The spindle pole body (SPB) is the microtubule organizing center in yeast cells, functionally equivalent to the centrosome.
The location of the inner and outer plaques in relation to the nuclear membranes is maintained during the entire cell cycle.
One side of the central plaque is associated with an electron-dense region of the nuclear envelope termed the half bridge.
The half-bridge is the site of new SPB assembly, and it also plays a role in cytoplasmic microtubule nucleation during G1 and karyogamy.
Two single-pass membrane proteins, Kar1p and Mps3p, localize to the half-bridge and are required to form and/or maintain the structure.
An additional half-bridge component, Sfi1p, shows ability to bind to Cdc31p through multiple conserved Cdc31-binding sites throughout its length.
Duplication of the SPB once, and only once, during each cell cycle is essential for formation of a bipolar mitotic spindle and accurate chromosome segregation.
In the same time satellite forms duplication plaque, a layered structure that is similar to the cytoplasmic half of a mature SPB.
SPB continues to grow until mitosis, so protein components are able to incorporate into both SPBs throughout the cell cycle.