What is a spindle pole body? The spindle pole body (SPB) or duplication plaque of fungi is the functional equivalent of the centrosome in higher eukaryotes. SPBs serve as microtubule organizing centres (MTOCs), sites where microtubules (MTs) are assembled from tubulin subunits. In contrast to centrosomes, SPBs do not contain centrioles. Instead, they are large, proteinaceous, multi-layered structures that are either continuously embedded in the nuclear envelope (NE) (budding yeast Saccharomyces cerevisiae) or become inserted into the NE before mitosis (fission yeast Schizzosaccharomyces pombe). Therefore, the SPB is able to organize two types of MTs, the nuclear and the cytoplasmic MTs with functions in chromosome segregation and nuclear positioning, respectively. Similar to centrosomes, SPBs duplicate conservatively and only once per cell cycle, producing a daughter next to the pre-existing mother SPB. SPBs also regulate late mitotic cell cycle events by forming an assembly platform of cell signalling networks like the mitotic exit network (MEN) or the septum initiation network (SIN) in budding and fission yeast, respectively.One of the best-studied MTOCs is the gigadalton SPB of S. cerevisiae (Figure 1Figure 1). It is composed of at least 18 different proteins. Electron microscopic analysis and interaction studies have generated a fine map of this compact organelle (Figure 1Figure 1). In haploid yeast cells the embedded SPB is a cylindrical organelle with a lateral diameter of around 100 nm. The SPB’s core components assemble into three main vertical layers or plaques, named outer, inner and central plaque. The central plaque spans the width of the SPB within the nuclear membrane, anchoring and interconnecting the outer and inner plaques. The central plaque contains a two-dimensional crystal of the SPB component Spc42. The outer layer faces the cytoplasm and organizes cytoplasmic MTs, whereas the inner plaque faces the nucleoplasm and organizes the nuclear MTs. The first and second intermediate layers are localized between the central and outer plaques.Figure 1Molecular composition and duplication cycle of the budding yeast SPB.Middle: a schematic representation of the SPB proteins and their location within the organelle. Nuclear and cytoplasmic microtubules (nMTs and cMTs) are illustrated and the distribution of their plus (+) and minus (–) ends is specified. Surrounding: the main steps of SPB duplication are shown. See text for details. IL 1, intermediate layer 1; IL 2, intermediate layer 2; cMT, cytoplasmic microtubules; nMT, nuclear microtubules.View Large Image | View Hi-Res Image | Download PowerPoint SlideAn important characteristic of the yeast SPB is an extension called the ‘half bridge’ that is important for SPB duplication. The half bridge protrusion is anchored between the central plaque and second intermediate layer, and forms a continuous coat on both sides of the NE.What are the functions of the SPB? The functions associated with SPBs are manifold. The major task of the SPB is the nucleation and anchorage of microtubules. The γ-tubulin complex, consisting of Spc97 (human GCP2), Spc98 (human GCP3) and Tub4 (γ-tubulin), is located at the inner and the outer plaque and enables the SPB to organize the two classes of MTs (Figure 1Figure 1). SPB-directed nucleation provides MT polarity with the MT minus ends at the SPB and the plus ends reaching into the cytoplasm and the nucleus.The SPB is also the loading site of MT-associated proteins. The protein Kar9 in complex with Bim1 (EB1 in mammalian cells), cyclin-dependent kinase (Cdk1) and the B-type cyclin Clb4 become loaded onto the SPB and then travel along cytoplasmic microtubules to the plus end where Cdk1–Clb4 regulates the interaction with the cortex of the mother cell.The SPB is also a site where signal transduction pathways are integrated. For example, the spindle positioning checkpoint (SPOC) surveys nuclear positioning during anaphase and blocks the MEN signalling cascade when the anaphase spindle is mis-positioned in the mother cell body. Components of the MEN interact with the conserved SPB core component Nud1, which functions as a scaffold for MEN pathway components. MEN inhibition prevents the transition from mitosis into G1 because the majority of the phosphatase Cdc14 remains entrapped in the nucleolus and so is unable to execute the dephosphorylation of Cdk1 substrates that drives mitotic exit. Consequently, mitotic Cdk1 activity remains high when MEN activity is compromised.The meiotic SPB is the site of pre-spore wall formation. Membranes are recruited to a modified meiotic outer plaque. This process resembles to a certain degree the assembly of membranes at basal bodies during cilia development.How is the SPB duplicated? To assemble a functional bipolar spindle and to ensure high accuracy of chromosome segregation, it is vital to restrict the duplication of the SPB to once per cell cycle. The initial step of this process takes place in early G1 phase, when the half bridge doubles its length and the so-called bridge structure is formed (Figure 1Figure 1). At its distal cytoplasmic tip, the satellite, a SPB precursor consisting mainly of cytoplasmic SPB components, assembles. The satellite expands into the duplication plaque, which reassembles the cytoplasmic part of the mature SPB. This layered structure is then inserted into the nuclear membrane. Once the insertion is completed, the nucleoplasmic SPB components, i.e. Spc110, Cmd1 (calmodulin) and the γ-tubulin complex, are recruited to the newly duplicated SPB forming the inner plaque. At the onset of S-phase, SPBs are separated by the cleavage of the bridge, leaving one half bridge with each SPB.What about SPBs and MTOCs in other organisms? Whereas baker’s yeast has only the SPB as its sole MTOC, the fission yeast S. pombe has three different MTOCs during its normal mitotic cell cycle. Besides SPBs, fission yeast forms at the end of mitosis equatorial MTOCs (eMTOCs) located at the later cell division site. In the subsequent interphase, MT growth occurs from the interphase MTOCs (iMTOCs). All of these MTOCs carry γ-tubulin. However, only the SPB duplicates in a cell cycle-dependent manner and contains γ-tubulin and centrin.Besides MT-dependent chromosome segregation, the S. pombe SPB has very similar functions to those of S. cerevisiae SPBs. For example, the S. pombe SPB regulates a late mitotic event, septation, through the equivalent of the MEN, the SIN. Furthermore, polo-like kinase Plo1 becomes activated at SPBs. This activation is part of the mechanism that regulates commitment to mitosis at the G2/M transition. Thus, like budding yeast’s SPB, the fission yeast’s SPB constitutes a platform where signalling pathways are integrated to generate a comprehensive cell cycle controlling machinery.MTOCs are also studied in other fungi, e.g. Aspergillus nidulans, Ustilago maydis and Ashbya gossypii. Much less is known about the composition of these MTOCs.Is the SPB equivalent to the centrosome? Although sharing no structural similarity, the yeast SPB is the functional equivalent of animal cells’ centrosomes. Whereas the centriole-less SPBs are situated in the NE during mitosis (fungi have mostly a closed mitosis) to organize the mitotic spindle, the centrosome, which consists of a pair of cylindrical centrioles surrounded by the pericentriolar material, is associated with the NE in interphase and then becomes released with nuclear envelope breakdown in early mitosis when the centrosome establishes the mitotic spindle. Despite these structural differences, a small set of components of the yeast SPB and the vertebrate centrosome are conserved, reflecting ancestral functions in duplication and MT organization.What can we learn from SPBs? SPBs have proven to be as good a model for addressing how a cell restricts the duplication of the MTOC to one event per cell cycle as they are for studying MT nucleation. Furthermore, SPBs explain to us how cells can fulfil many diverging processes with a limited set of protagonists.