Requirement for Nudel and dynein for assembly of the lamin B spindle matrix

Abstract

Guanosine triphosphatase Ran (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. We report here 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 independent of its function at kinetochores. Importantly, Nudel directly interacts 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 assembly of lamin B matrix. However, depleting lamin B still allowed 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.