Three-dimensional structure of maize α-zein proteins studied by small-angle X-ray scattering

Abstract

α-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 α-zeins was measured in 70% (v/v) aqueous ethanol containing β-mercaptoethanol or without reagent in a protein concentration range of 2.0 to 40.0 mg/ml. The overall radius of gyration of whole particles, Rg, and the corresponding radius of gyration of the cross-section, Rc, of reduced α-zeins are 4.00±0.03 nm and 1.39±0.05 nm, respectively, in the 70% (v/v) aqueous ethanol containing 2% (v/v) β-mercaptoethanol. Analyses using the Rg and Rc values indicate that reduced α-zeins exist as asymmetric particles with the length of about 13 nm in the solution. A structural model is developed under assumption that each of tandem repeats units forms single α-helix and they are joined by glutamine-rich `turns' or loops, 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 et 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 helices of tandem repeat in the direction perpendicular to the helical axis. The resultant model is presented by an elongated prism-like shape with an approximate axial ratio of 6:1.