Nuclear lamina

The nuclear lamina is a dense (~30 to 100 nm thick) fibrillar network inside the nucleus of most cells. It is composed of intermediate filaments and membrane associated proteins. Besides providing mechanical support, the nuclear lamina regulates important cellular events such as DNA replication and cell division. Additionally, it participates in chromatin organization and it anchors the nuclear pore complexes embedded in the nuclear envelope.

The nuclear lamina is associated with the inner face of the double bilayer nuclear envelope, whereas the outer face is continuous with the endoplasmic reticulum. The nuclear lamina is similar in structure to the nuclear matrix, but the latter extends throughout the nucleoplasm.

The nuclear lamina consists of two components, lamins and nuclear lamin-associated membrane proteins. The lamins are type V intermediate filaments which can be categorized as either A-type (lamin A, C) or B-type(lamin B₁, B₂) according to homology of their DNA sequences, biochemical properties and cellular localization during the cell cycle. Type V intermediate filaments differ from cytoplasmic intermediate filaments in the way that they have an extended rod domain (42 amino acid longer), that they all carry a nuclear localization signal (NLS) at their C-terminus and that they display typical tertiary structures. Lamin polypeptides have an almost complete α-helical conformation with multiple α-helical domains separated by non-α-helical linkers that are highly conserved in length and amino acid sequence. Both the C-terminus and the N-terminus are non α-helical, with the C-terminus displaying a globular structure. Their molecular weight ranges from 60 to 80 kilodaltons (kDa). In the amino acid sequence of a nuclear lamin, there are also two phosphoacceptor sites present, flanking the central rod domain. A phosphorylation event at the onset of mitosis leads to a conformational change which causes the disassembly of the nuclear lamina. (discussed later in the article)

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