Duplication of the Yeast Spindle Pole Body Once per Cell Cycle

Abstract

bThe yeast spindle pole body (SPB) is the functional equivalent of the mammalian centrosome. Centrosomes and SPBs duplicate exactly once per cell cycle by mechanisms that use the mother structure as a platform for the assembly of the daughter. The conserved Sfi1 and centrin proteins are essential components of the SPB duplication process. Sfi1 is an elongated molecule that has, in its center, 20 to 23 binding sites for the Ca 2؉ -binding protein centrin. In the yeast Saccharomyces cerevisiae, all Sfi1 N termini are in contact with the mother SPB whereas the free C termini are distal to it. During S phase and early mitosis, cyclin-dependent kinase 1 (Cdk1) phosphorylation of mainly serine residues in the Sfi1 C termini blocks the initiation of SPB duplication ("off" state). Upon anaphase onset, the phosphatase Cdc14 dephosphorylates Sfi1 ("on" state) to promote antiparallel and shifted incorporation of cytoplasmic Sfi1 molecules into the half-bridge layer, which thereby elongates into the bridge. The Sfi1 C termini of the two Sfi1 layers localize in the bridge center, whereas the N termini of the newly assembled Sfi1 molecules are distal to the mother SPB. These free Sfi1 N termini then assemble the new SPB in G 1 phase. Recruitment of Sfi1 molecules into the anaphase SPB and bridge formation were also observed in Schizosaccharomyces pombe, suggesting that the Sfi1 bridge cycle is conserved between the two organisms. Thus, restricting SPB duplication to one event per cell cycle requires only an oscillation between Cdk1 kinase and Cdc14 phosphatase activities. This clockwork regulates the "on"/"off" state of the Sfi1-centrin receiver.T he mammalian centrosome and the yeast spindle pole body (SPB) are both able to nucleate microtubules (MTs) from tubulin subunits and are therefore collectively named MT organizing centers (MTOCs). A second common feature is the duplication of the centrosome and SPB just once during each cell division cycle. In each case, it is the mother centrosome or SPB that provides the platform for the assembly of the daughter (1-3).In yeast, the SPB is absolutely critical for mitotic spindle formation and chromosome segregation. This is because alternative MT assembly pathways, such as the RCC1/Ran-GTP-or augmindependent MT nucleation pathways, are absent from yeast (4, 5). In contrast, human cells can assemble a mitotic spindle even in the absence of centrosomes because of centrosome-independent MT formation and the presence of spindle assembly pathways. Recent studies have shown that the ability of human cells to tolerate loss of centrosomes or centrosome overduplication is reliant upon the inactivation of the p53 tumor suppressor gene that otherwise arrests these abnormal cells in G 1 phase (6, 7). How cells sense centrosome number defects is currently not understood. Yet it is becoming clearer why such a surveillance mechanism is important. Centrosome overamplification may contribute to cell transformation or enhance the aggressive nature of already transformed cells (8, 9). It is therefore...

TheToxoplasmacentrocone houses cell cycle regulatory factors

“…The presence of multiple microtubule organizing centers in apicomplexan parasites (MTOCs; centrosome/centrocone and apical complex)(30, 31) sets these parasites apart from well studied eukaryotic models (yeast and mammalian cells) that have a single MTOC during interphase (32).…”

Comparative analysis of Synthetic Physical Interactions with the yeast centrosome

“…Further work is required to distinguish these models, in particular, assessment of the foci for presence of other SPB proteins and functionality of the foci as MTOCs is required to confirm them as real SPBs. If SPBs are created de novo we would expect that these strains would lose the requirement for proteins with an essential role in SPB duplication, 452 such as Cdc31 [Rüthnick and Schiebel, 2016]. Many mutants have been identified that fail to duplicate their SPBs, for example the original MPS (Mono-Polar Spindle) genes [Winey et al, 1991] however there are fewer cases of genetic perturbations that lead to SPB overduplication.…”

An ancestral role of pericentrin in centriole formation through SAS-6 recruitment

“…Instead of a canonical centrosome, yeasts have a spindle pole body (SPB), a layered structure composed of a centriole-less scaffold that similarly recruits γ-tubulin and nucleates microtubules (Cavanaugh and Jaspersen, 2017). The timing and regulation of SPB biogenesis are similar to the one observed in animal centrosomes (Lim et al, 2009;Kilmartin, 2014;Ruthnick and Schiebel, 2016). It is likely that the animal centrosome and yeast SPB evolved from a common ancestral structure that had centrioles ( Figure 1A), as early-diverged basal fungi such as chytrids (e.g.…”