The nuclear envelope forms a selectively permeable barrier between the nuclear and cytoplasmic compartments. Electron microscopy reveals that the envelope has two concentric mem branes separated by a narrow (30-50 nm) perinuclear space (Figures1 and 2). This space and the outer nuclear mem brane are continuous with the extensive cytoplasmic network of the rough endoplasmic reticulum (RER). Closely associated with the inner nuclear membrane is a highly organized mesh work of proteins called the nuclear lamina (Figure 3), which stabilizes the nuclear envelope. Major components of this layer are the class of intermediate filament proteins called lamins that bind to membrane proteins and associate with chromatin in nondividing cells.

Fig1. Relationship of nuclear envelope to the rough ER (RER).

Fig2. Ultrastructure of a nucleus.

Fig3. The nuclear envelope, nuclear lamina, and nuclear pore complexes.

The inner and outer nuclear membranes are bridged at nuclear pore complexes (Figures 1 through 5). Various core proteins of a nuclear pore complex, called nucleoporins, display eightfold symmetry around a lumen. Ions and small solutes pass through the lumen by simple diffusion, but the nucleoporin complex regulates the movement of macro molecules between nucleus and cytoplasm. A growing cell has 3000-4000 such channels, each providing passage for up to 1000 macromolecules per second. Individual pores permit molecular transfer in both directions simultaneously. Mac romolecules shipped out of the nucleus include ribosomal subunits and other RNAs associated with proteins, while inbound traffic consists of chromatin proteins, ribosomal proteins, transcription factors, and enzymes. Using mechanisms similar to that by which specific proteins are recognized and translocated across the RER membrane, proteins of complexes destined for the cytoplasm have specific nuclear export sequences and proteins to be imported have nuclear localization sequences. Such sequences bind specifically to transport proteins (importins) that in turn interact with proteins of the pore complexes for transfer across the nuclear envelope. Energy for the transport is derived from guanosine 5′-triphosphate (GTP), with specific GTPases helping provide directionality to the transfer.

Fig4. Nuclear pores.

Fig5. Cryofracture of nuclear envelope showing nuclear pores.

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