Nuclear Lamina - Structure and Composition

Structure and Composition

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 in sequence, 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 nuclear lamins, 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)

In the vertebrate genome, lamins are encoded by three genes. By alternative splicing, at least seven different polypeptides (splice variants) are obtained, some of which are specific for germ cells and play an important role in the chromatin reorganisation during meiosis. Not all organisms have the same number of lamin encoding genes; Drosophila melanogaster for example has only 2 genes, whereas Caenorhabditis elegans has only one. The presence of lamin polypeptides is an exclusive property of Metazoan organisms. Plants or single-cell Eukaryotic organisms such as Saccharomyces cerevisiae lack lamins.

The nuclear lamin-associated membrane proteins are either integral or peripheral membrane proteins. The most important are lamin associated polypeptide 1 and 2 (LAP₁, LAP2), emerin, lamin B-receptor (LBR), otefin and MAN₁. Due to their positioning within or their association with the inner membrane, they mediate the attachment of the nuclear lamina to the nuclear envelope.