
Lauwaet, T., C.C.L. Wong, D. Reiner, B.J. Davids, S.G. Svard, A.G. McArthur, J.R. Yates, & FD. Gillin. 2009. Biological surprises from the Giardia lamblia basal body proteome. Presentation at the 13th International Congress of Protistology, Rio de Janeiro, Brazil.
Encystation and excystation, which are crucial to the pathogenesis of Giardia lamblia, are marked by dramatic cytoskeletal remodeling. However, the underlying signaling pathways are not known. Although basal bodies have been studied for >100 years, their composition and functions remain incompletely understood. As an ancient protist that is motile, differentiates, and causes disease, Giardia makes a valuable model. Since calmodulin, which is needed for excystation, localizes exclusively to the basal bodies in giardial cytoskeletons, we used it as bait to affinity purify and identify interacting proteins that may help regulate differentiation. Among the proteins that bound to calmodulin-Sepharose in a Ca-dependent manner, we found known basal body proteins, validating the approach. We also found several unexpected proteins. Exploring their functions has produced complex and interesting findings. Ribosomal proteins found in basal body proteomes are assumed to be contaminants. However, when we epitope tagged one highly conserved ribosomal protein (RP) that affinity purified with calmodulin, we found that it localized to the nucleoli. Confocal microscopy using co-staining with centrin, showed that the nucleolar RP staining overlapped with a single basal body that had been shown by Benchimol to associate with a nucleus. This suggests distinct function of a single basal body and a novel association between a basal body and the nucleolus, another incompletely understood organelle.
We also identified a unique unknown Giardia protein that affinity-purified and co-localized with calmodulin in vegetative cells. Surprisingly it is targeted to variable numbers of encystation secretory vesicles in encysting cells. This may be the first hint that ESVs may be heterogeneous and is the first direct connection between calmodulin, centrosomes and cyst wall biosynthesis. “Biological surprises” such as these may offer unexpected insights into giardial differentiation and basic cell biology.




