H-1-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 r atio, as revealed by a complete analysis of its two-dimensional H-1-nm r spectra. The conformational heterogeneity arises from cis-trans isom erism about the Phe 12-Pro 13 peptide bond where the major form corres ponds to the cis conformation. The complete assignment of chemical shi fts and nuclear Overhauser effects (NOEs) of the two isoforms allows a detailed comparative analysis of their conformational properties, eve n though a complete calculation of their solution structures is not po ssible because of a somewhat limited number of NOE constraints. Struct ures for the two isomers could be modeled, however, on the basis of th e high structural homology between omega-hordothionin and related gamm a-thionins, and under the conditions of satisfying all observed experi mental data. The two isoforms adopt practically identical global folds and the structural changes imposed by cis-trans isomerization are con fined to the region proximal to Pro 13. The cis-trans isomerism occurs in a conserved loop connecting the first beta-strand of the triple-st randed antiparallel p-sheet and the oc-helix. A comparative analysis o f the sequences of this loop in the different thionins suggests that t he cis-trans equilibrium about the X-Pro peptide bond depends on the s ize 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-thio nins as well as with some scorpion toxins and insect defensins suggest s that these proteins may share a common mode of functional activity. (C) 1995 John Wiley & Sons, Inc.