In eukaryotic cells, the nuclear envelope consists
of inner and outer membranes which define the border between the nucleus
and cytoplasm.
The inner and outer nuclear membranes are joined by large, supramolecular
assemblies called nuclear pore complexes through which various molecules
pass into and out of the nucleus. The inner membrane is lined by a filamentous
meshwork called the nuclear lamina made up of proteins called lamins.
The attachment of the lamins to the inner nuclear membrane appears to
occur through their association with 4 related integral membrane proteins
called Lamin Associated Polypeptides (LAPs). LAP 1A, 1B & 1C share
a high degree of sequence homology while LAP 2 is distinct from the others.
LAP 1A & B bind lamins A, C and B1, while LAP 2 binds lamin B1 only.
In addition to lamin binding, LAP 2 has been shown to bind to chromosomes.
Phosphorylation of LAP 2 during mitosis inhibits binding to both lamins
and chromosomes suggesting that LAP 2 plays an important role in nuclear
envelope re-assembly.
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