A Link between Aurora Kinase and Clp1/Cdc14 Regulation Uncovered by the Identification of a Fission Yeast Borealin-Like Protein

Abstract

The chromosomal passenger complex (CPC) regulates various events in cell division. This complex is composed of a catalytic subunit, Aurora B kinase, and three nonenzymatic subunits, INCENP, Survivin, and Borealin. Together, these four subunits interdependently regulate CPC function, and they are highly conserved among eukaryotes. However, a Borealin homologue has never been characterized in the fission yeast, Schizosaccharomyces pombe. Here, we isolate a previously uncharacterized S. pombe protein through association with the Cdc14 phosphatase homologue, Clp1/Flp1, and identify it as a Borealin-like member of the CPC. Nbl1 (novel Borealin-like 1) physically associates with known CPC components, affects the kinase activity and stability of the S. pombe Aurora B homologue, Ark1, colocalizes with known CPC subunits during mitosis, and shows sequence similarity to human Borealin. Further analysis of the Clp1-Nbl1 interaction indicates that Clp1 requires CPC activity for proper accumulation at the contractile ring (CR). Consistent with this, we describe negative genetic interactions between mutant alleles of CPC and CR components. Thus, this study characterizes a fission yeast Borealin homologue and reveals a previously unrecognized connection between the CPC and the process of cytokinesis in S. pombe. INTRODUCTIONTo ensure successful cell division, sister chromatids must segregate to opposite poles of dividing cells in mitosis and be partitioned into two new daughter cells during cytokinesis (Pines and Rieder, 2001). Highly intricate mechanisms control these processes, with various macromolecular complexes coordinating their activities such that the integrity of cell division is maintained. The chromosomal passenger complex (CPC), which is composed of a catalytic subunit, Aurora B kinase, and three nonenzymatic subunits, INCENP, Survivin, and Borealin, functions as one of these critical regulatory complexes. As its name implies, this complex travels on chromosomes to various sites during cell division such that it can execute specific tasks at distinct locations and times (Earnshaw and Bernat, 1991). These functions include, but are not limited to, chromosome condensation, stabilization of the mitotic spindle, correction of improper kinetochore-microtubule attachments, and regulation of cytokinesis (for reviews, see Vagnarelli and Earnshaw, 2004;Vader et al., 2006;Ruchaud et al., 2007).The four CPC subunits are highly interdependent for proper localization, activity, and stability (Ruchaud et al., 2007). Consistent with such interdependency, these subunits are widely conserved among eukaryotes (Ruchaud et al., 2007). However, it has proven difficult to identify Borealin homologues in yeasts. Though some have speculated that yeast Borealin homologues might not exist because of fusion of the Borealin and Survivin subunits into a single yeast Survivin homologue (Vader et al., 2006), recent evidence suggests that there are in fact yeast Borealin homologues (Nakajima et al., 2009). Nonetheless, a Borealin homologue in th...

Effects of phosphatase and proteasome inhibitors on Borealin phosphorylation and degradation

“…Nbl1, the fission yeast Borealin orthologue, interacts with the Cdc14 phosphatase Clp1/Flp1 (16). Although over-expression of Cdc14B could accelerate the dephosphorylation of Borealin, human cells lacking either Cdc14A or Cdc14B did not show an increase in the amount of phosphorylated Borealin.…”

“…Potential candidates for Borealin dephosphorylation include Cdc14A or B, as well as PP2A, all of which have been implicated in dephosphorylating mitotic substrates (915). Clp1, a Cdc14 homologue of fission yeast is a bona fide interacting partner of the CPC, which co-localizes with the chromosomal passengers during mitosis (16). We now use somatic human knock-out cells to show that loss of either Cdc14A or Cdc14B does not affect Borealin dephosphorylation.…”

Arabidopsisα Aurora Kinases Function in Formative Cell Division Plane Orientation

“…Vertebrates evolved two subclades of Aurora kinase, A and B, with separate functions and interacting proteins (Carmena et al, 2009). Yeasts contain a single Aurora kinase resembling B-type Aurora (Bohnert et al, 2009;Nakajima et al, 2009), while the Dictyostelium discoideum (DdAurora) and Asterina pectinifera (ApAurora) Aurora kinases have properties of both A-and B-type Auroras (Li et al, 2008;Abe et al, 2010). Arabidopsis, like vertebrates, also contains three Aurora kinases that can be clustered into two groups that do not appear to resemble the A and B type (Demidov et al, 2005;Kawabe et al, 2005), highlighting the diversification of Aurora kinases throughout the eukaryotic kingdom.…”

Condensin phosphorylated by the Aurora-B-like kinase Ark1 is continuously required until telophase in a mode distinct from Top2

“…We focused on the three phosphorylated sites in the N-terminal region of Cnd2 (the Barren-like subunit), all of which belonged to the Aurora-B kinase consensus sequence. S. pombe Ark1 is the catalytic subunit of an Aurora-B-like kinase complex [which also comprises the survivin-like protein Bir1 (Cut17), INCENP-like Pic1 and Borealin-like Nbl1] (Bohnert et al, 2009;Leverson et al, 2002;Petersen et al, 2001) that behaves like a chromosome passenger complex (CPC) (Ruchaud et al, 2007). We succeeded in detecting the Ark1-mediated phosphorylation of S52 in Cnd2 by using an analog-sensitive ark1 mutant strain.…”