The Eukaryote Nucleus

BIO 224

Intro to Cell and Molecular Biology

Nucleus

• The control center of the cell• Holds the cell’s genetic information• Site of DNA replication, transcription, and

RNA processing• Defining feature of eukaryotic cells• Separates the genome translation

process, plays a major role in eukaryote gene expression

The Nuclear Envelope

• Separates the nuclear contents from the cytoplasm and provides structural support for the nucleus

• Maintains the nucleus as a distinct biochemical compartment

• Has two membranes, nuclear pore complexes, and an underlying lamina

Nuclear Envelope Membranes

• Each membrane is a phospholipid bilayer• Outer membrane is continuous with ER

– Has a similar function to the membrane of the ER – Has ribosomes on the cytoplasmic surface

• Inner membrane has nucleus specific proteins– The proteins bind it to the nuclear lamina

• Membranes are joined at pore complexes– The only place macromolecules and small polar

molecules may pass– Convey RNAs between the nucleus and cytoplasm

Nuclear Pore Complex

• The only channels for small polar molecules, ions, macromolecules (proteins and RNA) to travel between the nucleus and cytoplasm

• 120nm in diameter, 125 MD mass• The nuclear pore of vertebrates is a complex

of 30-50 nucleoporins• Control traffic between the nucleus and

cytoplasm, play a role in eukaryote cell physiology

Nuclear Lamina

• A fibrous meshwork that provides structural support to the nucleus

• Made of 60-80 kD intermediate protein filaments called lamins

• Filaments associate with one another to form complex higher order structures

• Associate with the internal face of the inner nuclear membrane via lipid and protein interactions

Model of lamin assembly

The nuclear lamina

Nuclear Protein Transport• Proteins imported from the cytoplasm to the nucleus are responsible for

genome structure and function– Histones, DNA and RNA polymerases, Transcription factors

• Proteins targeted to the nucleus by nuclear localization signals– Recognized by transport receptors and direct the transport of proteins

through the pore complex• Karyopherin family nuclear of transport receptors function as importins or exprotins

in response to recognition of the nuclear localization signals• Nuclear export signals target proteins for export from the nucleus

– Specific AA signals that are recognized by exportins• Ran is a protein that (in association with GTP) regulates passage of

macromolecules through the nuclear pore– Regulation of protein transport allows for control of nuclear proteins and gene

expression• Plays a role in cellular response to environmental change

– Allows cell surface signals to be transmitted to the nucleus– Regulates import of transcription factors and protein kinases

Nuclear Transport of RNA

• Most RNAs exported from nucleus to cytoplasm– mRNAs, rRNAs, tRNAs

• Transport receptors interact with the nuclear pore complex and use energy

• Importins and exportins transport most tRNAs, rRNAs and snRNAs using Ran/GTP pathway

• mRNAs exported by a 2 protein “mRNA exporter” complex independently of Ran

Nuclear Transport of RNA

• RNAs are transported as RNPs (ribonucleoprotein complexes)

• rRNAs are associated with ribosomal proteins and specific RNA processing proteins – Subunits exported to the cytoplasm

• Pre-mRNAs and mRNAs associate with a set of 20 proteins during processing and transport

• tRNAs are exported by exportin-t• Small RNAs (snRNAs & snoRNAs) function in the

nucleus to help with RNA processing– snRNAs exported to cytoplasm to associate with proteins to

becom snRNPs and reenter nucleus to function

Internal Nuclear Organization

• The nucleus’ internal structure organizes genetic material and localizes nuclear functions

• Nuclear lamins act as chromatin attachment sites and to organize proteins

• Lamin dependent complexes involved in DNA repair, chromatin organization, gene regulation, and signal transduction

• Chromatin organized into large loops of DNA

Chromosomes and Chromatin Structure

• Chromatin condenses during mitosis to form compact chromosomes of metaphase

• Heterochromatin remains highly condensed and transcriptionally inactive during interphase

• 2 types of heterochromatin• Constitutive: DNA sequences aren’t transcribed• Facultative: DNA sequences transcribed in some cell

types• Euchromatin decondenses and is distributed through

nucleus during interphase• Chromosomes in interphase chromatin arranged in

organized fashion and divided into functional domains with roles in regulating gene expression

Nuclear Sub-compartments• Most nuclear processes localized to distinct regions:

subnuclear bodies• It is not fully understood how or why• Mammal nuclei have clustered sites of DNA replication

where multiple molecules are replicated• mRNA processing machinery concentrated in discrete

bodies called nuclear speckles• PML bodies are sites of localization of transcriptional

regulatory proteins involved in acute promyelocytic leukemia– Interact with chromatin and are sites of accumulation of

transcription factors and chromatin modifying proteins• Cajal bodies have coilin protein and small RNPs

– Thought to be site of RNP assembly and snRNP processing

The Nucleolus• Site of rRNA transcription and processing and ribosome

assembly• General role in modification of RNA• Site of transcription of ribosomal subunit genes • Important in oocytes where genes need to be amplified

to support increased numbers of ribosomes needed in early embryonic development

• Divided into three regions: fibrillar center, dense fibrillar component, granular component– Sites of progressive stages of rRNA transcription, processing,

and ribosome assembly• Ribosomes assembled from ribosomal precursor RNA,

ribosomal proteins, and 5S rRNA

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