Lamin B1 Coordinates the Assembly of the Nuclear Lamina.

The nuclear lamina provides essential mechanical stability to protect the genome, yet how lamins target the nuclear rim and assemble into an organized meshwork remains unclear. To address this, we generated smooth muscle cells lacking all three lamin genes ("LKO cells") and expressed individual lamins at physiological levels. All lamins localized to the nuclear rim, but with distinct requirements: lamin B1 and lamin B2 required farnesylation, whereas prelamin A did not. Lamin C and mature lamin A also localized efficiently, demonstrating that C-terminal processing is not required for A-type lamin targeting. Despite correct targeting, lamins differed in their ability to form organized meshworks. Lamin B1 alone generated a normal fibrillar network, whereas lamin A, lamin C, and lamin B2 formed disorganized structures with large gaps. Co-expression of lamin A and lamin C did not rescue this defect. In contrast, co-expression with lamin B1 restored proper meshwork organization for all lamins and was essential for nuclear lamina assembly during cell division. These findings identify lamin B1 as the central organizer of nuclear lamina architecture.