The Three-dimensional Solution Structure by 1H NMR of a 6-Kda Proteinase Inhibitor Isolated from the Stigma of Nicotiana alata

The three-dimensional structure and disulfide connectivities of a 6-Kda protein isolated from the stigma of the ornamental tobacco Nicotiana alata has been determined by 1H NMR spectroscopy combined with simulated annealing calculations. The protein, termed Cl, is a chymotrypsin inhibitor and is one of five homologous proteinase inhibitors that are proteolytically cleaved from a 40·3-Kda precursor protein. The other four proteinase inhibitors (T1 to T4) contain reactive sites for trypsin. The three-dimensional structure of Cl is generally well defined and contains a triple stranded β-sheet as the dominant secondary structural feature. Several turns and a short region of 310 helix are also present. The putative chymotrypsin reactive site is present on an exposed loop which is less defined than the rest of the protein. The overall shape of Cl is disc-like and the N and C termini are exposed, supporting the proposal that this protein results from post-translational processing of the 40·3-Kda precursor protein.