title: SmCL3, a gastrodermal cysteine protease of the human blood fluke Schistosoma mansoni.
abstract: BACKGROUND
  Blood flukes of the genus Schistosoma are platyhelminth parasites that infect 200 million people worldwide. Digestion of nutrients from the host bloodstream is essential for parasite development and reproduction. A network of proteolytic enzymes (proteases) facilitates hydrolysis of host hemoglobin and serum proteins.
  
  
  RESULTS
  We identified a new cathepsin L termed SmCL3 using PCR strategies based on S. mansoni EST sequence data. An ortholog is present in Schistosoma japonicum. SmCL3 was heterologously expressed as an active enzyme in the yeast, Pichia pastoris. Recombinant SmCL3 has a broad pH activity range against peptidyl substrates and is inhibited by Clan CA protease inhibitors. Consistent with a function in degrading host proteins, SmCL3 hydrolyzes serum albumin and hemoglobin, is localized to the adult gastrodermis, and is expressed mainly in those life stages infecting the mammalian host. The predominant form of SmCL3 in the parasite exists as a zymogen, which is unusual for proteases. This zymogen includes an unusually long prodomain with alpha helical secondary structure motifs. The striking specificity of SmCL3 for amino acids with large aromatic side chains (Trp and Tyr) at the P2 substrate position, as determined with positional scanning-synthetic combinatorial library, is consistent with a molecular model that shows a large and deep S2 pocket. A sequence similarity network (SSN) view clusters SmCL3 and other cathepsins L in accordance with previous large-scale phylogenetic analyses that identify six super kingdoms.
  
  
  CONCLUSIONS
  SmCL3 is a gut-associated cathepsin L that may contribute to the network of proteases involved in degrading host blood proteins as nutrients. Furthermore, this enzyme exhibits some unusual sequence and biophysical features that may result in additional functions. The visualization of network inter-relationships among cathepsins L suggests that these enzymes are suitable 'marker sequences' for inclusion in future phylogenetic analyses.
authors.affiliation: Sandler Center for Basic Research in Parasitic Diseases, California Institute for Quantitative Biosciences, University of California San Francisco, San Francisco, California, United States of America. hdvorak76@centrum.cz
authors.forename: Jan
authors.initials: J
authors.lastname: Dvorák
authors.forename: Susan T
authors.initials: ST
authors.lastname: Mashiyama
authors.forename: Mohammed
authors.initials: M
authors.lastname: Sajid
authors.forename: Simon
authors.initials: S
authors.lastname: Braschi
authors.forename: Melaine
authors.initials: M
authors.lastname: Delcroix
authors.forename: Eric L
authors.initials: EL
authors.lastname: Schneider
authors.forename: Wilson H
authors.initials: WH
authors.lastname: McKerrow
authors.forename: Mahmoud
authors.initials: M
authors.lastname: Bahgat
authors.forename: Elizabeth
authors.initials: E
authors.lastname: Hansell
authors.forename: Patricia C
authors.initials: PC
authors.lastname: Babbitt
authors.forename: Charles S
authors.initials: CS
authors.lastname: Craik
authors.forename: James H
authors.initials: JH
authors.lastname: McKerrow
authors.forename: Conor R
authors.initials: CR
authors.lastname: Caffrey
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D056668:Cathepsin L / Q000235:genetics / Q000302:isolation & purification* / Q000378:metabolism*; D006224:Cricetinae; D016054:DNA, Protozoan / Q000235:genetics; D004795:Enzyme Stability; D020224:Expressed Sequence Tags; D041981:Gastrointestinal Tract / Q000737:chemistry; D015870:Gene Expression; D006454:Hemoglobins / Q000378:metabolism; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010802:Phylogeny; D010843:Pichia / Q000235:genetics; D016133:Polymerase Chain Reaction / Q000379:methods; D011480:Protease Inhibitors / Q000494:pharmacology; D012550:Schistosoma mansoni / Q000201:enzymology* / Q000235:genetics; D017386:Sequence Homology, Amino Acid; D012709:Serum Albumin / Q000378:metabolism; D012908:Snails; D013379:Substrate Specificity
chemical_list: D016054:DNA, Protozoan; D006454:Hemoglobins; D011480:Protease Inhibitors; D012709:Serum Albumin; D056668:Cathepsin L