title: Absence of a conventional spindle mitotic checkpoint in the binucleated single-celled parasite Giardia intestinalis.
abstract: The spindle assembly checkpoint (SAC) joins the machinery of chromosome-to-spindle microtubule attachment with that of the cell cycle to prevent missegregation of chromosomes during mitosis. Although a functioning SAC has been verified in a limited number of organisms, it is regarded as an evolutionarily conserved safeguard mechanism. In this report, we focus on the existence of the SAC in a single-celled parasitic eukaryote, Giardia intestinalis. Giardia belongs to Excavata, a large and diverse supergroup of unicellular eukaryotes in which SAC control has been nearly unexplored. We show that Giardia cells with absent or defective mitotic spindles due to the inhibitory effects of microtubule poisons do not arrest in mitosis; instead, they divide without any delay, enter the subsequent cell cycle and even reduplicate DNA before dying. We identified a limited repertoire of kinetochore and SAC components in the Giardia genome, indicating that this parasite is ill equipped to halt mitosis before the onset of anaphase via SAC control of chromosome-spindle microtubule attachment. Finally, based on overexpression, we show that Giardia Mad2, a core SAC protein in other eukaryotes, localizes along intracytoplasmic portions of caudal flagellar axonemes, but never within nuclei, even in mitotic cells with blocked spindles, where the SAC should be active. These findings are consistent with the absence of a conventional SAC, known from yeast and metazoans, in the parasitic protist Giardia.
authors: Department of Tropical Medicine, 1st Faculty of Medicine, Charles University in Prague, Studnickova 7, Prague 2, Czech Republic.
authors: Kristyna
authors: K
authors: Markova
authors: Department of Tropical Medicine, 1st Faculty of Medicine, Charles University in Prague, Studnickova 7, Prague 2, Czech Republic.
authors: Magdalena
authors: M
authors: Uzlikova
authors: Department of Tropical Medicine, 1st Faculty of Medicine, Charles University in Prague, Studnickova 7, Prague 2, Czech Republic.
authors: Pavla
authors: P
authors: Tumova
authors: Department of Tropical Medicine, 1st Faculty of Medicine, Charles University in Prague, Studnickova 7, Prague 2, Czech Republic.
authors: Klara
authors: K
authors: Jirakova
authors: Institute of Cellular Biology and Pathology, 1st Faculty of Medicine, Charles University in Prague, Albertov 4, Prague 2, Czech Republic.
authors: Guy
authors: G
authors: Hagen
authors: Department of Parasitology, Faculty of Science, Charles University in Prague, Vinicna 7, Prague 2, Czech Republic.
authors: Jaroslav
authors: J
authors: Kulda
authors: Department of Tropical Medicine, 1st Faculty of Medicine, Charles University in Prague, Studnickova 7, Prague 2, Czech Republic. Electronic address: enohy@lf1.cuni.cz.
authors: Eva
authors: E
authors: Nohynkova
mesh_terms: D000818:Animals; D018797:Cell Cycle Proteins / Q000378:metabolism*; D016829:Giardia lamblia / Q000235:genetics / Q000302:isolation & purification / Q000502:physiology*; D018386:Kinetochores / Q000502:physiology; D059566:M Phase Cell Cycle Checkpoints / Q000502:physiology*; D008941:Spindle Apparatus / Q000502:physiology*
chemical_list: D018797:Cell Cycle Proteins
keywords: Albendazole; Giardia; Kinetochore; Mad2; Single-celled eukaryote; Spindle assembly checkpoint