Adding 'splice' to protein engineering

Characterization and Recombinant Expression of Terebrid Venom Peptide from Terebra guttata

Venom peptides found in terebrid snails expand the toolbox of active compounds that can be applied to investigate cellular physiology and can be further developed as future therapeutics. However, unlike other predatory organisms, such as snakes, terebrids produce very small quantities of venom, making it difficult to obtain sufficient amounts for biochemical characterization. Here, we describe the first recombinant expression and characterization of terebrid peptide, teretoxin Tgu6.1, from Terebra guttata. Tgu6.1 is a novel forty-four amino acid teretoxin peptide with a VI/VII cysteine framework (C-C-CC-C-C) similar to O, M and I conotoxin superfamilies. A ligation-independent cloning strategy with an ompT protease deficient strain of E. coli was employed to recombinantly produce Tgu6.1. Thioredoxin was introduced in the plasmid to combat disulfide folding and solubility issues. Specifically Histidine-6 tag and Ni-NTA affinity chromatography were applied as a purification method, and enterokinase was used as a specific cleavage protease to effectively produce high yields of folded Tgu6.1 without extra residues to the primary sequence. The recombinantly-expressed Tgu6.1 peptide was bioactive, displaying a paralytic effect when injected into a Nereis virens polychaete bioassay. The recombinant strategy described to express Tgu6.1 can be applied to produce high yields of other disulfide-rich peptides.

Phosphorylation of eIF4E attenuates its interaction with mRNA 5' cap analogs by electrostatic repulsion: intein-mediated protein ligation strategy to obtain phosphorylated protein

Phosphorylation of the eukaryotic initiation factor eIF4E in response to mitogenic stimuli and cytokines is implicated in the regulation of the initiation step of translation. It still remains unclear how the phosphorylation of eIF4E regulates the translation. To address this problem, we applied a unique technique in protein engineering, intein-mediated protein ligation, to synthesize eIF4E, which is selectively phosphorylated at Ser 209. Using selectively chosen synthetic cap analogs, we compared quantitatively the cap affinity for phosphorylated and unphosphorylated eIF4E by a fluorometric time-synchronized titration method. A 1.5- to 4.5-fold reduction of the cap affinity for phosphorylated eIF4E was observed, depending on the negative charge of the 5'-to-5' phosphate chains as well as the presence of a longer tetraribonucleotide strand. Possible implications for understanding the regulation of eIF4E functioning, cap complex formation, and stability, are discussed.

Selective in vitro glycosylation of recombinant proteins: semi-synthesis of novel homogeneous glycoforms of human erythropoietin

BACKGROUND

A natural glycoprotein usually exists as a spectrum of glycosylated forms, where each protein molecule may be associated with an array of oligosaccharide structures. The overall range of glycoforms can have a variety of different biophysical and biochemical properties, although details of structure-function relationships are poorly understood, because of the microheterogeneity of biological samples. Hence, there is clearly a need for synthetic methods that give access to natural and unnatural homogeneously glycosylated proteins. The synthesis of novel glycoproteins through the selective reaction of glycosyl iodoacetamides with the thiol groups of cysteine residues, placed by site-directed mutagenesis at desired glycosylation sites has been developed. This provides a general method for the synthesis of homogeneously glycosylated proteins that carry saccharide side chains at natural or unnatural glycosylation sites. Here, we have shown that the approach can be applied to the glycoprotein hormone erythropoietin, an important therapeutic glycoprotein with three sites of N-glycosylation that are essential for in vivo biological activity.

RESULTS

Wild-type recombinant erythropoietin and three mutants in which glycosylation site asparagine residues had been changed to cysteines (His(10)-WThEPO, His(10)-Asn24Cys, His(10)-Asn38Cys, His(10)-Asn83CyshEPO) were overexpressed and purified in yields of 13 mg l(-1) from Escherichia coli. Chemical glycosylation with glycosyl-beta-N-iodoacetamides could be monitored by electrospray MS. Both in the wild-type and in the mutant proteins, the potential side reaction of the other four cysteine residues (all involved in disulfide bonds) were not observed. Yield of glycosylation was generally about 50% and purification of glycosylated protein from non-glycosylated protein was readily carried out using lectin affinity chromatography. Dynamic light scattering analysis of the purified glycoproteins suggested that the glycoforms produced were monomeric and folded identically to the wild-type protein.

CONCLUSIONS

Erythropoietin expressed in E. coli bearing specific Asn-->Cys mutations at natural glycosylation sites can be glycosylated using beta-N-glycosyl iodoacetamides even in the presence of two disulfide bonds. The findings provide the basis for further elaboration of the glycan structures and development of this general methodology for the synthesis of semi-synthetic glycoproteins.

Intein-mediated protein ligation: harnessing nature's escape artists

Inteins are naturally occurring proteins that are involved in the precise cleavage and formation of peptide bonds in a process known as protein splicing. Genetic engineering has allowed the controllable cleavage of peptide bonds at either the N- or C-terminus of the intein. Inteins displaying controllable cleavage have been used in the isolation of bacterially expressed proteins possessing either a C-terminal thioester or an N-terminal cysteine. The specific placement of these reactive groups has allowed either protein-protein or protein-peptide condensation through a native peptide bond. This review describes the methods used to specifically generate these reactive groups on bacterially expressed proteins and some applications of this technique, known as intein-mediated protein ligation. Furthermore, a versatile two intein (TWIN) system will be described which enables the circularization and polymerization of bacterially expressed proteins or peptides.

InBase, the Intein Database

InBase, the Intein Database (http://www.neb.com/neb/inteins.html ), is a comprehensive on-line resource that includes the Intein Registry. Inteins are protein splicing elements that mediate a self-catalytic protein splicing reaction. InBase presents general information as well as detailed data for each intein, including tabu-lated comparisons and a comprehensive bibliography.

The cyclization and polymerization of bacterially expressed proteins using modified self-splicing inteins

Mini-inteins derived from Synechocystis sp. (Ssp DnaB intein) and Mycobacterium xenopi (Mxe GyrA intein) that have been modified to cleave peptide bonds at their C and N termini, respectively, were cloned in-frame to the N and C termini of a target protein. Peptide bond cleavage of the modified inteins generated an N-terminal cysteine and a C-terminal thioester on the same protein. These complementary reactive groups underwent intra- or intermolecular condensation to generate circular or polymeric protein species with a new peptide bond at the site of ligation. Three cyclic peptides, BBP, an organ specific localization peptide; RGD, an inhibitor of platelet aggregation; and CDR-H3/C2, which inhibits HIV-1 replication, were isolated using the two-intein system. BBP, RGD, and CDR-H3/C2 had masses of 977.1, 1119.9, and 2098.6 g/mol, respectively, as determined by matrix-assisted laser desorption-time of flight mass spectrometry, which agreed well with the values of 977.2, 1120.3, and 2098.3 g/mol, respectively, predicted for the cyclic species. This system was used to cyclize proteins as large as 395 amino acids. Furthermore, multimers of thioredoxin were formed upon concentration of the reactive species, indicating the potential to form novel biomaterials based on fibrous proteins.

The in vitro ligation of bacterially expressed proteins using an intein from Methanobacterium thermoautotrophicum

The smallest known intein, found in the ribonucleoside diphosphate reductase gene of Methanobacterium thermoautotrophicum (Mth RIR1 intein), was found to splice poorly in Escherichia coli with the naturally occurring proline residue adjacent to the N-terminal cysteine of the intein. Splicing proficiency increased when this proline was replaced with an alanine residue. However, constructs that displayed efficient N- and C-terminal cleavage were created by replacing either the C-terminal asparagine or N-terminal cysteine of the intein, respectively, with an alanine. Furthermore, these constructs were used to specifically generate complementary reactive groups on protein sequences for use in ligation reactions. Reaction between an intein-generated C-terminal thioester on E. coli maltose-binding protein (43 kDa) and an intein-generated cysteine at the N terminus of either T4 DNA ligase (56 kDa) or thioredoxin (12 kDa) resulted in the ligation of the proteins through a native peptide bond. Thus the smallest of the known inteins is capable of splicing and its unique properties extend the utility of intein-mediated protein ligation to include the in vitro fusion of large, bacterially expressed proteins.