CLONING AND MOLECULAR CHARACTERIZATION OF A HUMAN INTRACELLULAR SERINE PROTEINASE-INHIBITOR

We describe a cDNA encoding a serine proteinase inhibitor present in p lacental tissue and the cytosolic fraction of K562 cells. On the basis of its interaction with thrombin, through which it was discovered, th e inhibitor has been operationally named the placental thrombin inhibi tor (PTI). Amino acid sequence comparisons suggest that its reactive c enter is located at Arg-341 and Cys-342, that it lacks a classical N-t erminal signal sequence, and that it has the highest degree of similar ity to intracellular serine proteinase inhibitors (serpins), such as t he human monocyte/neutrophil elastase inhibitor and the equine leukocy te elastase inhibitor. PTI also resembles these inhibitors in that it contains oxidation-sensitive residues adjacent to the reactive site. T he PTI cDNA was expressed in rabbit reticulocyte lysate and in COS-7 c ells and a 42-kDa protein was produced. Recombinant PTI formed a 67-kD a complex when incubated with thrombin. The ability of native PTI to b ind thrombin was destroyed by incubation with iodoacetamide. Analysis of human tissue mRNA indicated that PTI is expressed widely with the h ighest levels in cardiac and skeletal muscle and placenta. We conclude that PTI is a member of an emerging class of intracellular serpins.