The small guanosine triphosphatase Ran loaded with GTP (RanGTP) can stimulate assembly of the type V intermediate filament protein lamin B into a membranous lamin B spindle matrix, which is required for proper microtubule organization during spindle assembly. Microtubules in turn enhance assembly of the matrix. Here we report that the isolated matrix contains known spindle assembly factors such as dynein and Nudel. Using spindle assembly assays in Xenopus egg extracts, we show that Nudel regulates microtubule organization during spindle assembly independently of its function at kinetochores. Importantly, Nudel interacts directly with lamin B to facilitate the accumulation and assembly of lamin-B-containing matrix on microtubules in a dynein-dependent manner. Perturbing either Nudel or dynein inhibited the assembly of lamin B matrix. However, depleting lamin B still allowed the formation of matrices containing dynein and Nudel. Therefore, dynein and Nudel regulate assembly of the lamin B matrix. Interestingly, we found that whereas depleting lamin B resulted in disorganized spindle and spindle poles, disrupting the function of Nudel or dynein caused a complete lack of spindle pole focusing. We suggest that Nudel regulates microtubule organization in part by facilitating assembly of the lamin B spindle matrix in a dynein-dependent manner.
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