Studies on a Neurotoxic Protein: the 5 Å Structure of α-Bungarotoxin

Author: Spencer, Steven Andrew

Year: 1978

Degree: Dissertation (Ph.D.)

Advisor: Stroud, Robert M.

Committee Member: Unknown, Unknown

Option: Chemistry

DOI: 10.7907/1xys-8s92

Abstract

The neurotoxic venom from the Formosan Banded Krait, Bungarus multicinctus, was fractionated into nineteen components by ion-exchange and gel-filtration chromatography. The thirteen main protein components were nm on SDS-polyacrylamide gels. Six of these components produced banding patterns si1nilar to a-bungarotoxin (a-BuTX), while four produced banding patterns similar to S-bungarotoxin. Three components did not fit either pattern.

Crystallization studies of α-BuTX were conducted, and crystals were obtained throughout the pH range 2.5 to 7.5. However, only near pH 4 were crystals routinely obtained which were large enough to allow the α-BuTX structure to be determined by x-ray diffraction methods. The pH 4 crystals contained four molecules in the asymmetric

The pH 4 crystals contained four molecules in the asymmetric unit. The unit cell dimensions were 69.9 x 76.7 x 44.8 Å. The crystals appeared to orthorhombic, but space group identification was difficult because the l = odd reflections were generally weak throughout reciprocal space . These weak reflections resulted from the presence within the unit cell of two subcells, each of which had approximately P212121 symmetry. This feature of the crystals considerably complicated the structure determination process . Further complications arose from the presence within the asymmetric unit of a noncrystallographic symmetry axis.

The α-BuTX structure was solved at 5 Å resolution by the method of multiple isomorphous replacement . Except for a long tail at the carboxyl terminus, the toxin molecule fit within an ellipsoid approximately 40 x 28 x 21 Å. The main features of the electron density map were interpreted by comparison to the structure of another neurotoxin, erabutoxin b.

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