Crambin in phospholipid vesicles: Circular dichroism analysis of crystal structure relevance

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Abstract
Crambin, a hydrophobic plant [Crambe abyssinica] seed protein that exhibits sequence homology to membrane-active plant toxins, was incorporated into phospholipid vesicles. Circular dichroism spectroscopy indicates that its structure in vesicles is nearly identical to its structure in 60% ethanol solution, the solvent from which the protein was crystallized. The secondary structure predicted from the circular dichroism data of the ethanol solution closely resembles that determined by X-ray diffraction of the crystals. This agreement suggests that the X-ray structure may form a useful basis for remodeling the structure and behavior of lipophilic plant toxins. Finally, because the structure of crambin has been determined in an organic solvent medium, it provids a protein standard for examination of the effect of solvent dipole moment on the circular dichroism spectra of proteins, which may be important for interpretation of data for membrane proteins.