THE SPINDLE POLE BODY OF SCHIZOSACCHAROMYCES-POMBE ENTERS AND LEAVES THE NUCLEAR-ENVELOPE AS THE CELL-CYCLE PROCEEDS

The cycle of spindle pole body (SPB) duplication, differentiation, and segregation in Schizosaccharomyces pombe is different from that in so me other yeasts. Like the centrosome of vertebrate cells, the SPB of S . pombe spends most of interphase in the cytoplasm, immediately next t o the nuclear envelope. Some gamma-tubulin is localized on the SPB, su ggesting that it plays a role in the organization of interphase microt ubules (MTs), and serial sections demonstrate that some interphase MTs end on or very near to the SPB. gamma-Tubulin is also found on osmiop hilic material that lies near the inner surface of the nuclear envelop e, immediately adjacent to the SPB, even though there are no MTs in th e interphase nucleus. Apparently, the MT initiation activities of gamm a-tubulin in S. pombe are regulated. The SPB duplicates in the cytopla sm during late G(2) phase, and the two resulting structures are connec ted by a darkly staining bridge until the mitotic spindle forms. As th e cell enters mitosis, the nuclear envelope invaginates beside the SPB , forming a pocket of cytoplasm that accumulates dark amorphous materi al. The nuclear envelope then opens to form a fenestra, and the duplic ated 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 the ir fenestrae, bound to the polar ends of spindle MTs; at about this ti me, a small bundle of cytoplasmic MTs forms in association with each S PB. 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 clos e, and the SPBs are extruded back into the cytoplasm. These observatio ns define new fields of enquiry about the control of SPB duplication a nd the dynamics of the nuclear envelope.