Chromosome passenger complexes (CPCs) are composed of the Aurora B kinase and associated proteins that are believed to regulate the activity and localization of the kinase. The dynamic spatial and temporal localization of CPCs suggests that they perform multiple mitotic functions. Before anaphase, passengers associate with chromosomes and kinetochores where they correct errant kinetochore–microtubule attachments and, thus, ensure the bivalent attachment of chromatids to the mitotic spindle. After anaphase onset, chromosome passengers localize to the anaphase spindle (Cooke et al., 1987
; Adams et al., 2000
). Although the precise role on the spindle is not clear, evidence from a variety of systems have implicated CPCs in maintaining the spindle midzone and in ensuring completion of cytokinesis. Despite a large number of functional studies in multiple systems, it is unclear how CPCs carry out their multiple roles during mitosis.
Our understanding of CPC function is largely informed by analysis of the Aurora B kinase (Ipl1 in budding yeast). The physical association of Aurora B with other passengers, including inner centromere protein, Survivin, and Borealin (Sli15, Bir1, and Nbl1, respectively, in budding yeast), has been linked to its catalytic activity (Gassmann et al., 2004
; Nakajima et al., 2009
). Thus, one idea is that the other passengers act to target Aurora B to its appropriate mitotic substrates. However, not all CPC functions require the Aurora B kinase. For example, in budding yeast, mutations that disrupt the interaction between Sli15 and Bir1 inhibit septin ring disassembly at the end of mitosis, yet ipl1-321
has no effect on septins (Thomas and Kaplan, 2007
). Therefore, a second possibility is that discrete CPCs carry out different mitotic functions, some of which may not require the catalytic activity of Aurora B. To further elucidate the mitotic roles and mechanisms of CPC function, we focus on the role of CPCs on the anaphase spindle.
Mutant analysis in budding yeast suggests that CPCs play a role in spindle midzone organization (Mackay et al., 1998
; Khmelinskii and Schiebel, 2008
). In budding yeast, the spindle midzone is a poorly defined region of overlapping interpolar microtubules where multiple microtubule-associated proteins (MAPs) contribute to stability and outward spindle forces. CPCs associate with the spindle through the simultaneous interaction of Sli15 with microtubules and with Ipl1 and Bir1; Bir1 in turn recruits CBF3 (Kim et al., 1999
; Kang et al., 2001
; Pereira and Schiebel, 2003
; Bouck and Bloom, 2005
; Widlund et al., 2006
; Thomas and Kaplan, 2007
). The sli15-3
mutant prevents the midzone protein Slk19 from properly localizing (Pereira and Schiebel, 2003
), which in turn alters the localization of other midzone proteins, such as Cin8 (kinesin-5 family), Bim1 (EB1 family), and Stu2 (Dis1/XMAP215; Khmelinskii et al., 2007
). Although the changes in sli15-3
are not sufficient to cause spindle instability in anaphase, spindles become fragile when anaphase is initiated artificially by the engineered cleavage of sister chromatid cohesion (Pereira and Schiebel, 2003
). To better understand the role of CPCs during normal anaphase, we used live-cell imaging to quantify spindle behavior in a series of CPC mutants.