Dimeric ?-cobratoxin X-ray structure: localization of intermolecular disulfides and possible mode of binding to nicotinic acetylcholine receptors.

ABSTRACT: In Naja kaouthia cobra venom, we have earlier discovered a covalent dimeric form of ?-cobratoxin (?CT-?CT) with two intermolecular disulfides, but we could not determine their positions. Here, we report the ?CT-?CT crystal structure at 1.94 ? where intermolecular disulfides are identified between Cys(3) in one protomer and Cys(20) of the second, and vice versa. All remaining intramolecular disulfides, including the additional bridge between Cys(26) and Cys(30) in the central loops II, have the same positions as in monomeric ?-cobratoxin. The three-finger fold is essentially preserved in each protomer, but the arrangement of the ?CT-?CT dimer differs from those of noncovalent crystallographic dimers of three-finger toxins (TFT) or from the ?-bungarotoxin solution structure. Selective reduction of Cys(26)-Cys(30) in one protomer does not affect the activity against the ?7 nicotinic acetylcholine receptor (nAChR), whereas its reduction in both protomers almost prevents ?7 nAChR recognition. On the contrary, reduction of one or both Cys(26)-Cys(30) disulfides in ?CT-?CT considerably potentiates inhibition of the ?3?2 nAChR by the toxin. The heteromeric dimer of ?-cobratoxin and cytotoxin has an activity similar to that of ?CT-?CT against the ?7 nAChR and is more active against ?3?2 nAChRs. Our results demonstrate that at least one Cys(26)-Cys(30) disulfide in covalent TFT dimers, similar to the monomeric TFTs, is essential for their recognition by ?7 nAChR, although it is less important for interaction of covalent TFT dimers with the ?3?2 nAChR.

SUBMITTER: Osipov AV 

PROVIDER: S-EPMC3307301 | biostudies-literature | 2012 Feb

REPOSITORIES: biostudies-literature