A primitive enzyme for a primitive cell: the protease required for excystation of Giardia

Cell. 1997 May 2;89(3):437-44. doi: 10.1016/s0092-8674(00)80224-x.


Protozoan parasites of the genus Giardia are one of the earliest lineages of eukaryotic cells. To initiate infection, trophozoites emerge from a cyst in the host. Excystation is blocked by specific cysteine protease inhibitors. Using a biotinylated inhibitor, the target protease was identified and its corresponding gene cloned. The protease was localized to vesicles that release their contents just prior to excystation. The Giardia protease is the earliest known branch of the cathepsin B family. Its phylogeny confirms that the cathepsin B lineage evolved in primitive eukaryotic cells, prior to the divergence of plant and animal kingdoms, and underscores the diversity of cellular functions that this enzyme family facilitates.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Animals
  • Cathepsin B / genetics
  • Cathepsin B / metabolism
  • Cell Division / physiology
  • Cloning, Molecular
  • Cysteine Endopeptidases / genetics*
  • Cysteine Endopeptidases / isolation & purification
  • Cysteine Endopeptidases / metabolism*
  • Cysteine Proteinase Inhibitors / pharmacology
  • Fluorescent Dyes
  • Giardia / cytology
  • Giardia / enzymology*
  • Giardia / growth & development*
  • Ketones / pharmacology
  • Molecular Sequence Data
  • Phylogeny
  • Protozoan Proteins / genetics
  • Protozoan Proteins / metabolism
  • Sequence Homology, Amino Acid
  • Vacuoles / enzymology


  • Cysteine Proteinase Inhibitors
  • Fluorescent Dyes
  • Ketones
  • Protozoan Proteins
  • Cysteine Endopeptidases
  • Cathepsin B

Associated data

  • GENBANK/U83275
  • GENBANK/U83276
  • GENBANK/U83277