The Nuclear Envelope and Traffic between the Nucleus and the Cytoplasm PDF

The Nuclear Envelope and Traffic between the Nucleus and the Cytoplasm Asst. Prof. Burcu TÜRKGENÇ Dept. of Medical Biology 1  NUCLEUS ( is present in nearly all eukaryotic cells) There are some exceptional Anucleate cells, Mammalian erythrocytes Bl...

The Nuclear Envelope and Traffic between the Nucleus and the Cytoplasm Asst. Prof. Burcu TÜRKGENÇ Dept. of Medical Biology 1  NUCLEUS ( is present in nearly all eukaryotic cells) There are some exceptional Anucleate cells, Mammalian erythrocytes Blood platelets (thrombocytes) They are severly limited in their metabolic activities they can not undergo cell division Nucleus is stained with basic dyes Hematoxylin: purple Kernechtrot: red Carmin: red Methylene blue: blue. 2  NUCLEUS Functions DNA replication Transcription Within Nucleus RNA processing Translation Within cytoplasm 3  Gene expression is composed of two steps 1-Transcription (process of RNA synthesis) 2-Translation (Protein synthesis) 1-Transcription (process of RNA synthesis) In the nucleus gene sequence is copied into mRNA synthesis molecule 2-Translation (Protein synthesis) In the Cytoplasm The process of translating the sequence of a (mRNA) molecule to a sequence of amino acids 4  NUCLEUS 1-Nuclear envelope 2-Chromatin (DNA+ Proteins: Histon, nonhiston) 3-Nucleolus 4-Nucleoplasm (nuclear matrix) 5  NUCLEAR ENVELOPE Nucleus is surrounded by two parallel membranes inner membrane and outer membrane separated by perinuclear space Nuclear envelope separates the contents of the nucleus from the cytoplasm. 6  NUCLEAR ENVELOPE Ribosomes are attached to the outer membrane Outer membrane sometimes continuous with Rough endoplasmic reticulum Around the nuclear envelope there are circular pores Nuclear pores provide communication between nucleus and cytoplasm Lining the inner nuclear membrane is a protein meshwork called nuclear lamina. 7  NUCLEAR ENVELOPE It acts as a selective permeability barrier. Prevents free passage of most molecules between the nucleus and the cytoplasm. Maintains the nucleus as a distinct biochemical compartment. The nuclear envelope regulates the transport of molecules into and out of the nucleus. 8  9 A cross-sectional view of a typical cell nucleus  NUCLEAR LAMINA Closely associated with the inner membrane Network-like protein structure; Nuclear lamina Nuclear lamina is composed of 3 different polypeptides; Lamins Lamin A,B,C Lamins are related to the intermediate filament 10 11  Nuclear Lamina Function: Provide a structural support (scaffold beneath the nuclear membrane) Serve as linkers between the nuclear envelope and chromatin fibers. Inner nuclear membrane contains several integral proteins such as Emerin and Lamin B receptor (LBR) that interact with nuclear lamins The lamins also bind chromatin 12 Lamin dimers and formation of nuclear lamina 13  Nuclear Lamina May have a role in regulation of transcription and DNA replication Mutations of Lamin A : Result in breakdown of nuclear envelope Cause muscular dystrophy. 14  Diseases of Nuclear Lamina Mutations of LMNA gene (encodes lamin A and C) result in muscular dystrophy; Charcot-Marie-Tooth disease Emery-Dreifus muscular dystrophy Hutchinson-Gilford progeria syndrome 15 Charcot-Marie-Tooth disease (nerve damage, atrophy of muscles in the feet, legs, and hands. 16 Emery-Dreifus muscular dystrophy affect skeletal muscles and cardiac muscle 17 Hutchinson-Gilford progeria syndrome (rapid appearance of aging beginning in childhood.) 18  Nuclear Pores Around the nuclear envelope there are circular pores where the outer and inner membranes fuse. Nuclear pores transverse the nuclear envelope and provide communication between the nucleoplasm and cytoplasm. 19 Nuclear pores 20  Nuclear Pore Complex (NPC) In a typical nucleus; There are 15-20 pore complexes per µm2 (Total number of NPC is 3000-4000) The number varies with the activity of the cell Nucleus of an oocyte contains 50 million pore complexes Frog erythrocyte nucleus (inactive nucleus) 1-3 pore complexes per µm2 *Highly active cells like lymphocytes or oocytes (which require extensive protein synthesis and gene expression) have a large number of nuclear pore complexes 21  During replication 1 million histon molecules are transported to the nucleus per 3 min. Each NPC transports100 histon per min. In a rapidly growing cell, Each NPC transports 6 large and small ribosomal subunits per minute from nucleus to the cytosol. 22  Nuclear pore complex (in negatively stained preparations); NPC consists of Eight peripheral granules, A central granule Contains 50-100 types of proteins called nucleoporins. It contains a water-filled cylindrical channel about 9nm in diameter and 15nm long. 23 24 25 26  Nuclear pore complex Permeability of the nuclear pores to molecules is variable molecules of about 5000 daltons can freely enter nucleus 60 000 dalton or larger molecules can not pass through NPC but all pores are permeable to some macromolecules. e.g. mRNAs, DNA polymerase, RNA polymerase ( 100.000-200.000 Dalton ) (Active transport) Nuclear pores allow the regulated exchange of molecules (selective traffic of proteins and RNAs) Passage of large molecules through pore complex requires energy. 27 Traffic of molecules through nuclear pore complexes 28  Selective import of proteins to the nucleus Histones DNA polymerases RNA polymerases Transcription factors Splicing factors Ribosomal proteins These proteins are targeted to the nucleus by specific amino acid sequences called nuclear localization signals that are recognized by transport receptors and direct the transport of the proteins through the nuclear pore complex. 29  Nuclear Localization Signals Nuclear localization signals are recognized by proteins that function as nuclear transport receptors. Most are members of the karyopherin protein family and function either as importins, which transport macromolecules to the nucleus from the cytoplasm, or exportins, which transport macromolecules from the nucleus to the cytoplasm 30  Nuclear transport DOI: 10.1146/annurev.biochem.76.052705.161529 Figure: Schematic drawing of nuclear import and export processes mediated by karyopherins (also known as importins and exportins). The two nuclear pore complexes (NPCs) show the characteristic nuclear basket and cytoplasmic fibrils. (bottom left) An importin binds a cargo in the cytoplasm and releases it upon binding RanGTP in the nucleus ( upper left ). ( upper right ) An exportin binds both cargo and RanGTP in the nucleus and releases them upon conversion of RanGTP 31 into RanGDP (bottom right). Export of RNA’s from nucleus to the cytoplasm mRNA (messenger RNA) rRNA (ribozomal RNA)+Ribosomal proteins (Subunits) tRNA (transfer RNA) miRNA (microRNA) Export of all RNAs via exportins is an active energy- dependent process.