The nuclear lamina is a protein meshwork, typically 20 to 40 nm thick, composed of type v intermediate filament proteins called nuclear lamins are generally divided into two families. Lamin A is encoded by a gene that gives rise to four polypeptides by alternative splicing. Members of the lamin B family are the products of two distinct genes.
Lamin gene expression depends on the cell type and stage of development. All nuclei of higher eukaryotes, including early embryos, have a lamina that contains lamin B-family subunits, loss of which is lethal. Lamins A and C typically appear only later in development as cells begin to differentiate. This variation in lamina composition may affect patterns chromo-some organization, possibly contributing to different of gene expression.
Like other intermediate filament proteins, nuclear lam-ins have a central, rod-like domain that is largely a-helical. The basic building block of lamin assembly is an α-helical coiled- coil of two identical parallel polypeptides. Two large globular C-terminal domains protrude from one end. Lamin dimers self- associate end to end to form polymers. In some cases, these polymers grow as thick as 10-nmintermediate filaments.
The C-terminal globular domain contains a nuclear localization sequence that ensures the rapid import of newly synthesized lamin precursors into the nucleus through nuclear pores. Most lamin subunits acquire a hydrophobic posttranslational modification that targets them to the nuclear membrane. The modification involves the enzymatic addition of a hydrocarbon tail, a C15-isoprenoid group called farnesyl. The farnesyl group is added to a characteristic amino acid motif called the CaaX box at the carboxyl terminus of the protein. This motif was first recognized in the Ras proteins. Lamin subunits lacking a CaaX box form aggregates in the nuclear interior. Once at the nuclear membrane lamin A is processed by a specialized protease called FACE-1 that clips off the C-terminal 18 amino acids, removing the farnesyl group. The aaX residues are removed from B-type lamins, leaving a protein with the farnesyl group on its carboxyl terminal cysteine.
The assembled lamina appears to be tethered to the inner nuclear membrane by interactions with integral membrane proteins. The surface of the lamina facing the nuclear interior also interacts with the chromosomes. Thus, the lamina and its associated proteins not only may serve as a structural support for the nuclear envelope but also may influence chromosome distribution and function within the nucleus.
A diffuse network of lamins is spread throughout the nucleus of most cells, and local concentrations appear at particular times during the cell cycle. Intranuclear lamin A spots are most prominent in G1, whereas those with lamin Bare most prominent during S, when they colocalize with sites of DNA replication. Lamin B spots do not colocalize with lamin A spots. The role of these intranuclear concentrations of lamins is unknown, but they might be localized regions of nuclear matrix with roles in RNA transcription and DNA replication.