Reinvestigation of the proposed folding and self-association of the Neuropeptide Head Activator

An engineered tryptophan zipper-type peptide as a molecular recognition scaffold

In an effort to develop a structured peptide scaffold that lacks a disulfide bond and is thus suitable for molecular recognition applications in the reducing environment of the cytosol, we investigated engineered versions of the trpzip class of beta-hairpin peptides. We have previously shown that even most highly folded members of the trpzip class (i.e. the 16mer peptide HP5W4) are substantially destabilized by the introduction of mutations in the turn region and therefore not an ideal peptide scaffold. To address this issue, we used a FRET-based live cell screening system to identify extended trpzip-type peptides with additional stabilizing interactions. One of the most promising of these extended trpzip-type variants is the 24mer xxtz1-peptide with the sequence KAWTHDWTWNPATGKWTWLWRKNK. A phage display library of this peptide with randomization of six residues with side chains directed towards one face of the hairpin was constructed and panned against immobilized streptavidin. We have also explored the use of xxtz1-peptide for the presentation of an unstructured peptide 'loop' inserted into the turn region. Although NMR analysis provided no direct evidence for structure in the xxtz1-peptide with the loop insertion, we did attempt to use this construct as a scaffold for phage display of randomized peptide libraries. Panning of the resulting libraries against streptavidin resulted in the identification of peptide sequences with submicromolar affinities. Interestingly, substitution of key residues in the hairpin-derived portion of the peptide resulted in a 400-fold decrease in K(d), suggesting that the hairpin-derived portion plays an important role in preorganization of the loop region for molecular recognition.

Gas phase dimerization of neuropeptide head activator analogs useful for the noncovalent constraint of peptides

We have used electrospray mass spectrometry to examine the dimerization of EFLIVKS, a reversed sequence analog of part of the neuropeptide head activator, and other similar analogs. Observation of the EFLIVKS gas phase dimer is concentration-dependent, with a half-saturation concentration for relative dimer formation of 7.8 microM, similar to that of SKVILFE of 12 microM. The lowest energy conformers from quenched molecular dynamics simulations suggest EFLIVKS may dimerize in the gas phase by formation of multiple ion pairs across the dimer interface. Alanine-scan mutants also dimerize in the gas phase, with replacement of the interior residues FLIVK diminishing dimerization. The concentration-dependence of the EFLIVKS circular dichroism spectrum at pH 7.5 suggests the existence of different conformation states at different concentrations, but does not provide evidence supporting the saturable dimer formation in solution. Different analogs of EFLIVKS, when fused to each end of a 18mer unfolded peptide, induce solution structures with T(m)s of 42-50 degrees C. These peptides and analogs may thus be useful for the noncovalent constraint of peptides and peptide library members used in cellular screens.

Current strategies for the development of peptide-based anti-cancer therapeutics

The completion of the human genome sequence and the development of new techniques, which allow the visualisation of comprehensive gene expression patterns, has led to the identification of a large number of gene products differentially expressed in tumours and corresponding normal tissues. The task at hand is the sorting of these genes into correlative and causative ones. Correlative genes are merely changed as a consequence of transformation and have no decisive effects upon transformation. In contrast, causative genes play a direct role in the process of cellular transformation and the maintenance of the transformed state, which can be exploited for therapeutic purposes. Oncogenes and tumour suppressor genes are prime targets for the development of new inhibitors and gene therapeutic strategies. However, many target oncogene products do not exhibit enzymatic activity that can be inhibited by conventional small molecular weight compounds. They exert their functions through regulated protein-protein or protein-DNA interactions and might require other compounds for efficient interference with such functions. Peptides are emerging as a novel class of drugs for cancer therapy, which could fulfil these tasks. Peptide therapy aims at the specific inhibition of inappropriately activated oncogenes. This review will focus on the selection procedures, which can be employed to identify useful peptides for the treatment of cancer. Before peptide-based therapeutics can become useful, it will be necessary to increase their stability by modifications or the use of scaffolds. Additionally, various delivery methods including liposomes and particularly the use of protein transduction domains (PTDs) have to be explored. These strategies will yield highly specific and more effective peptides and improve the potential of peptide-based anti-cancer therapeutics.