Structural analysis of a biologically active echistatin analogue des(46-49)-[Ala8,37]-echistatin gamma with three disulfide bonds by 2D-NMR and computer graphics

Expression in Pichia pastoris and characterization of echistatin, an RGD-containing short disintegrin

Echistatin (Ech) is a potent inhibitor of integrins and was isolated from snake Echis carinatus. To facilitate the study on the molecular determinants of integrin-ligand interactions, we expressed recombinant Ech and its mutants in the Pichia pastoris (P. pastoris) expression system and purified them to homogeneity with the yields of 2-7 mg/L. Ech produced in P. pastoris inhibited platelet aggregation with the IC(50) value of 210.5 nM. The sequential assignment and structure analysis of recombinant Ech were obtained by 2D and 3D (15)N-edited NMR spectra. These data suggests that Ech produced in P. pastoris retained its function and native fold. The results presented here provide the evidences that four disulfide-bonded peptide inhibitor of integrin, Ech, can be expressed in P. pastoris with correct fold and high yield. Platelet aggregation analysis of Ech mutants showed that the length of C-terminus and the K45 residue of Ech are important for interacting with integrin αIIbβ3. We also found that recombinant Ech can inhibit the migration of human A375 melanoma cell. These findings may serve as the basis for understanding the activities of Ech.

Role of disulfide bridges in the folding, structure and biological activity of omega-conotoxin GVIA

Omega-Conotoxin GVIA (GVIA), an N-type calcium channel blocker from the cone shell Conus geographus, is a 27 residue polypeptide cross-linked by three disulfide bonds. Here, we report the synthesis, structural analysis by (1)H NMR and bioassay of analogues of GVIA with disulfide bridge deletions and N- and C-terminal truncations. Two analogues that retain the crucial Lys-2 and Tyr-13 residues in loops constrained by two native disulfide bridges were synthesised using orthogonal protection of cysteine residues. In the first analogue, the Cys-15-Cys-26 disulfide bridge was deleted (by replacing the appropriate Cys residues with Ser), while in the second, this disulfide bridge and the eight C-terminal residues were deleted. No activity was detected for either analogue in a rat vas deferens assay, which measures N-type calcium channel activity in sympathetic nerve, and NMR studies showed that this was due to a gross loss of secondary and tertiary structure. Five inactive analogues that were synthesised without orthogonal protection of Cys residues as part of a previous study (Flinn et al. (1995) J. Pept. Sci. 1, 379-384) were also investigated. Three had single disulfide deletions (via Ser substitutions) and two had N- or C-terminal deletions in addition to the disulfide deletion. Peptide mapping and NMR analyses demonstrated that at least four of these analogues had non-native disulfide pairings, which presumably accounts for their lack of activity. The NMR studies also showed that all five analogues had substantially altered tertiary structures, although the backbone chemical shifts and nuclear Overhauser enhancements (NOEs) implied that native-like turn structures persisted in some of these analogues despite the non-native disulfide pairings. This work demonstrates the importance of the disulfides in omega-conotoxin folding and shows that the Cys-15-Cys-26 disulfide is essential for activity in GVIA. The NMR analyses also emphasise that backbone chemical shifts and short- and medium-range NOEs are dictated largely by local secondary structure elements and are not necessarily reliable monitors of the tertiary fold.

Role of the 6-20 disulfide bridge in the structure and activity of epidermal growth factor

Two synthetic analogues of murine epidermal growth factor, [Abu6, 20] mEGF4-48 (where Abu denotes amino-butyric acid) and [G1, M3, K21, H40] mEGF1-48, have been investigated by NMR spectroscopy. [Abu6, 20] mEGF4-48 was designed to determine the contribution of the 6-20 disulfide bridge to the structure and function of mEGF. The overall structure of this analogue was similar to that of native mEGF, indicating that the loss of the 6-20 disulfide bridge did not affect the global fold of the molecule. Significant structural differences were observed near the N-terminus, however, with the direction of the polypeptide chain between residues four and nine being altered such that these residues were now located on the opposite face of the main beta-sheet from their position in native mEGF. Thermal denaturation experiments also showed that the structure of [Abu6, 20] mEGF4-48 was less stable than that of mEGF. Removal of this disulfide bridge resulted in a significant loss of both mitogenic activity in Balb/c 3T3 cells and receptor binding on A431 cells compared with native mEGF and mEGF4-48, implying that the structural changes in [Abu6, 20] mEGF4-48, although limited to the N-terminus, were sufficient to interfere with receptor binding. The loss of binding affinity probably arose mainly from steric interactions of the dislocated N-terminal region with part of the receptor binding surface of EGF. [G1, M3, K21, H40] mEGF1-48 was also synthesized in order to compare the synthetic polypeptide with the corresponding product of recombinant expression. Its mitogenic activity in Balb/c 3T3 cells was similar to that of native mEGF and analysis of its 1H chemical shifts suggested that its structure was also very similar to native.