CHAPTER 2 Integral Proteins of the Nuclear Pore Membrane

The nuclear envelope contains three distinct membrane domains. The outer nuclear membrane faces the cytoplasm and is continuous with the rough endoplasmic reticulum (ER). Like the rough ER, the nuclear outer membrane is covered with ribosomes engaged in translating secreted and integral membrane proteins. The inner nuclear membrane faces the nucleoplasm, has its own unique protein composition and interacts with the fibrous meshwork of the nuclear lamina (reviewed in ref 6). The inner and outer nuclear membranes fuse to form the third membrane domain, termed the pore membrane domain. Nuclear pore complexes (NPCs) are anchored at the pore membrane domain and mediate both passive diffusion and active nucleocytoplasmic transport. Aaive transport requires signals on the imported or ex-ported macromolecules, termed nuclear localization signals (NLS) and nuclear export signals (NES), respectively. Transport is mediated by soluble NLS and NES receptors (termed importins/ exportins/karyopherins/transportins), whose direction of movement is determined by Ran, a small GTP-binding protein (reviewed in refs. 22, 48 and 50). NPC structure includes soluble proteins, termed nucleoporins (nups) and integral membrane proteins, termed POMs. The NPC is anchored to the pore membrane by binding to POMs. ̂ " ^ POMs are also proposed to have roles in nuclear pore assembly, nucleocytoplasmic transport and NPC organization (see below)