Читать книгу Principles of Virology - Jane Flint, S. Jane Flint - Страница 249

The nuclear envelope is composed of two typical lipid bilayers separated by a luminal space. Like all other cellular membranes, it is impermeable to macromolecules such as proteins. However, the nuclear pore complexes that stud the nuclear envelopes of all eukaryotic cells provide aqueous channels that span both the inner and outer nuclear membranes for exchange of small molecules, macromolecules, and macromolecular assemblies between nuclear and cytoplasmic compartments. Numerous experimental techniques, including direct visualization of gold particles attached to proteins or RNA molecules as they are transported, have established that nuclear proteins enter and RNA molecules exit the nucleus by transport through the nuclear pore complex. The functions of the nuclear pore complex in these processes are not completely understood, not least because this important cellular machine is large (molecular mass, approximately 125 × 10⁶ kDa in vertebrates), built from many different proteins, architecturally elaborate, and dynamic (Fig. 5.24).

The nuclear pore complex allows passage of cargo in and out of the nucleus by either passive diffusion or facilitated translocation. Passive diffusion does not require interaction between the cargo and components of the nuclear pore complex and becomes inefficient as molecules approach 9 nm in diameter. Objects as large as 39 nm in diameter can pass through nuclear pore complexes by facilitated translocation via specific interactions between the cargo and components of the nuclear pore complex. Many subviral particles are too large to pass through the nuclear pore complex, but several strategies overcome this limitation.