Nuclear Mitotic Apparatus Protein NuMA

NuMA, an abundant 240-kDa protein, binds to microtubules via its carboxyl terminal domain. During interphase, it is found in the nucleus, but during early mitosis it redistributes to the separating centrosomes. As a mitotic component it is essential for the organization and stabilization of spindle poles up to the onset of anaphase. The cell cycle-dependent distribution and function of NuMA is regulated by phos-phorylation (in mitosis) and dephosphorylation.

NuMA has been shown not only to bind S/MARs (Luderus et al. 1994), but also to be part of nuclear matrix core filaments (Zeng et al. 1994). Its predicted structural features are a globular head, tail domains and a central two-stranded a-helical rod similar to members of IF family, suggesting that NuMA can form filamentous structures via coiled-coil interactions. In line with this expectation an extensive filamentous network of interconnected 5-nm fibers could be demonstrated upon transient over-expression (Harborth et al.1999). Antibodies against NuMA label portions of the nuclear matrix, but not on the 10-nm filaments. Since the actin-related nuclear proteins myosin and structural protein 4.1 bind to NuMA, this interaction may link actin-related and NuMA-related nuclear structures.

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