SPBs

Spindle pole bodies (SPBs) are the principal microtubule organizing centers (MTOCs) of many fungi, including budding and fission yeasts. They are embedded in the nuclear envelope, with parts facing the nucleoplasm and the cytoplasm, and they organize the assembly and anchoring of microtubules needed for chromosome segregation during cell division. Unlike animal centrosomes, SPBs remain within intact nuclei during a closed mitosis characteristic of many fungi.

The SPB is a multilayer structure consisting of a central plaque and inner and outer plaques that face the nucleoplasm and cytoplasm, respectively. Core components include Spc42, which forms a scaffold at the central plaque, and Spc110, which helps recruit the gamma-tubulin ring complex to nucleate microtubules. Additional factors such as Nud1, Mps2, and Mps3 participate in linking the SPB to the nuclear envelope and organizing cytoplasmic microtubules. Duplication of the SPB involves a set of proteins including Cdc31 (centrin) and Sfi1, with Kar1 also contributing to the assembly of a new SPB adjacent to the old one.

Duplication occurs once per cell cycle and is followed by maturation and insertion of the new SPB into the nuclear envelope to establish a bipolar spindle. During mitosis in yeast, the spindle forms and chromosomes segregate within the nucleus without breakdown of the nuclear envelope, a process in which SPBs serve as the primary organizing centers for both nuclear and, via cytoplasmic connections, astral microtubules.

SPBs are characteristic of fungi and represent a distinct class of MTOCs compared with centrosomes in animals. They have been central to studies on MTOC assembly, duplication, and spindle dynamics, contributing broadly to understanding of eukaryotic cell division.