H-1 NMR-BASED DETERMINATION OF THE SECONDARY STRUCTURE OF PORCINE PANCREATIC SPASMOLYTIC POLYPEPTIDE - ONE OF A NEW FAMILY OF TREFOIL MOTIF CONTAINING CELL-GROWTH FACTORS

Two-dimensional H-1 NMR spectroscopy has been used to obtain comprehensive sequence-specific resonance assignments for the putative cell growth factor porcine pancreatic spasmolytic polypeptide, which is a 106-residue protein containing two "trefoil" domains. The patterns of sequential (i,i + 1), medium-range (i,i < 5), and long-range NH to NH, alpha-CH to NH, and alpha-CH to alpha-CH nuclear Overhauser effects clearly show that the protein's two trefoil domains adopt essentially the same secondary structure in solution. The main feature of each domain is a seven-residue helix followed by a short antiparallel beta-sheet formed from two strands of four amino acids each. This is a novel supersecondary structure, which clearly identifies the trefoil motif as a new class of growth factor associated module, distinct from other types of highly disulfide cross-linked domains, such as those found in epidermal growth factor and insulin-like growth factor I.