1H-nmr studies on the structure of a new thionin from barley endosperm.
A new thionin from barley, omega-hordothionin, has been shown to exist in aqueous solution as a mixture of two different isoforms in a 3:2 ratio, as revealed by a complete analysis of its two-dimensional 1H-nmr spectra. The conformational heterogeneity arises from cis-trans isomerism about the Phe 12-Pro 13 peptide bond, where the major form corresponds to the cis conformation. The complete assignment of chemical shifts and nuclear Overhauser effects (NOEs) of the two isoforms allows a detailed comparative analysis of their conformational properties, even though a complete calculation of their solution structures is not possible because of a somewhat limited number of NOE constraints. Structures for the two isomers could be modeled, however, on the basis of the high structural homology between omega-hordothionin and related gamma-thionins, and under the conditions of satisfying all observed experimental data. The two isoforms adopt practically identical global folds and the structural changes imposed by cis-trans isomerization are confined to the region proximal to Pro 13. The cis-trans isomerism occurs in a conserved loop connecting the first beta-strand of the triple-stranded antiparallel beta-sheet and the alpha-helix. A comparative analysis of the sequences of this loop in the different thionins suggests that the cis-trans equilibrium about the X-Pro peptide bond depends on the size of the side chain of X (X = Gly in gamma-thionins and Phe in omega-thionin). The structural homology of this new thionin with gamma-thionins as well as with some scorpion toxins and insect defensins suggests that these proteins may share a common mode of functional activity.
Bruix
M
M
Instituto de Estructura de la Materia, CSIC, Madrid, Spain.
González
C
C
Santoro
J
J
Soriano
F
F
Rocher
A
A
Méndez
E
E
Rico
M
M
D000595:Amino Acid Sequence; D023181:Antimicrobial Cationic Peptides; D001467:Hordeum / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D008969:Molecular Sequence Data; D010940:Plant Proteins / Q000737:chemistry*; D011487:Protein Conformation; D017386:Sequence Homology, Amino Acid
D023181:Antimicrobial Cationic Peptides; D010940:Plant Proteins; C036092:hordothionin protein, Hordeum vulgare