The Structure and Function of the Nuclear Envelope

1. The NucleusNuclear OrganizationNuclear Envelope and Molecular TraffickingNucleolus and rRNA Processing

2. The Structure and Function of the Nuclear Envelope The nuclear envelope has a complex structure consisting of two nuclear membranes, an underlying nuclear lamina, and nuclear pore complexes. Nuclear membranes are a system of two concentric membranes (inner and outer) that surround the nucleus. The nuclear envelope separates the contents of the nucleus from the cytoplasm and provides the structural framework of the nucleus.

3. 9.1 The nuclear envelope • The selective traffic of proteinsand RNAs through the nuclear pore complexes establish the internal composition of the nucleus and also play a critical role in regulating eukaryotic gene expression.

4. Structure of the Nuclear Envelope The critical function of the nuclear membranes is to act as a barrier that separates the contents of the nucleus from the cytoplasm. The nuclear lamina underlies the inner nuclear membrane and is a fibrous meshwork that provides structural support to the nucleus. Lamins are 60- to 80-kilodalton fibrous proteins that make up the nuclear lamina.

5. Structure of the Nuclear Envelope Lamins are composed of intermediate filament proteins which combine with each other to form higher-order structures. Attachment of Lamins to the nuclear envelope is mediated by prenylation and binding to specific inner nuclear membrane proteins such as emerin and the lamin B receptor. Nuclear Lamina diseases: Hutchinson-Gilford progeria

6. Nuclear Lamina Defects • X-linked Emery-Dreifuss muscular Dystrophy • Results form mutations in the nuclear envelope protein, emerin. • Non-Sex Linked Emery Dreifuss Muscular Dystrophy • Results from mutations in the LMNA gene which is a single gene that encodes lamin A and C. • Hutchinson-Guilford Progeria Disease • Results from a 150 bp deletion within the lamin A gene.

7. The Nuclear Pore Complex A nuclear pore complex consists of a structure with eightfold symmetry organized around a large central channel.

8. The Nuclear Pore Complex Nuclear pore complexes are the only channels through which small polar molecules, ions, and macromolecules can travel between the nucleus and the cytoplasm. Depending on their size, molecules can travel through the nuclear pore complex by one of two different mechanisms. Passive transport Active transport –energy dependent

9. Selective Transport of Proteins to and from the Nucleus • Nuclear export signals are specific amino acid sequences that target proteins for export from the nucleus. • Protein import through the nuclear pore complex begins when a specific importin binds to the nuclear localization signal of a cargo protein in the cytoplasm.

10. Selective Transport of Proteins to and from the Nucleus Nuclear localization signals are specific amino acid sequences that are recognized by transport receptors and direct the transport of proteins through the nuclear pore complex. Nuclear localization signals have been identified in many proteins.

11. Selective Transport of Proteins to and from the Nucleus Nuclear transport receptors are proteins that recognize nuclear localization signals and mediate transport across the nuclear envelope. Karyopherin is a nuclear transport receptor. Importins transport macromolecules to the nucleus from the cytoplasm. Exportins transport macromolecules from the nucleus to the cytoplasm.

12. 9.10 Distribution of Ran/GTP across the nuclear envelope • Ran is one of several types of small GTP-binding proteins that regulate movement of macromolecules through the nuclear pore.

13. 9.12 Nuclear export

14. Regulation of Nuclear Protein Import The transport of proteins to the nucleus is yet another level at which the activities of nuclear proteins can be controlled. In one mechanism of regulation, transcription factors associate with cytoplasmic proteins that mask their nuclear localization signals.

15. Transport of RNAs Most RNAs are exported from the nucleus to the cytoplasm. RNAs are transported across the nuclear envelope as ribonucleoprotein complexes, or RNPs.

16. Internal Organization of the Nucleus The nucleus has an internal structure that organizes the genetic material and localizes nuclear functions. Chromatin within the nucleus is organized into large loops of DNA, and specific regions of these loops are bound to the lamin matrix by lamin-binding proteins in the chromatin.

17. 9.16 Heterochromatin in interphase nuclei • The chromosomes in interphase chromatin are actually arranged in an organized fashion. • Heterochromatin is condensed, transcriptionally inactive chromatin. • Euchromatin is decondensed, transcriptionally active interphase chromatin that is distributed throughout the nucleus.

18. Chromosomes and Higher-Order Chromatin Structure Individual chromosomes also occupy distinct territories within the nuclei of mammalian cells. The chromatin in interphase nuclei is organized into looped domains of about 50 to 100 kb of DNA.

19. Sub-Compartments within the Nucleus • Many proteins of the nucleus are localized to discrete, sub-nuclear bodies that have a low-density, sponge-like structure that allows macromolecules from the rest of the nucleus to move in and out. • The nuclei of mammalian cells contain clustered sites of DNA replication within which the replication of multiple DNA molecules takes place.

20. Sub-Compartments within the Nucleus Components of the mRNA splicing machinery are concentrated in discrete nuclear bodies termed nuclear speckles. Nuclei contain several other types of distinct structures, such as PML bodies and cajal bodies. The function of PML bodies remains largely unknown. The nucleolus is the most prominent nuclear body and is the site of rRNA transcription and processing as well as aspects of ribosome assembly

21. Ribosomal RNA Genes and the Organization of the Nucleolus The nucleolus is associated with the chromosomal regions that contain the genes for the 5.8S, 18S, and 28S rRNAs. To meet the need for transcription of large numbers of rRNA molecules, all cells contain multiple copies of the rRNA genes.

22. Ribosomal RNA Genes and the Organization of the Nucleolus Morphologically, nucleoli consist of three distinguishable regions: the fibrillar center, dense fibrillar component, and granular component. Nucleolar organizing regions are where nucleoli become associated with the chromosomal regions that contain the 5.8S, 18S, and 28S rRNA genes.

23. Transcription and Processing of rRNA Each nucleolar organizing region contains a cluster of tandemly repeated rRNA genes separated from each other by nontranscribed spacer DNA. In higher eukaryotes the primary transcript of the rRNA genes is the large 45S pre-rRNA, which contains the 18S, 5.8S, and 28S rRNAs as well as transcribed spacer regions.

24. Transcription and Processing of rRNA The processing of pre-rRNA involves a substantial amount of base modification resulting both from the addition of methyl groups to specific bases and ribose residues and from the conversion of uridine to pseudouridine. Small nucleolar RNAs, or snoRNAs, are complexed with proteins and function in pre-rRNA processing.

25. Transcription and Processing of rRNA Small nucleolar RNAs or snoRNAs complex with proteins to generate snoRNPs which regulate processing of rRNAs. Most snoRNAs function as guide RNAs to direct the specific base modifications of pre-rRNA, including methylation of specific ribose residues and the formation of pseudouridines.

26. Ribosome Assembly The formation of ribosomes involves the assembly of the ribosomal precursor RNA with both ribosomal proteins and 5S rRNA. The association of ribosomal proteins with rRNA begins while the pre-rRNA is still being synthesized. The final stages of ribosome maturation follow the export of pre-ribosomal particles to the cytoplasm, forming the active 40S and 60S subunits of eukaryotic ribosomes.