Centrosomes

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Formation of an ordered structure of the spindle depends on the presence of microtubule organizing centers (MTOC). Centrosomes and their functional equivalent in yeast, spindle pole body (SPB), are the first structures identified as important MTOCs. Centrosomes were first observed by Walther Flemming over a century ago and described by Theodor Boveri as structures positioned at the geometrical center of the interphase cell and at the poles of the mitotic spindle. Centrosomes nucleate microtubules and organize them into arrays. This organization has a specific role in interphase and in mitosis (see below, 1.1.1.1. and 1.1.3.3.; Bettencourt-Dias and Glover, 2007). Centrosomes had long been thought of as the crucial spindle MTOCs in mitosis due to their distinctive position and structure. This view has recently been challenged (see 1.1.2.2.; Basto et al., 2006). Nevertheless, centrosomes are important and dominant MTOCs in mitosis. Increased number of centrosomes has been related to many cancers (Nigg, 2002; Silkworth et al., 2009).

Centrosomes are composed of two cylindrical structures, called the centrioles, surrounded by electron-dense pericentriolar material, PCM (Bettencourt-Dias and Glover, 2007). Centrioles are short cylinders oriented perpendicularly to each other. They consist of nine triplet microtubules arranged radially. While centrioles function as a basal body for cilia and flagella formation is known, their exact function in microtubule organization is not clear. Centrioles have been proposed to recruit PCM (Bornens, 2002).

It is the PCM that is responsible for nucleation and arrangement of microtubules into arrays in the centrosome (Bornens, 2002). Pericentrin and proteins belonging to AKAP450 family are the components of PCM. Supposedly, they form a lattice-like structure which functions as a docking site for other PCM components and proteins responsible for microtubule nucleation, such as γ-tubulin (Takahashi et al., 2002; Dictenberg et al., 1998; Zimmerman et al., 2004; Zheng et al., 1995).

γ-tubulin is a key component of MTOCs. It is a highly conserved protein, which acts as a template for polymerization of αβ-tubulin dimers (Oakley and Oakley, 1989; Horio et al, 1991; Joshi et al, 1992; Teixido-Travesa et al., 2012). γ-tubulin functions in a complex with proteins called the gamma tubulin complex proteins, GCP (Gunawardane et al., 2000; Murphy et al., 2001; Teixido-Travesa et al., 2012). Two molecules of γ-tubulin associate into a tetrameric complex with GCP2 and GCP3, collectively known as the γ-tubulin small complex, γTuSC (Oegema et al., 1999, Kollman et al., 2010). Several γTuSCs further assemble into a ring held by additional GCP proteins, together called the γ-tubulin ring complex, γTuRC (Moritz et al., 1995; Oegema et al., 1999; Zheng et al., 1995; Murphy et al., 2001). While γTuSC is essential and sufficient for microtubule nucleation, γTuRC components seem to increase the efficiency of this process (Verollet et al., 2006). γTuSCs and γTuRCs are found also in the cytoplasm, unassociated with centrosomes (see 1.1.2.3.; Lüders and Stearns, 2007).

Nucleation of microtubules by centrosomes is regulated by Polo-like kinase-1 and Aurora A, as well as phophatases, such as PP1 (protein phosphatase-1), PP4 and others (Bladgen and Glover, 2003).

Organization of microtubule arrays by centrosomes involves stabilization of microtubule minus ends (see 1.1.1.1.) and anchoring microtubules at the centrosomes. Several anchoring mechanisms have been proposed in which, both, centrioles and PCM play a role (Bornens, 2002). Proteins crucial for anchorage include ninein, dynactin and EB1 (Mogensen et al., 2000; Askham et al., 2002). In addition to the role in nucleation, the γTuRC also prevents microtubule depolymerization by capping the minus ends (Wiese and Zheng, 2000; Anders and Sawin, 2011).

The arrangement of a microtubule array by centrosomes is important for the spindle assembly in mitosis (see 1.1.3.3.). Centrosomes duplicate in S phase and split before NEB (Bettencourt-Dias and Glover, 2007; Tanenbaum and Medema, 2010). Centrosomes nucleate microtubules and simultaneously migrate to the opposite sides of the nucleus as a result of antiparallel microtubule sliding and cortical forces. This results in formation of two radial arrays (Fig.3.). Position of centrosomes at the centers of the asters pre-determines the spindle bipolarity. Microtubules radiating from centrosomes are highly dynamic. They explore the cytoplasm by growing in random directions, shrinking and regrowing (see 1.1.1.1.). Microtubules become stabilized after attachment to the kinetochore (see 1.1.3.3.). Kirschner and Mitchison (1986) proposed that, with time, more microtubules attach to kinetochores and the number of astral microtubules decreases proportionally. This way the two arrays become a fusiform structure of a spindle.