A multienzyme network functions in intestinal protein digestion by a platyhelminth parasite.
Proteases frequently function not only as individual enzymes but also in cascades or networks. A notable evolutionary switch occurred in one such protease network that is involved in protein digestion in the intestine. In vertebrates, this is largely the work of trypsin family serine proteases, whereas in invertebrates, cysteine proteases of the papain family and aspartic proteases assume the role. Utilizing a combination of protease class-specific inhibitors and RNA interference, we deconvoluted such a network of major endopeptidases functioning in invertebrate intestinal protein digestion, using the parasitic helminth, Schistosoma mansoni as an experimental model. We show that initial degradation of host blood proteins is ordered, occasionally redundant, and substrate-specific. Although inhibition of parasite cathepsin D had a greater effect on primary cleavage of hemoglobin, inhibition of cathepsin B predominated in albumin degradation. Nevertheless, in both cases, inhibitor combinations were synergistic. An asparaginyl endopeptidase (legumain) also synergized with cathepsin B and L in protein digestion, either by zymogen activation or facilitating substrate cleavage. This protease network operates optimally in acidic pH compartments either in the gut lumen or in vacuoles of the intestinal lining cells. Defining the role of each of these major enzymes now provides a clearer understanding of the function of a complex protease network that is conserved throughout invertebrate evolution. It also provides insights into which of these proteases are logical targets for development of chemotherapy for schistosomiasis, a major global health problem.
Delcroix
Melaine
M
Department of Pathology, Tropical Disease Research Unit and Sandler Center for Basic Research in Parasitic Diseases, University of California, San Francisco, California 94158, USA. jmck@cgl.ucsf.edu
Sajid
Mohammed
M
Caffrey
Conor R
CR
Lim
Kee-C
KC
Dvorák
Jan
J
Hsieh
Ivy
I
Bahgat
Mahmoud
M
Dissous
Colette
C
McKerrow
James H
JH
D000818:Animals; D002401:Cathepsin B / Q000037:antagonists & inhibitors; D002402:Cathepsin D / Q000037:antagonists & inhibitors; D056668:Cathepsin L; D002403:Cathepsins / Q000037:antagonists & inhibitors; D003546:Cysteine Endopeptidases / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006454:Hemoglobins / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D007422:Intestines / Q000469:parasitology*; D051379:Mice; D008810:Mice, Inbred C57BL; D008954:Models, Biological; D009097:Multienzyme Complexes / Q000378:metabolism*; D012330:RNA, Double-Stranded / Q000737:chemistry; D012550:Schistosoma mansoni / Q000378:metabolism*; D000099240:Asparaginyl Endopeptidase
D002401:Cathepsin B; D002402:Cathepsin D; D056668:Cathepsin L; D002403:Cathepsins; D003546:Cysteine Endopeptidases; D004791:Enzyme Inhibitors; D006454:Hemoglobins; D009097:Multienzyme Complexes; D012330:RNA, Double-Stranded; D000099240:Asparaginyl Endopeptidase; C534278:Ctsl protein, mouse