Nuclear Lamins: Thin Filaments with Major Functions

The nuclear lamina is a nuclear peripheral meshwork that is mainly composed of nuclear lamins, although a small fraction of lamins also localizes throughout the nucleoplasm. Lamins are classified as type V intermediate filament (IF) proteins. Mutations in lamin genes cause at least 15 distinct human diseases, collectively termed laminopathies, including muscle, metabolic, and neuronal diseases, and can cause accelerated aging. Most of these mutations are in the LMNA gene encoding A-type lamins. A growing number of nuclear proteins are known to bind lamins and are implicated in both nuclear and cytoskeletal organization, mechanical stability, chromatin organization, signaling, gene regulation, genome stability, and cell differentiation. Recent studies reveal the organization of the lamin filament meshwork in somatic cells where they assemble as tetramers in cross-section of the filaments. Lamins are nuclear IFs that make a meshwork of filaments at the nuclear periphery. Each major lamin isoform forms a separate meshwork. The lamin filaments are organized in somatic cells as protofilaments with a diameter of 3.5 nm in mammalian cells and 4–6 nm in C. elegans. Mutations in lamin A and B1 cause numerous laminopathies affecting muscle, adipose, nerve, bone, and skin, and can also cause early-aging diseases. These mutations can modify lamin filament organization and nuclear mechanical properties. Lamin filament organization in vitro is different from that observed in vivo, which is probably due to lamin-binding proteins and lamin post-translational modifications. Lamin-binding proteins are also involved in mediating lamin functions such as signaling, cell-cycle regulation, and chromatin organization.