The spindle pole body (SPB) is the
microtubule organizing center The microtubule-organizing center (MTOC) is a structure found in eukaryotic cells from which microtubules emerge. MTOCs have two main functions: the organization of eukaryotic flagella and cilia and the organization of the mitotic and meiotic spind ...
in yeast cells, functionally equivalent to the
centrosome
In cell biology, the centrosome (Latin centrum 'center' + Greek sōma 'body') (archaically cytocentre) is an organelle that serves as the main microtubule organizing center (MTOC) of the animal cell, as well as a regulator of cell-cycle prog ...
. 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.
SPB structure in ''Saccharomyces cerevisiae''
The
molecular mass
The molecular mass (''m'') is the mass of a given molecule: it is measured in daltons (Da or u). Different molecules of the same compound may have different molecular masses because they contain different isotopes of an element. The related quanti ...
of a diploid SPB, including
microtubules
Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nm and have an inner diameter between 11 an ...
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
In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as organs are to the body, hence ''organelle,'' the ...
. 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
The cell cycle, or cell-division cycle, is the series of events that take place in a cell that cause it to divide into two daughter cells. These events include the duplication of its DNA (DNA replication) and some of its organelles, and subs ...
. 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
In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is mainta ...
. 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
A coiled coil is a structural motif in proteins in which 2–7
alpha helix, alpha-helices are coiled together like the strands of a rope. (Protein dimer, Dimers and Protein trimer, trimers are the most common types.) Many coiled coil-type protei ...
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 ''Mlu''I 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
In biochemistry, a kinase () is an enzyme that catalyzes the transfer of phosphate groups from high-energy, phosphate-donating molecules to specific substrates. This process is known as phosphorylation, where the high-energy ATP molecule don ...
, because the SPB is highly phosphorylated.
The main central plaque component is coiled-coil protein Spc42p (for spindle pole body component) also found to be a part of satellite, that forms a core crystal of SPB. The Spc42p protein is involved in initiation of SPB assembly and its duplication.
The Spc42p associates with Spc110p and Spc29p, two other essential coiled-coil proteins that localize to the nuclear face of the SPB. Spc110 localizes to the central plaque and is thought to bind to Spc29p and calmodulin (Cmd1p). The role of Spc110p is a spacer molecule between the central and inner plaque and γ-tubilin complex binding protein. The essential function of calmodulin is at the SPB where it has been proposed to regulate binding of Spc110p to Spc29p. Spc29 forms in the central plaque a repeating structure. Spc98p and Spc97p are two similar yeast γ –tubulin (Tub4p) binding proteins required for microtubule nucleation. Spc98p, Spc97p and Tub4p are found at the inner and outer plaques of SPB and are involved in microtubules organization. Spc42 faces the cytoplasm and binds to coiled-coil Cnm67p (chaotic nuclear migration). Cnm67p forms dimers and functions as a spacer between IL2 and IL1. Cnm67 binds to the outer plaque protein Nud1p, a SPB protein required for exit from mitosis. Another coiled-coil protein, Spc72p, is also found in the outer plaque. Spc72p associates with Nud1p and to components of the γ-tubulin complex.
The half-bridge is the site of new SPB assembly, and it also plays a role in cytoplasmic microtubule nucleation during G1 and karyogamy. Both sides of the half-bridge are not equivalent. Two single-pass membrane proteins, Kar1p and Mps3p, localize to the half-bridge and are required to form and/or maintain the structure. Both proteins bind to Cdc31p, the yeast centrin homolog, which also localizes to the half-bridge and is required for half-bridge integrity. An additional half-bridge component, Sfi1p, shows ability to bind to Cdc31p through multiple conserved Cdc31-binding sites throughout its length. Kar1p is also involved in connecting the half-bridge to the core SPB via its interaction with Bbp1p. In addition, Kar1p plays a role in reorganization of the SPB during G1.
SPB duplication pathway in ''Saccharomyces cerevisiae''
Duplication of the SPB once, and only once, during each
cell cycle
The cell cycle, or cell-division cycle, is the series of events that take place in a cell that cause it to divide into two daughter cells. These events include the duplication of its DNA (DNA replication) and some of its organelles, and subs ...
is essential for formation of a bipolar
mitotic spindle
In cell biology, the spindle apparatus refers to the cytoskeletal structure of eukaryotic cells that forms during cell division to separate sister chromatids between daughter cells. It is referred to as the mitotic spindle during mitosis, a pr ...
and accurate chromosome segregation. SPB duplication in ''S. cerevisiae'' can be divided into several steps. The first step occurs early in
G1, when satellite material forms on cytoplasmic tip of half-bridge. During the second step half-bridge elongates and completes its nuclear and cytoplasmic faces fusion. In the same time satellite forms duplication plaque, a layered structure that is similar to the cytoplasmic half of a mature SPB. The last step of SPB duplication is insertion of the duplication plaque into the
nuclear envelope and assembly of nuclear SPB components. At the end of G1 yeast cells contain two duplicated side-by-side SPBs connected by a complete bridge. Then bridge separates and SPB nucleates bipolar spindle. SPB continues to grow until
mitosis
In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is mainta ...
, so protein components are able to incorporate into both SPBs throughout the cell cycle.
References
External links
Sue L. Jaspersen and Mark Winey THE BUDDING YEAST SPINDLE POLE BODY: Structure, Duplication, and FunctionAstrid Helfant Composition of the spindle pole body of Saccharomyces cerevisiae and the proteins involved in its duplication{{Organelles
Cytoskeleton
Organelles