Summary The double membrane of the nuclear envelope is a formidable barrier separating the nucleus and cytoplasm of eukaryotic cells. However, movement of specific macromolecules across the nuclear envelope is critical for embryonic development, cell growth and differentiation. Transfer of molecules between the nucleus and cytoplasm occurs through the aqueous channel formed by the nuclear pore complex (NPC)". Although small molecules may simply diffuse across the NPC, transport of large proteins and RNA requires specific transport signals and is energy dependent. A family of pore glycoproteins modified by 0-linked N-acetylglucosamine moieties are essential for transport through the NPC. Recent evidence suggests that the regulation of nuclear transport may also involve the interaction of RNA and nuclear proteins with specific binding proteins that recognize these transport signals. Are these nuclear pore glycoproteins and signal binding proteins the 'gatekeepers' that control access to the genetic material? Recent evidence obtained from a combination of biochemical and genetic approaches suggests - perhaps.
Introduction Following mitosis in eukaryotic cells, the processes of DNA replication and RNA transcription are sequestered in the nucleus away from the cytoplasmic translation machinery and other metabolic processes. Although there is no complete answer as to why such strict compartmentalization is necessary, this physical separation of transcription from translation dictates that only mature, fully processed mRNA is translated into protein. Furthermore, the ability to control the exchange of proteins and RNA between the nuclear and cytoplasmic compartments may provide an additional level of regulation over the processes of DNA replication and gene expression. The portal through which transport across the nuclear envelope occurs is the multiprotein assembly termed the nuclear pore complex (NPC). *Abbreviations: NPC, nuclear pore complex; GlcNAc, N-acetylglucosamine; WGA, wheat germ agglutinin.
Structure of the Nuclear Pore Complex Nuclear pores have been found embedded in the nuclear envelope of all eukaryotic cells. The number of pores per nucleus varies greatly depending on the cell type, ranging from hundreds to several million per nucleus. Generally speaking, rapidly metabolizing cells contain a greater density of pores than relatively inert cells. For example, Xenopus oocyte nuclei contain 50-60 pores/micron and rat liver nuclei contain 15-20 pores/micron while the nuclei of avian red blood cells contain
Introduction Following mitosis in eukaryotic cells, the processes of DNA replication and RNA transcription are sequestered in the nucleus away from the cytoplasmic translation machinery and other metabolic processes. Although there is no complete answer as to why such strict compartmentalization is necessary, this physical separation of transcription from translation dictates that only mature, fully processed mRNA is translated into protein. Furthermore, the ability to control the exchange of proteins and RNA between the nuclear and cytoplasmic compartments may provide an additional level of regulation over the processes of DNA replication and gene expression. The portal through which transport across the nuclear envelope occurs is the multiprotein assembly termed the nuclear pore complex (NPC). *Abbreviations: NPC, nuclear pore complex; GlcNAc, N-acetylglucosamine; WGA, wheat germ agglutinin.
Structure of the Nuclear Pore Complex Nuclear pores have been found embedded in the nuclear envelope of all eukaryotic cells. The number of pores per nucleus varies greatly depending on the cell type, ranging from hundreds to several million per nucleus. Generally speaking, rapidly metabolizing cells contain a greater density of pores than relatively inert cells. For example, Xenopus oocyte nuclei contain 50-60 pores/micron and rat liver nuclei contain 15-20 pores/micron while the nuclei of avian red blood cells contain
