nalyzed for CD69 expression. Found at: doi:10.1371/journal.pone.0005000.s001 Ckb Modulates TCR Signaling Acknowledgments We thank Dr. R. Bosselut for helpful discussion and D. Li for instructive comments on the manuscript; Q. Yuan for animal husbandry and Z. Tan for cell sorting. DP thymocytes. Total thymocytes from TCR transgenic or Ckb and TCR double transgenic mice were stained for surface CD4, CD8 and either CD5 or CD69 and then analyzed by flow cytometry. Litt, littermate; CkbTg, Ckb transgenic. Found at: doi:10.1371/journal.pone.0005000.s003 Trypanosoma brucei, is an early divergent protozoan parasite that causes African sleeping sickness in human and nagana in livestock. It has a complex biphasic life cycle that allows the cells to multiply in both the mammalian host and the insect vector tsetse flies. There are many unique features in the cell cycle progression in T. brucei when compared with that in metazoa. For instance, cytokinesis in the bloodstream-form T. brucei is controlled by mitosis whereas that in the insect form is driven primarily by the duplication and LY2109761 cost segregation of basal bodies and its associated mitochondrial DNA complexes, the kinetoplasts. Therefore, procyclic form cells can undergo cytokinesis in the absence of mitosis, whereas a mitotic arrest in bloodstream form cells inhibits cytokinesis with continued kinetoplast replication and segregation and nuclear DNA synthesis, which implicates fundamental differences in cell cycle controls between different life cycle forms of T. brucei and potential absence of the key cell cycle checkpoints. Cell division in both forms of T. brucei proceeds longitudinally along the dorsal line from the anterior to the posterior end of the cell, which contrasts significantly from that in metazoa. The mechanism of this distinctive 
form of cell division in T. brucei is initiated by a trans-localization of the chromosome passenger complex from the midzone of central spindle across the nuclear envelope to the flagellar attachment zone during the final stage of mitosis. The CPC then moves along the FAZ to the anterior end of the dividing cell and slides back toward the posterior end along the FAZ in an unzipping action to separate the dividing mother from the daughter. The cell cycle of T. brucei 18334597 has thus become one of the most intriguing subjects for further investigation in recent years. The progression from metaphase to anaphase during mitosis of T. brucei appears, however, somewhat similar to that observed in other eukaryotes. The chromosomes in the nucleus of T. brucei are replicated during S-phase and attached to the mitotic spindle and aligned in two closely associated parallel rows during metaphase. The chromosomal duplexes are then pulled apart by the mitotic spindle into two separate entities during anaphase. Metaphase-anaphase transition and mitotic exit in metazoa and yeast are controlled primarily by the anaphase promoting complex/cyclosome regulated by periodic association and dissociation with the mitotic checkpoint complex. 10604535 APC/C is a multi-subunit E3 ubiquitin ligase consisting of 13 core subunit proteins in yeast that is inactivated by association with the effector proteins of the MCC complex in the prometaphase. When proper alignment and attachment of the duplicated chromosomes to the mitotic spindle The APC/C of Trypanosoma brucei are achieved toward the end of metaphase, MCC and APC/C are dissociated from each other leaving a single MCC subunit protein, CDC20, with
