3-DIMENSIONAL STRUCTURE OF MAIZE ALPHA-ZEIN PROTEINS STUDIED BY SMALL-ANGLE X-RAY-SCATTERING

alpha-zeins of maize (Zea mays) that are storage proteins contain nine or ten tandem repeats comprising of about 20 amino acids. Small-angle X-ray scattering (SAXS) of alpha-zeins was measured in 70% (v/v) aque ous ethanol containing beta-mercaptoethanol or without reagent in a pr otein concentration range of 2.0 to 40.0 mg/ml. The overall radius of gyration of whole particles, Rg, and the corresponding radius of gyrat ion of the cross-section, Re, of reduced alpha-zeins are 4.00 +/- 0.03 nm and 1.39 +/- 0.05 nm, respectively, in the 70% (v/v) aqueous ethan ol containing 2% (v/v) beta-mercaptoethanol. Analyses using the Rg and Re values indicate that reduced cr-zeins exist as asymmetric particle s with the length of about 13 nm in the solution. A structural model i s developed under assumption that each of tandem repeats units forms s ingle alpha-helix and they are joined by glutamine-rich 'turns' or loo ps, as employed by Argos et al., [Argos, O., Pedersen, K., Marks, M.D. and Larkins, B.A. (1982) J. Biol. Chem. 257, 9984-9990] and Garratt e t al, [Garratt, R., Oliva, G., Caracelli, I., Leite, A. and Arruda, P. (1993) Proteins Struc. Func. Genet. 15, 88-99], and that the longest dimension of 13 nm comes from linear stacking of the anti-parallel hel ices of tandem repeat in the direction perpendicular to the helical ax is. The resultant model is presented by an elongated prism-like shape with an approximate axial ratio of 6:1.