Reliable identification of cross-linked products in protein interaction studies by 13C-labeled p-benzoylphenylalanine.

We describe the use of the (13)C-labeled artificial amino acid p-benzoyl-L-phenylalanine (Bpa) to improve the reliability of cross-linked product identification. Our strategy is exemplified for two protein-peptide complexes. These studies indicate that in many cases the identification of a cross-link without additional stable isotope labeling would result in an ambiguous assignment of cross-linked products. The use of a (13)C-labeled photoreactive amino acid is considered to be preferred over the use of deuterated cross-linkers as retention time shifts in reversed phase chromatography can be ruled out. The observation of characteristic fragment ions additionally increases the reliability of cross-linked product assignment. Bpa possesses a broad reactivity towards different amino acids and the derived distance information allows mapping of spatially close amino acids and thus provides more solid structural information of proteins and protein complexes compared to the longer deuterated amine-reactive cross-linkers, which are commonly used for protein 3D-structure analysis and protein-protein interaction studies.

Internalization routes of cell-penetrating melanoma antigen peptides into human dendritic cells.

Optimized delivery of antigens combined with sustainable maturation of dendritic cells (DCs) is crucial for generation of effective antitumoral immune responses. Multiple approaches for ex vivo antigen loading and improvement in immunogenicity have been described. We have recently established a single-step protocol consisting of a fusion peptide (a sequence of the melanoma antigen Melan-A and a cationic cell-penetrating HIV TAT domain) bound in complexes with a toll-like receptor agonist. As the exact cellular uptake mechanisms of TAT-coupled antigens have been a matter of considerable debate and significantly depend on cell type, cargo and concentrations, we evaluated internalization routes into human immature DCs in comparison with non-phagocytic cell lines. We found that Melan-A-TAT fusion peptide uptake by DCs is mainly energy dependent, superior compared with polylysine-coupled Melan-A and significantly higher in DCs as compared with Jurkat cells or HUVECs. Furthermore, we could track the uptake of the fusion peptide exclusively through early endosomes to lysosome compartments after 90 min by fluorescence microscopy and immunoelectron microscopy. Specific endocytosis inhibitors revealed major internalization of the fusion peptide by DCs via clathrin-mediated endocytosis, whereas uptake by non-phagocytic HUVECs differed significantly, involving macropinocytosis as well as clathrin-mediated endocytosis. As our understanding of the processes involved in internalization of TAT-coupled cargos by human DCs broadens, and DC activation becomes available by single-step procedures as described, further development of simultaneous DC maturation and intra-cellular peptide targeting is warranted.

Three-amino acid motifs of urocortin II and III determine their CRF receptor subtype selectivity.

Corticotropin-releasing factor (CRF) and the CRF-like peptide urocortin I (UcnI) exert their activity through two different CRF receptors, CRF1 and CRF2. Recently, UcnII and UcnIII have been discovered as potential endogenous agonists selective for CRF2 known to be involved in brain functions such as learning and anxiety, as well as in cardiovascular functions. A structure-affinity relationship study using chimeric peptides was designed to characterize mouse UcnII (mUcnII) and mUcnIII further and to investigate the structural basis of their receptor subtype selectivity. In the framework of this study, mUcnII (IC50 = 4.4 nM) but not mUcnIII was identified as high-affinity ligand for the rat CRF binding protein. Such affinity had previously not been observed for the human version of this protein. On the basis of secondary structure predictions, it was hypothesized that the amino acid motifs Pro-Ile-Gly of mUcnII and Pro-Thr-Asn of mUcnIII decrease alpha-helicity and thereby impair binding to CRF1. In support of this hypothesis, binding affinity to CRF1 of the chimeric peptides [Pro11Ile12Gly13]h/rCRF, [Pro11Thr12Asn13]h/rCRF, and the corresponding rUcnI analogs was found to be decreased by three orders of magnitude, whereas binding affinity to CRF2 was much less affected. The dramatic decrease in binding affinity to CRF1 correlated with a decrease in alpha-helicity as indicated by the data of circular dichroism spectroscopy.