Spindle Pole Bodies Exploit the Mitotic Exit Network in Metaphase to Drive Their Age-Dependent Segregation

Abstract

Like many asymmetrically dividing cells, budding yeast segregates mitotic spindle poles nonrandomly between mother and daughter cells. During metaphase, the spindle positioning protein Kar9 accumulates asymmetrically, localizing specifically to astral microtubules emanating from the old spindle pole body (SPB) and driving its segregation to the bud. Here, we show that the SPB component Nud1/centriolin acts through the mitotic exit network (MEN) to specify asymmetric SPB inheritance. In the absence of MEN signaling, Kar9 asymmetry is unstable and its preference for the old SPB is disrupted. Consistent with this, phosphorylation of Kar9 by the MEN kinases Dbf2 and Dbf20 is not required to break Kar9 symmetry but is instead required to maintain stable association of Kar9 with the old SPB throughout metaphase. We propose that MEN signaling links Kar9 regulation to SPB identity through biasing and stabilizing the age-insensitive, cyclin-B-dependent mechanism of symmetry breaking.

Spatiotemporal analysis of organelle and macromolecular complex inheritance

“…New NPCs are assembled de novo using new nucleoporins. The SPB is a large and also stable multiprotein complex, yet, unlike the NPC, the already-assembled SPB allows incorporation of new proteins, an observation differing from previous descriptions (27,29). Pereira and coworkers (27) exploited the slow maturation of RFP to follow old Spc42, reasoning that new Spc42-RFP acquires fluorescence only during the next cell cycle.…”

“…One known exception to equal sharing between subsequent generations is the SPB-the centrosome in yeast (27)(28)(29). The SPB is a large protein complex embedded in the nuclear envelope (30).…”

Spindle Pole Body History Intrinsically Links Pole Identity with Asymmetric Fate in Budding Yeast

“…Furthermore, the respective impact of those modifications on Kar9-Bim1 complex formation, its dynamics, or Kar9 polarized localization remains unclear [34][35][36]. Recently, phosphorylation by a mitotic exit network (MEN) kinase somehow linked to the MEN-specific function of Nud1 [37] was implicated in Kar9 retention by the old SPB. An alternative cytoskeletal-centric proposal favors instead the possibility that asymmetric aMT organization links Kar9 bias to the old SPB.…”

Mitotic Exit and Separation of Mother and Daughter Cells

“…These include a possible role in orientation of the mitotic spindle in metaphase (Hotz et al 2012) and stabilization of the early mitotic transcripts CLB2 and SWI5 (Trcek et al 2011). In the latter case, a fraction of Dbf2 and Dbf20 associate with the mRNAs and suppress their degradation.…”