Complete assignment of the 1H nuclear magnetic resonance spectrum of French bean plastocyanin. Sequential resonance assignments, secondary structure and global fold.

Sequence-specific proton nuclear magnetic resonance (n.m.r.) assignments for all 99 amino acid residues of French bean Cu(I) plastocyanin are described. The assignments were made using standard sequential assignment procedures and were greatly facilitated by the availability of complete spin system assignments. The characteristic short NOE connectivities between backbone protons, the values of 3JHN alpha, and the locations of slowly exchanging backbone amide protons, identify and define the elements of regular secondary structure. Eight well-defined beta-strands, a small helical segment and eight tight turns can be identified unambiguously. On the basis of a very extensive set of inter-strand NOE connectivities, the beta-strands can be packed into two distinct beta-sheets. Over 80% of the residues in the protein can be assigned to some regular element of secondary structure. The n.m.r. data is sufficient to define the chain folding topology, which is that of a Greek key beta-barrel, and provides a qualitative description of the global fold. The overall structure of French bean plastocyanin in solution is very similar to that of poplar plastocyanin in crystals. Significant local differences are, however, observed, particularly in the loops connecting some of the beta-strands.