THE 3-DIMENSIONAL SOLUTION STRUCTURE BY H-1-NMR OF A 6-KDA PROTEINASE-INHIBITOR ISOLATED FROM THE STIGMA OF NICOTIANA-ALATA

The three-dimensional structure and disulfide connectivities of a 6-kD a protein isolated from the stigma of the ornamental tobacco Nicotiana alta has been determined by H-1 NMR spectroscopy combined with simula ted 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 (Tl to T4) contain reactive sites for tryps in. The three-dimensional structure of Cl is generally well defined an d contains a triple stranded P-sheet as the dominant secondary structu ral feature. Several turns and a short region of 3(10) helix are also present. The putative chmotrypsin reactive site is present on an expos ed loop which is less defined than the rest of the protein. The overal l shape of Cl is disc-like and the N and C termini are exposed, suppor ting the proposal that this protein results from post-translational pr ocessing of the 40.3-kDa precursor protein.