Involvement of the lamin rod domain in heterotypic lamin interactions important for nuclear organization

The nuclear lamina is a meshwork of intermediate-type filament proteins (la mins) that lines the inner nuclear membrane. The lamina is proposed to be a n important determinant of nuclear structure, but there has been little dir ect testing of this idea. To investigate lamina functions, we have characte rized a novel lamin B1 mutant lacking the middle similar to4/5 of its alpha -helical rod domain. Though retaining only lo heptads of the rod, this mut ant assembles into intermediate filament-like structures in vitro. When exp ressed in cultured cells, it concentrates in patches at the nuclear envelop e. Concurrently, endogenous lamins shift from a uniform to a patchy distrib ution and lose their complete colocalization, and nuclei become highly lobu lated. In vitro binding studies suggest that the internal rod region is imp ortant for heterotypic associations of lamin B1, which in turn are required for proper organization of the lamina, Accompanying the changes in lamina structure induced by expression of the mutant, nuclear pore complexes and i ntegral membrane proteins of the inner membrane cluster, principally at the patches of endogenous lamins, Considered together, these data indicate tha t lamins play a major role in organizing other proteins in the nuclear enve lope and in determining nuclear shape.