- Spindle pole body
The spindle pole body (SPB) is the
microtubule organizing centerin 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.
SPB structure in "Saccharomyces cerevisiae"
molecular massof a diploid SPB, including microtubulesand microtubule associated proteins, is estimated to be 1–1.5 GDa whereas a core SPB is 0.3–0.5 GDa. The SPB is 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 it's 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 DNAcontent. A larger SPB diameter increases microtubule nucleation capacity of the SPB, which is important for chromosome segregation.
All SPB proteins can be divided into free groups: core components, half-bridge components and components needed for connection with NE. There is no noun 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-coilmotifs, that allow to form dimmers, 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 promoterelements 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.
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 dimmers 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 cycleis essential for formation of a bipolar mitotic spindleand 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 envelopeand 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, so protein components are able to incorporate into both SPBs throughout the cell cycle.
[http://arjournals.annualreviews.org/doi/abs/10.1146%2Fannurev.cellbio.20.022003.114106 Sue L. Jaspersen and Mark Winey THE BUDDING YEAST SPINDLE POLE BODY: Structure, Duplication, and Function]
[http://www.springerlink.com/content/vv9v8ar5qy58x14c/ Astrid Helfant Composition of the spindle pole body of Saccharomyces cerevisiae and the proteins involved in its duplication]
Wikimedia Foundation. 2010.
Look at other dictionaries:
pole — 1. One of the two points at the extremities of the axis of any organ or body. 2. Either of the two points on a sphere at the greatest distance from the equator. 3. One of the two points in a magnet or an electric battery or cell having extremes… … Medical dictionary
pole — pole1 poleless, adj. /pohl/, n., v., poled, poling. n. 1. a long, cylindrical, often slender piece of wood, metal, etc.: a telephone pole; a fishing pole. 2. Northeastern U.S. a long, tapering piece of wood or other material that extends from the … Universalium
pole — I. /poʊl / (say pohl) noun 1. a long, rounded, usually slender piece of wood, metal, etc. 2. the long tapering piece of wood extending from the front axle of a vehicle, between the animals drawing it. 3. Nautical a light spar. 4. a unit of length … Australian English dictionary
pole — I [[t]poʊl[/t]] n. v. poled, pol•ing 1) a long, cylindrical, often slender piece of wood, metal, etc 2) dial. a tapering piece of wood or other material that extends from the front axle of a vehicle between the animals drawing it 3) spo the… … From formal English to slang
pole plate — noun Etymology: pole (I) + plate 1. : a horizontal timber resting on the tie beams of a roof rather than on the wall and supporting the ends of the rafters compare plate 5a(1); see roof illustration 2. [ … Useful english dictionary
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… … Wikipedia
Centrosome — Schematic of typical animal cell, showing subcellular components. Organelles: (1) Nucleolus (2) Nucleus (3) Ribosomes (little dots) (4) Vesicle (5) Rough endoplasmic reticulum (ER) (6) Golgi apparatus (7) Cytoskeleton (8) Smooth ER (9)… … Wikipedia
Cell nucleus — HeLa cells stained for the cell nucleus DNA with the Blue Hoechst dye. The central and rightmost cell are in interphase, thus their entire nuclei are labeled. On the left, a cell is going through mitosis and its DNA has condensed ready for… … Wikipedia
Centriole — Schematic of centriole showing microtubule triplets Three dimensi … Wikipedia
Punto de control de la mitosis — La comprensión de cómo las células mantienen constante el número apropiado de cromosomas después de cada división celular es una cuestión fascinante. Un único error que dé lugar a células con más o menos cromosomas de los que deber tener (una… … Wikipedia Español