Davids, B.J., D.S. Reiner, M.A. Gilbert, A.G. McArthur, & F.D. Gillin. 2009. Transcriptomics of the Giardia lamblia degradome. Presentation at the 49th Annual Meeting of the American Society for Cell Biology, San Diego, California.Proteolytic activity is important in the life cycle of parasites and in the interactions with their hosts. While the main class of proteases studied in the protozoan parasite Giardia lamblia has been the cysteine type, the activity of other protease classes have been implicated in the cell cycle and differentiation.
To define the complete set of Giardia proteases (degradome), we queried the Giardia genome database for peptidase homologues and we profiled their transcription expression in the different stages of the Giardia life cycle by serial analysis of gene expression (SAGE). We uncovered 87 candidate peptidase genes that were distributed over 5 catalytic classes: cysteine (43.68%), metallo (21.8%), serine (16.1%), threonine (16.1%), aspartic (1.15%), and unknown type (1.15%). Out of the 87 protease ORFs, 11 did not have SAGE tags and 14 had significant transcriptional variation. Only one protease transcript in the serine catalytic family was upregulated in encystation specifically. This serine protease was most similar to eukaryotic subtilisin-like proprotein convertases (SPC). However, this giardial SPC (gSPC) was annotated as a ‘non-peptidase homologue’ by MEROPS because a classic catalytic triad was not evident. To analyze gSPC protein expression in encystation, we expressed gSPC under its own promoter in a vector with an AUI epitope tag. Western blot analyses showed that the epitope-tagged gSPC protein was upregulated and processed during encystation with highest expression in cysts. Gelatin zymography showed catalytic activity during encystation.
Overall our data show that 1.74% of the annotated Giardia genome is composed of peptidases (1.18%) and their homologues (0.56%), which is in range of any cellular genome. Despite the lack of a conserved catalytic domain, the gSPC homologue is catalytically active supporting the inclusion of ‘non-peptidase’ homologues in our degradome data set. This study is the most complete quantitative analysis of protease gene expression in Giardia to date. These data give direction for future research of specific protease families and their functions.
