Comparative proteomic analysis of trophozoites versus cysts of Giardia lamblia

The proteome of Giardia lamblia at its cyst stage was compared with that of trophozoites by using two-dimensional SDS-PAGE gel electrophoresis. Protein spots that increased in the extracts of cysts compared to trophozoites were identified by MALDI-TOF mass spectroscopy and categorized as cytoskeletal proteins, metabolic enzymes, a cell-cycle-specific kinase, stress resistance proteins, and a protein involved in translation. Expression patterns of five of the identified proteins were examined during encystation by real-time PCR. Expression of cwp1 (encoding cyst wall protein 1), a marker for encystation, was increased 11-fold. In contrast, tim (encoding triose-1-phosphate isomerase) was expressed constitutively during encystation, and its transcription level was therefore used as a mRNA loading control. Expression of three genes encoding β-tubulin, vacuolar ATPase, and never-in-mitosis-A-related protein kinase did not vary significantly during encystation. Interestingly, genes encoding two heat shock proteins (Hsp70 and Hsp90) showed increased expression during encystation, suggesting that this differentiation process accompanies a cellular response to stress in G. lamblia.