Tryptophan and tyrosine to terbium fluorescence resonance energy transfer as a method to “map” aromatic residues and monitor docking

Doubling the cross-linking interface of a rationally designed beta roll peptide for calcium-dependent proteinaceous hydrogel formation

We have rationally engineered a stimulus-responsive cross-linking domain based on a repeats-in-toxin (RTX) peptide to enable calcium-dependent formation of supramolecular hydrogel networks. The peptide isolated from the RTX domain is intrinsically disordered in the absence of calcium. In calcium rich environments, the peptide binds Ca(2+) ions and folds into a beta roll (β-roll) secondary structure composed to two parallel β-sheet faces. Previously, we mutated one of the faces to contain solvent exposed leucine side chains which are localized only in the calcium-bound β-roll conformation. We demonstrated the ability of this mutant peptide to self-assemble into hydrogels in the presence of calcium with the aid of additional peptide-based cross-linking moieties. Here, we have expanded this approach by engineering both β-roll faces to contain leucine residues, thereby doubling the cross-linking interface for each monomeric building block. These leucine rich surfaces impart a hydrophobic driving force for self-assembly. Extensive characterization was performed on this double-faced mutant to ensure the retention of calcium affinity and subsequent structural rearrangement similar to that of the wild type domain. We genetically fused an α-helical leucine zipper capable of forming tetrameric coiled-coil bundles to the peptide and the resulting chimeric protein self-assembles into a hydrogel only in calcium rich environments. Since this new mutant peptide enables cross-linking on both surfaces simultaneously, a higher oligomerization state was achieved. To further investigate the cross-linking capability, we constructed concatemers of the β-roll with maltose binding protein (MBP), a monomeric globular protein, without the leucine zipper domains. These concatemers show a similar sol-gel transition in response to calcium. Several biophysical techniques were used to probe the structural and mechanical properties of the mutant β-roll domain and the resulting supramolecular networks including circular dichroism, fluorescence resonance energy transfer, bis-ANS binding, and microrheology. These results demonstrate that the engineered β-roll peptides can mediate calcium-dependent cross-linking for protein hydrogel formation without the need for any other cross-linking moieties.

Oral gene delivery with cyclo-(D-Trp-Tyr) peptide nanotubes

The feasibility of cyclo-(D-Trp-Tyr) peptide nanotubes (PNTs) as oral gene delivery carriers was investigated in nude mice with eight 40 μg doses of pCMV-lacZ in 2 days at 3 h intervals. The association between DNA and PNTs, the DNase I stability of PNTs-associated DNA, and in vitro permeability of DNA were estimated. The results showed that the cyclo-(D-Trp-Tyr) PNTs self-associated at concentrations above 0.01 mg/mL. Plasmid DNA associated with PNTs with a binding constant of 3.2 × 10(8) M(-1) calculated by a fluorescence quenching assay. PNTs were able to protect DNA from DNase I, acid, and bile digestion for 50 min, 60 min, and 180 min, respectively. The in vitro duodenal apparent permeability coefficient of pCMV-lacZ calculated from a steady state flux was increased from 49.2 ± 21.6 × 10(-10) cm/s of naked DNA to 395.6 ± 142.2 × 10(-10) cm/s of pCMV-lacZ/PNT formulation. The permeation of pCMV-lacZ formulated with PNTs was found in an energy-dependent process. Furthermore, β-galatosidase (β-Gal) activity in tissues was quantitatively assessed using chlorophenol red-β-D-galactopyranoside (CPRG) and was significantly increased by 41% in the kidneys at 48 h and by 49, 63, and 46% in the stomach, duodenum, and liver, respectively, at 72 h after the first dose of oral delivery of pCMV-lacZ/PNT formulation. The organs with β-Gal activity were confirmed for the presence of pCMV-lacZ DNA with Southern blotting analysis and intracellular tracing the TM-rhodamine-labeled DNA and the presence of mRNA by reverse transcription-real time quantitative PCR (RT-qPCR). Another plasmid (pCMV-hRluc) encoding Renilla reniformis luciferase was used to confirm the results. An increased hRluc mRNA and luciferase in stomach, duodenum, liver, and kidney were detected by RT-qPCR, ex vivo bioluminescence imaging, luciferase activity quantification, and immunostaining, respectively.