During M phase, Endosulfine (Endos) family proteins are phosphorylated by Greatwall kinase (Gwl), and the resultant pEndos inhibits the phosphatase PP2A-B55, which would otherwise prematurely reverse many CDK-driven phosphorylations. We show here that PP2A-B55 is the enzyme responsible for dephosphorylating pEndos during M phase exit. The kinetic parameters for PP2A-B55’s action on pEndos are orders of magnitude lower than those for CDK-phosphorylated substrates, suggesting a simple model for PP2A-B55 regulation that we call inhibition by unfair competition. As the name suggests, during M phase PP2A-B55’s attention is diverted to pEndos, which binds much more avidly and is dephosphorylated more slowly than other substrates. When Gwl is inactivated during the M phase-to-interphase transition, the dynamic balance changes: pEndos dephosphorylated by PP2A-B55 cannot be replaced, so the phosphatase can refocus its attention on CDK-phosphorylated substrates. This mechanism explains simultaneously how PP2A-B55 and Gwl together regulate pEndos, and how pEndos controls PP2A-B55.DOI: http://dx.doi.org/10.7554/eLife.01695.001

The atypical AGC kinase Greatwall (Gwl) mediates a pathway that prevents the precocious removal of phosphorylations added to target proteins by M phase-promoting factor (MPF); Gwl is thus essential for M phase entry and maintenance. Gwl itself is activated by M phase-specific phosphorylations that are investigated here. Many phosphorylations are nonessential, being located within a long nonconserved region, any part of which can be deleted without effect. Using mass spectrometry and mutagenesis, we have identified 3 phosphorylation sites (phosphosites) critical to Gwl activation (pT193, pT206, and pS883 in Xenopus laevis) located in evolutionarily conserved domains that differentiate Gwl from related kinases. We propose a model in which the initiating event for Gwl activation is phosphorylation by MPF of the proline-directed sites T193 and T206 in the presumptive activation loop. After this priming step, Gwl can intramolecularly phosphorylate its C-terminal tail at pS883; this site probably plays a role similar to that of the tail/Z motif of other AGC kinases. These events largely (but not completely) explain the full activation of Gwl at M phase.

Microtubule plus-end tracking proteins (+TIPs) are a diverse group of molecules that regulate microtubule dynamics and interactions of microtubules with other cellular structures. Many +TIPs have affinity for each other but the functional significance of these associations is unclear. Here we investigate the physical and functional interactions among three +TIPs in S. cerevisiae, Stu2, Bik1, and Bim1. Two-hybrid, coimmunoprecipitation, and in vitro binding assays demonstrate that they associate in all pairwise combinations, although the interaction between Stu2 and Bim1 may be indirect. Three-hybrid assays indicate that these proteins compete for binding to each other. Thus, Stu2, Bik1, and Bim1 interact physically but do not appear to be arranged in a single unique complex. We examined the functional interactions among pairs of proteins by comparing cytoplasmic and spindle microtubule dynamics in cells lacking either one or both proteins. On cytoplasmic microtubules, Stu2 and Bim1 act cooperatively to regulate dynamics in G1 but not in preanaphase, whereas Bik1 acts independently from Stu2 and Bim1. On kinetochore microtubules, Bik1 and Bim1 are redundant for regulating dynamics, whereas Stu2 acts independently from Bik1 and Bim1. These results indicate that interactions among +TIPS can play important roles in the regulation of microtubule dynamics.

In vertebrates, mitotic and meiotic M phase is facilitated by the kinase Greatwall (Gwl), which phosphorylates a conserved sequence in the effector Endosulfine (Endos). Phosphorylated Endos inactivates the phosphatase PP2A/B55 to stabilize M-phasespecific phosphorylations added to many proteins by cyclin-dependent kinases (CDKs). We show here that this module functions essentially identically in Drosophila melanogaster and is necessary for proper mitotic and meiotic cell division in a wide variety of tissues. Despite the importance and evolutionary conservation of this pathway between insects and vertebrates, it can be bypassed in at least two situations. First, heterozygosity for loss-of-function mutations of twins, which encodes the Drosophila B55 protein, suppresses the effects of endos or gwl mutations. Several types of cell division occur normally in twins heterozygotes in the complete absence of Endos or the near absence of Gwl. Second, this module is nonessential in the nematode Caenorhaditis elegans. The worm genome does not contain an obvious ortholog of gwl, although it encodes a single Endos protein with a surprisingly well-conserved Gwl target site. Deletion of this site from worm Endos has no obvious effects on cell divisions involved in viability or reproduction under normal laboratory conditions. In contrast to these situations, removal of one copy of twins does not completely bypass the requirement for endos or gwl for Drosophila female fertility, although reducing twins dosage reverses the meiotic maturation defects of hypomorphic gwl mutants. These results have interesting implications for the function and evolution of the mechanisms modulating removal of CDKdirected phosphorylations. ENTRY into M phase of the cell cycle is driven by the rapid activation of the kinase MPF (M phase promoting factor; Cdk1/cyclin B), which then phosphorylates hundreds of target proteins at proline-directed phosphosites (S/T P) (Dephoure et al. 2008;Lindqvist et al. 2009). These sites must remain phosphorylated during M phase, but then be dephosphorylated upon M-phase exit. It would make sense that the phosphatase(s) directed against mitotic phosphosites should be inhibited during M phase, because if not, these key phosphorylations would be prematurely removed, preventing cells from entering M phase.Recent findings support this hypothesis. During M phase in Saccharomyces cerevisiae, the phosphatase Cdc14p, whose activity against MPF targets is required for mitotic exit, is sequestered in the nucleolus from its substrates (reviewed in Mocciaro and Schiebel 2010). In Xenopus egg extracts or in

Bim1 promotes microtubule assembly in vitro, primarily by decreasing the frequency of catastrophes. In contrast, Bik1 inhibits microtubule assembly by slowing growth and, consequently, promoting catastrophes. These proteins interact to form a complex that affects microtubule dynamics in much the same way as Bim1 alone.