The spindle pole body of Schizosaccharomyces pombe enters and leaves the nuclear envelope as the cell cycle proceeds.

Abstract

The cycle of spindle pole body (SPB) duplication, differentiation, and segregation in Schizosaccharomyces pombe is different from that in some other yeasts. Like the centrosome of vertebrate cells, the SPB of S. pombe spends most of interphase in the cytoplasm, immediately next to the nuclear envelope. Some y-tubulin is localized on the SPB, suggesting that it plays a role in the organization of interphase microtubules (MTs), and serial sections demonstrate that some interphase MTs end on or very near to the SPB. y-Tubulin is also found on osmiophilic material that lies near the inner surface of the nuclear envelope, immediately adjacent to the SPB, even though there are no MTs in the interphase nucleus. Apparently, the MT initiation activities of y-tubulin in S. pombe are regulated. The SPB duplicates in the cytoplasm during late G2 phase, and the two resulting structures are connected by a darkly staining bridge until the mitotic spindle forms. As the cell enters mitosis, the nuclear envelope invaginates beside the SPB, forming a pocket of cytoplasm that accumulates dark amorphous material. The nuclear envelope then opens to form a fenestra, and the duplicated SPB settles into it. Each part of the SPB initiates intranuclear MTs, and then the two structures separate to lie in distinct fenestrae as a bipolar spindle forms. Through metaphase, the SPBs remain in their fenestrae, bound to the polar ends of spindle MTs; at about this time, a small bundle of cytoplasmic MTs forms in association with each SPB. These MTs are situated with one end near to, but not on, the SPBs, and they project into the cytoplasm at an orientation that is oblique to the spindle axis. As anaphase proceeds, the nuclear fenestrae close, and the SPBs are extruded back into the cytoplasm. These observations define new fields of enquiry about the control of SPB duplication and the dynamics of the nuclear envelope.

cut11+: A Gene Required for Cell Cycle-dependent Spindle Pole Body Anchoring in the Nuclear Envelope and Bipolar Spindle Formation inSchizosaccharomyces pombe

“…The ratio of cells with and without spots was similar to the ratio of interphase and mitotic cells in a log phase culture of fission yeast (Mitchison and Nurse, 1985). The predominance of two spots versus one spot is consistent with the association of unseparated and separated mitotic SPBs within the nuclear envelope (Ding et al, 1997). Therefore, the bright spot(s) were interpreted as mitotic spindle pole bodies.…”

“…The mitotic stages described for images in Figure 9 are inferred from the configuration of the SPBs and the nuclear envelope at specific times as previously reported (Ding et al, 1993(Ding et al, , 1997. These stages of mitosis correspond to the time when the SPB is anchored in the nuclear envelope (Ding et al, 1997).Cut11p has the same localization pattern in meiotic cells as that described for vegetative growth. cut11: GFP cells of the opposite mating type were induced to mate by starving for nitrogen on malt agar plates (Moreno et al, 1991) and were observed at various stages of the meiotic life cycle.…”

“…Serial sections of this sample revealed one SPB complex that was duplicated but unseparated ( Figure 5A, a and b) and an additional SPB lying beside it ( Figure 5A, c). The duplicated SPBs look indistinguishable from wild-type SPBs (Ding et al, 1997), but the unduplicated SPB shows no evidence of a bridge structure on any portion of its surface. The position of these SPBs relative to each other and to the nuclear envelope is diagrammed in Figure 5A, d.…”

“…Although these two organelles are structurally distinct, genetic and biochemical approaches have identified several common components of centrosomes and SPBs, including ␥-tubulin (reviewed in Kellogg et al, 1994), CDC31/centrin (reviewed in Schiebel and Bornes, 1995), and p34 cdc2 (Bailly et al, 1989; Raibowol et al, 1989). Thus, analyses of SPBs have been informative about centrosomes in general.Recent work has demonstrated that the SPB of Schizosaccharomyces pombe is a dynamic organelle, undergoing significant changes in morphology and cellular localization as cells progress through their growth and division cycle (Ding et al, 1997). The nature of these changes distinguishes the fission yeast centrosome from that of other organisms.…”

Shape Transformation of the Nuclear Envelope during Closed Mitosis

“…It is widely known that such shape evolution is driven by the poleward force generated by kinesins along with other microtubule-associated proteins that reside at the overlapping region of antiparallel spindle microtubules (MTs) nucleated from the two spindle pole bodies (SPBs) at both ends of a dividing nucleus (5)(6)(7)(8). Throughout interphase, there is only one SPB in the cytoplasm that starts to duplicate in G1/S phase, resulting in two SPBs embedded in the NE during mitosis (9). Besides generating forces, spindle MTs are also responsible for moving sister chromatids toward the opposite poles through capturing the kinetochore, a large proteinaceous complex on chromosomes (10).…”