-reducing sodium dodecylsulfate and native polyacrylamide gels. Ion spray mass spectrometry showed that the two forms had a mass difference of one to three multiples of 97 +/- 2 D. Gas chromatography-mass spectrometry analysis revealed the presence of phosphoric acid in the hydrolysate and in the intact protein. The more slowly migrating protein species had trapped molecules of phosphoric acid during the protein extraction. Most of the trapped phosphoric acid was loosely associated with the protein. One to three molecules were tightly, but non-covalently linked per receptor molecule. Phosphoric acid entrapment did not affect biological activity and most likely did not affect protein conformation. The species carrying phosphoric acid showed higher solubility. Trapping of phosphoric acid by proteins may be a general phenomenon and the results reported here thus useful in the characterization of other recombinant proteins.

Rapid and quantitative analysis of recombinant protein expression using pyrolysis mass spectrometry and artificial neural networks: application to mammalian cytochrome b5 in Escherichia coli

Recombinant Escherichia coli clones encoding between 0 and 6 copies of the mammalian cytochrome b5 gene were subjected to pyrolysis mass spectrometry (PyMS). To deconvolute the pyrolysis mass spectra so as to obtain quantitative information on the amount of cytochrome b5 produced fully-interconnected feedforward artificial neural networks (ANNs) were studied. It was found that the combination of PyMS and ANNs could be used to predict the amount of cytochrome b5 expressed in E. coli. PyMS is a novel, convenient and rapid method for the screening and analysis of microbial and other cultures producing recombinant proteins.