Cell Biology Lecture 6

Nucleus Structural components (molecular structure and function of each component) 1. Nuclear envelope 2.Nucleoskeleton 3.Nuclear pores 4.Chromatin 5.Nucleolus 6.Sub-nuclear bodies: Cajal bodies, PIKA, PML bodies, Paraspeckles, Speckles Functions: 1.Gene expression regulation 2.Nuclear transport 3.Cell compartmentalization 4.Processing of pre-mRNA Disease and dynamics: OUTLINE

The NUCLEAR COMPARTMENT nucleus or nuculeus, meaning kernel Largest most obvious double membrane-bound cellular organelle in animals. Dense, roughly spherical In mammalian cells, the average diameter 6 micrometers (μm), 10% of total cell volume. Nucleoplasm, similar in composition to the cytosol maintains the integrity of genes controls the cellular activities by regulating gene expression  production of other cell components  cell's growth  reproduction A nucleated and multinucleated cells RBC Skeletal muscle, filamentous fungi Functions as “The control center of the cell “

The Structural components

Nuclear envelope The nuclear envelope consists of 1.two nuclear membranes, arranged parallel to one another 2.an underlying nuclear lamina 3.nuclear pore complexes completely encloses the nucleus, separates the cell's genetic material from the surrounding cytoplasm, serving as a barrier to prevent macromolecules from diffusing freely between the nucleoplasm and the cytoplasm.

The outer nuclear membrane is continuous with the membrane of the rough endoplasmic reticulum (RER), and is similarly studded with ribosomes. the outer nuclear membrane contains various proteins found in far higher concentrations than the endoplasmic reticulum The Inner membrane The inner nuclear membrane encloses the nucleoplasm, and is covered by the nuclear lamina, It is connected to the outer membrane by nuclear pores which penetrate the membranes. the inner nuclear membrane carries unique proteins that are specific to the nucleus. The space between the membranes is called the perinuclear space (It is typically about 20–40 nm wide) and is continuous with the RER lumen. Nuclear envelope

Nucleoskeleton: NUCLEAR LAMINA Underlying the inner nuclear membrane, a fibrous network within the nucleus that 1.adds mechanical support, much like the cytoskeleton, which supports the cell as a whole 2.involved in chromatin function and gene expression. The nuclear lamina is composed of one or more related proteins called lamins. Most mammalian cells contain four different lamins, designated A, B 1, B 2, and C. All the lamins are 60- to 80-kilodalton (kd) fibrous proteins that are related to the intermediate filament proteins of the cytoskeleton.

The association of lamins with the inner nuclear membrane is facilitated by the post translational addition of lipid—in particular, prenylation of C-terminal cysteine residues In addition, the lamins bind to inner nuclear membrane proteins, which may help organize the lamin filaments into a meshwork and mediate their attachment to the membrane. lamins associate with each other to form filaments 1.Two lamins form a dimer in which the α- helical regions of two polypeptide chains are wound around each other in a structure called a coiled coil. 2.Two of these dimer structures then join side by side, to form a tetramer called a protofilament 3.Eight of these protofilaments form a lateral arrangement that is twisted to form a ropelike filament. Structural organization

Medical implication: Mutations in lamin genes leading to defects in filament assembly are known as laminopathies. The most notable laminopathy is the family of diseases known as progeria, which causes the appearance of premature aging in its sufferers. The exact mechanism by which the associated biochemical changes give rise to the aged phenotype is not well understood.

NUCLEAR PORE COMPLEX Because the nuclear membrane is impermeable to large molecules, nuclear pores are required that regulate nuclear transport of molecules across the envelope. The nucleus of a typical mammalian cell will have about 3000 to 4000 pores throughout its envelope. Pore complex is composed of 50 to 100 different proteins collectively referred to as nucleoporins. 125 mega Daltons in molecular weight, about 30 times the size of a ribosome The pores are 100 nm in total diameter; however, the gap through which molecules freely diffuse is only about 9 nm wide, due to the presence of regulatory systems within the center of the pore. Movement of large molecules such as proteins and RNA through the pores is required for both gene expression and the maintenance of chromosomes. They must be actively transported by carrier proteins while allowing free movement of small molecules and ions. WHY: This size allows the not-free passage of small water-soluble molecules while preventing larger molecules, such as nucleic acids and larger proteins, from in-appropriately entering or exiting the nucleus. These large molecules must be actively transported into the nucleus instead.

A ring-shaped structure at a position where the inner and outer membranes fuse. Attached to the ring is nuclear basket that extends into the nucleoplasm, and a series of filamentous extensions that reach into the cytoplasm. Both structures serve to mediate binding to nuclear transport proteins. The central channel is approximately 40 nm in diameter, which is wide enough to accommodate the largest particles able to cross the nuclear envelope. It contains a structure called the central transporter, through which the active transport of macromolecules is thought to occur. STRUCTURE

1.Ran (RAs-related Nuclear protein) …interacting with karyopherins and changing their ability to bind or release cargo molecules. 2.Cargo proteins containing a nuclear localization signal (NLS) are bound by importins and transported into the nucleus. 3.Inside the nucleus, RanGTP binds to importin and releases the import cargo. 4.Cargo that needs to get out of the nucleus (carrying NES) into the cytoplasm binds to exportin in a ternary complex with RanGTP. Upon hydrolysis of RanGTP to RanGDP outside the nucleus, the complex dissociates and export cargo is released. Transport Mechanism  importins depend on RanGTP to dissociate from their cargo, exportins require RanGTP in order to bind to their cargo

1.The nuclear envelope separates the contents of the nucleus from the cytoplasm 2.provides the structural framework of the nucleus. 3.acting as barriers that prevent the free passage of molecules between the nucleus and the cytoplasm, maintain the nucleus as a distinct biochemical compartment. 4.Phospholipid bilayers, which are permeable only to small nonpolar molecules, Other molecules are unable to diffuse through the phospholipid bilayer. 5.Nuclear pore complex sole channel, allow the regulated exchange of molecules between the nucleus and cytoplasm. 6.The selective traffic of proteins and RNAs through the nuclear pore complexes not only establishes the internal composition of the nucleus, but also plays a critical role in regulating eukaryotic gene expression. FUNCTIONS OF NUCLEAR ENVELOPE

Assignment 4 classes of ATP powered pumps Write in your words Paraphrase