1H 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 1H 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+l), medium-range (i,i less than 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.