The spindle matrix

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Several studies have suggested presence of a stable non-microtubule structure, providing a scaffold for microtubule network and supporting chromosome movement in anaphase, called the spindle matrix (Pickett-Heaps and Forer, 2009; Zheng 2010; Yao et al., 2012). Investigators of the spindle matrix believe that a supportive mechanism in chromosome segregation to microtubule-based would be evolutionary preferred (Pickett-Heaps and Forer, 2009). The main evidence for presence of the matrix was provided by localization of the component of the presumptive matrix, Skeletor, upon removal of spindle microtubules (Walker et al., 2000). Skeletor showed localization coincident with the Drosophila mitotic spindle before microtubule depolymerization and retained the spindle shape after the treatment. Nevertheless, the nature and the exact role of the spindle matrix are unknown.

Among the proposed components of the matrix are Megator and Chromator, which showed similar localization to Skeletor in Drosophila mitotic cells (Qi et al., 2004; Rath et al., 2004). Titin has also been proposed to be a component of the spindle matrix in insect spermatocytes (Fabian et al., 2007). Actin and myosin have been suggested to support motility of chromosomes in anaphase (Pickett-Heaps and Forer, 2009).

Lamin B, a component of the nuclear envelope, has been identified to localize and behave as a component of the spindle matrix in Xenopus (see 1.2.2.; Tsai et al., 2006). Disruption of Lamin B function results in failure in proper spindle assembly. Lamin B function in spindle assembly is regulated by RanGTP (see 1.1.2.2.). Microtubule depolymerization does not affect lamin B localization. Interestingly, other RanGTP-dependent spindle assembly factors (SAFs), such as XMAP215, NuMa and Eg5 are found in the lamin B-containing matrix.