SOLUTION STRUCTURE OF A TREFOIL-MOTIF-CONTAINING CELL-GROWTH FACTOR, PORCINE SPASMOLYTIC PROTEIN

The porcine spasmolytic protein (pSP) is a 106-residue cell growth fac tor that typifies a family of eukaryotic proteins that contain at leas t one copy of an approximate to 40-amino acid protein domain known as the trefoil moth. In fact, PSP contains two highly homologous trefoil domains. We have determined the complete three-dimensional solution st ructure of pSP by using a combination of two- and three-dimensional H- 1 NMR spectroscopy and distance geometry calculations. pSP is a relati vely elongated molecule, consisting of two compact globular domains jo ined via a small interface. The protein's two trefoil domains adopt th e same tertiary structure and contain a core C-terminal two-stranded a ntiparallel beta sheet, preceded by a 6-residue helix that packs again st the N-terminal beta-strand. The remainder of the protein backbone i s taken up by two short loops that lie on either side of the beta-hair pin and are linked by an extended region that wraps around the C-termi nal beta-strand. The topology of the protein backbone observed for the trefoil domains in pSP represents an unusual polypeptide fold. A stri king feature of both trefoil domains is a surface patch formed from fi ve conserved residues that have no obvious structural role. The two pa tches are located at the far ends of the protein molecule, and we prop ose that these residues form at least part of the receptor binding sit e, or sites, on pSP.