Spindle pole body

The spindle pole body (SPB) is the microtubule organizing center in yeast cells, functionally equivalent to the centrosome. Unlike the centrosome the SPB does not contain centrioles. The SPB organises the microtubule cytoskeleton which plays many roles in the cell. It is important for organising the spindle and thus in cell division.

The molecular mass of a diploid SPB, including microtubules and microtubule associated proteins, is estimated to be 1–1.5 GDa whereas a core SPB is 0.3–0.5 GDa. The SPB is a cylindrical multilayer organelle. These layers are: an outer plaque (OP), which connects to the cytoplasmic microtubules (cMT); a first intermediate layer (IL1) and an electrondense second intermediate layer (IL2); an electrondense central plaque (CP), which is at the level of the nuclear envelope and is connected to it by hook-like structures, an ill-defined inner plaque (IP); and a layer of the inner plaque that contains capped nuclear microtubules (nMT) ends. The central plaque and IL2 appeared as distinct but highly ordered layers. The other layers (MT ends, IP, IL1, and OP) do not show ordered packing. 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 SPB has constant height size (the inner plaque to outer plaque distance) for about 150 nm, but its diameter changes during cell cycle, e.g. in haploid cells, the SPB grows in diameter from 80 nm in G1 to 110 nm in mitosis. The SPB diameter depends on DNA content. A larger SPB diameter increases microtubule nucleation capacity of the SPB, which is important for chromosome segregation.

All SPB proteins can be divided into three groups: core components, half-bridge components and components needed for connection with NE. There is no known motif or structure, that makes a protein belong to SPB, but analysis of known SPB proteins and their genes shows several common features. The core contains mostly proteins with coiled-coil motifs, that allow to form dimers, either with themselves or with others proteins and maintain regular structures (e.g. CP, IL2). Many SPB genes contain MluI cell cycle boxes in their promoter elements that lead to G1 specific gene transcription. The primary sequence of SPB components should contain consensus phosphorylation sites for mitotic kinases, because the SPB is highly phosphorylated.

...
Wikipedia