Two-dimensional 1H-NMR studies of maize lipid-transfer protein. Sequence-specific assignment and secondary structure.
Résumé
Correlation spectroscopy (COSY), total correlation spectroscopy (TOCSY) and NOE spectroscopy (NOESY) experiments have been used to assign sequentially the 1H 500-MHz NMR spectra of a non-specific (ns) lipid-transfer protein extracted from maize seeds. The spin-system identification and sequential assignment were combined with secondary-structure determination to identify most of the proton resonances of this 93-residue protein. From the sequential connectivities it was established that the secondary structure mainly involved four helical fragments: H1, H2, H3 and H4. This secondary structure was compared with that of wheat ns-lipid-transfer protein recently determined. The four helices are located in nearly the same regions, but helix H4 is appreciably longer in the maize protein than in the wheat protein. Comparison of the transfer activities reveals that the maize protein is more efficient than the wheat ns-lipid-transfer protein and that this difference is probably due to the affinity of the lipid for the binding site and not to the interfacial activation, i.e. adsorption of the ns-lipid-transfer protein to the membrane. From these results, it is suggested that helix H4 is a part of the lipid-binding site or contributes to the folding of this site. The present data define the basis for a further modelling of the three-dimensional structure of the maize ns-lipid-transfer protein which will be compared with that of the wheat ns-lipid-transfer protein in order to establish structure/activity relationships for this class of carriers by using natural ns-lipid-transfer protein mutants.