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A cytosolic granzyme B inhibitor related to the viral apoptotic regulator cytokine response modifier A is present in cytotoxic lymphocytes.


Using a polymerase chain reaction strategy we identified a serine proteinase inhibitor (serpin) in human bone marrow that is related to the cellular serpin proteinase inhibitor 6 (PI-6) and the viral serpin cytokine response modifier A (CrmA). This serpin, proteinase inhibitor 9 (PI-9), has an unusual reactive center P1(Glu)-P1'(Cys), which suggests that it inhibits serine proteinases that cleave after acidic residues. The only known serine proteinase with this specificity is granzyme B, a granule cytotoxin produced by cytotoxic lymphocytes. To test the interaction of PI-9 with granzyme B we prepared recombinant hexa-histidine tagged PI-9 in a yeast expression system. Addition of the recombinant protein to native granzyme B resulted in an SDS-resistant complex typical of serpin-serine proteinase interactions. Further analysis showed that complex formation followed bimolecular kinetics with a second order rate constant of 1.7 +/- 0.3 x 10(6) M-1 s-1, which is in the range for a physiologically significant serpin-proteinase interaction. Recombinant PI-9 also completely abrogated granzyme B and perforin-mediated cytotoxicity in vitro. Examination of PI-9 mRNA distribution demonstrated that it is expressed in immune tissue, primarily in lymphocytes. The highest levels of PI-9 mRNA and protein were observed in natural killer cell leukemia cell lines and in interleukin-2 stimulated peripheral blood mononuclear cells, which also produce granzyme B. Like PI-6, PI-9 was shown to be a cytosolic protein that is not secreted. Fractionation of natural killer cells and stimulated peripheral blood mononuclear cells demonstrated that PI-9 is in a separate subcellular compartment to granzyme B. These results suggest that PI-9 serves to inactivate misdirected granzyme B following cytotoxic cell degranulation. This may explain why cytotoxic cells are not damaged by their own granzyme B during destruction of abnormal cells.

Pubmed ID: 8910377


  • Sun J
  • Bird CH
  • Sutton V
  • McDonald L
  • Coughlin PB
  • De Jong TA
  • Trapani JA
  • Bird PI


The Journal of biological chemistry

Publication Data

November 1, 1996

Associated Grants


Mesh Terms

  • Amino Acid Sequence
  • Base Sequence
  • Binding Sites
  • Bone Marrow
  • Cell Line
  • Cytotoxicity, Immunologic
  • DNA Primers
  • Granzymes
  • Humans
  • Molecular Sequence Data
  • Recombinant Proteins
  • Sequence Homology, Amino Acid
  • Serine Endopeptidases
  • Serpins
  • T-Lymphocytes, Cytotoxic
  • Transcription, Genetic
  • Viral Proteins