Protein SUMOylation is Involved in Cell-cycle Progression and Cell Morphology in Giardia lamblia.

The unicellular protozoa Giardia lamblia is a food- and waterborne parasite that causes giardiasis. This illness is manifested as acute and self-limited diarrhea and can evolve to long-term complications. Successful establishment of infection by Giardia trophozoites requires adhesion to host cells and colonization of the small intestine, where parasites multiply by mitotic division. The tight binding of trophozoites to host cells occurs by means of the ventral adhesive disc, a spiral array of microtubules and associated proteins such as giardins. In this work we show that knock down of the Small Ubiquitin-like MOdifier (SUMO) results in less adhesive trophzoites, decreased cell proliferation and deep morphological alterations, including at the ventral disc. Consistent with the reduced proliferation, SUMO knocked-down trophozoites were arrested in G1 and in S phases of the cell cycle. Mass spectrometry analysis of anti-SUMO immunoprecipitates was performed to identify SUMO substrates possibly involved in these events. Among the identified SUMOylation targets, α-tubulin was further validated by Western blot and confirmed to be a SUMO target in Giardia trophozoites.

Infection Strategies of Intestinal Parasite Pathogens and Host Cell Responses.

Giardia lamblia, Cryptosporidium sp., and Entamoeba histolytica are important pathogenic intestinal parasites and are amongst the leading causes worldwide of diarrheal illness in humans. Diseases caused by these organisms, giardiasis, cryptosporidiosis, and amoebiasis, respectively, are characterized by self-limited diarrhea but can evolve to long-term complications. The cellular and molecular mechanisms underlying the pathogenesis of diarrhea associated with these three pathogens are being unraveled, with knowledge of both the strategies explored by the parasites to establish infection and the methods evolved by hosts to avoid it. Special attention is being given to molecules participating in parasite-host interaction and in the mechanisms implicated in the diseases' pathophysiologic processes. This review focuses on cell mechanisms that are modulated during infection, including gene transcription, cytoskeleton rearrangements, signal transduction pathways, and cell death.

The Gcn2 Regulator Yih1 Interacts with the Cyclin Dependent Kinase Cdc28 and Promotes Cell Cycle Progression through G2/M in Budding Yeast.

The Saccharomyces cerevisiae protein Yih1, when overexpressed, inhibits the eIF2 alpha kinase Gcn2 by competing for Gcn1 binding. However, deletion of YIH1 has no detectable effect on Gcn2 activity, suggesting that Yih1 is not a general inhibitor of Gcn2, and has no phenotypic defect identified so far. Thus, its physiological role is largely unknown. Here, we show that Yih1 is involved in the cell cycle. Yeast lacking Yih1 displays morphological patterns and DNA content indicative of a delay in the G2/M phases of the cell cycle, and this phenotype is independent of Gcn1 and Gcn2. Accordingly, the levels of phosphorylated eIF2α, which show a cell cycle-dependent fluctuation, are not altered in cells devoid of Yih1. We present several lines of evidence indicating that Yih1 is in a complex with Cdc28. Yih1 pulls down endogenous Cdc28 in vivo and this interaction is enhanced when Cdc28 is active, suggesting that Yih1 modulates the function of Cdc28 in specific stages of the cell cycle. We also demonstrate, by Bimolecular Fluorescence Complementation, that endogenous Yih1 and Cdc28 interact with each other, confirming Yih1 as a bona fide Cdc28 binding partner. Amino acid substitutions within helix H2 of the RWD domain of Yih1 enhance Yih1-Cdc28 association. Overexpression of this mutant, but not of wild type Yih1, leads to a phenotype similar to that of YIH1 deletion, supporting the view that Yih1 is involved through Cdc28 in the regulation of the cell cycle. We further show that IMPACT, the mammalian homologue of Yih1, interacts with CDK1, the mammalian counterpart of Cdc28, indicating that the involvement with the cell cycle is conserved. Together, these data provide insights into the cellular function of Yih1/IMPACT, and provide the basis for future studies on the role of this protein in the cell cycle.